Cell
Biology
Glossary
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CELL BIOLOGY
CELL BIOLOGY
Glossary
A coiled secondary structure of a polypeptide chain formed by hydrogen bonding between amino acids
separated by four residues.
ABC transporters A large family of membrane transport proteins characterized by a highly conserved ATP
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Cell
Biology
Glossary
binding domain.
actin An abundant 43-kd protein that polymerizes to form cytoskeletal filaments.
actin bundle Actin filaments that are crosslinked into closely packed arrays.
actin network Actin filaments that are crosslinked into loose three-dimensional meshworks.
activation energy The energy required to raise a molecule to its transition state to undergo a chemical reaction.
active site The region of an enzyme that binds substrates and catalyzes an enzymatic reaction.
active transport The transport of molecules in an energetically unfavorable direction across a membrane
coupled to the hydrolysis of ATP or other source of energy.
adaptin A protein that binds to membrane receptors and mediates the formation of clathrin-coated vesicles.
adenine A purine that base-pairs with either thymine or uracil.
adenoma A benign tumor arising from glandular epithelium.
adenylyl cyclase An enzyme that catalyzes the formation of cyclic AMP from ATP.
adherens junction A region of cell-cell adhesion at which the actin cytoskeleton is anchored to the plasma
membrane.
Akt A protein-serine/threonine kinase that is activated by PIP3 and plays a key role in signaling cell survival.
allele One copy of a gene.
allosteric regulation The regulation of enzymes by small molecules that bind to a site distinct from the active
site, changing the conformation and catalytic activity of the enzyme.
alternative splicing The generation of different mRNAs by varying the pattern of pre-mRNA splicing.
amino acid Monomeric building blocks of proteins, consisting of a carbon atom bound to a carboxyl group, an
amino group, a hydrogen atom, and a distinctive side chain.
aminoacyl tRNA synthetase An enzyme that joins a specific amino acid to a tRNA molecule carrying the
correct anticodon sequence.
amphipathic A molecule that has both hydrophobic and hydrophilic regions.
anaphase The phase of mitosis during which sister chromatids separate and move to opposite poles of the
spindle.
anaphase A The movement of daughter chromosomes toward the spindle poles during mitosis.
anaphase B The separation of the spindle poles during mitosis.
anaphase-promoting complex A ubiquitin ligase that triggers progression from metaphase to anaphase by
signaling the degradation of cyclin B and cohesins.
angiogenesis The formation of new blood vessels.
antibody A protein produced by B lymphocytes that binds to a foreign molecule.
anticodon The nucleotide sequence of transfer RNA that forms complementary base pairs with a codon
sequence on messenger RNA.
antigen A molecule against which an antibody is directed.
antiport The transport of two molecules in opposite directions across a membrane.
AP endonuclease A DNA repair enzyme that cleaves next to apyrimidinic or apurinic sites in DNA.
apical domain The exposed free surface of a polarized epithelial cell
apoptosis An active process of programmed cell death, characterized by cleavage of chromosomal DNA,
chromatin condensation, and fragmentation of both the nucleus and the cell.
Arabidopsis thaliana A small flowering plant used as a model for plant molecular biology and development.
archaebacteria One of two major groups of prokaryotes; many species of archaebacteria live in extreme
conditions similar to those prevalent on primitive Earth.
ARF A GTP-binding protein required for vesicle budding from the trans-Golgi network.
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Cell
Biology
Glossary
astral microtubules Microtubules of the mitotic spindle that extend to the cell periphery.
ATP (adenosine 5´-triphosphate) An adenine-containing nucleoside triphosphate that serves as a store of free
energy in the cell.
ATP synthase A membrane spanning protein complex that couples the energetically favorable transport of
protons across a membrane to the synthesis of ATP.
autocrine signaling A type of cell signaling in which a cell produces a growth factor to which it also responds.
autophagy The degradation of cytoplasmic proteins and organelles by their enclosure in vesicles from the
endoplasmic reticulum that fuse with lysosomes.
autoradiography The detection of radioisotopically labeled molecules by exposure to X-ray film.
axonemal dynein The type of dynein found in cilia and flagella.
axoneme The fundamental structure of cilia and flagella composed of a central pair of microtubules surrounded
by nine microtubule doublets.
β—sheet A sheetlike secondary structure of a polypeptide chain, formed by hydrogen bonding between amino
acids located in different regions of the polypeptide.
bacteriophage A bacterial virus.
basal body A structure similar to a centriole that initiates the growth of axonemal microtubules and anchors cilia
and flagella to the surface of the cell.
basal lamina A sheetlike extracellular matrix that supports epithelial cells and surrounds muscle cells, adipose
cells, and peripheral nerves.
base-excision repair A mechanism of DNA repair in which single damaged bases are removed from a DNA
molecule.
basement membrane See basal lamina.
basolateral domain The surface region of a polarized epithelial cell that is in contact with adjacent cells or the
extracellular matrix.
benign tumor A tumor that remains confined to its site of origin.
cadherins A group of cell adhesion molecules that form stable cell-cell junctions at adherens junctions and
desmosomes.
Caenorhabditis elegans A nematode used as a simple multicellular model for development.
calmodulin A calcium-binding protein.
Calvin cycle A series of reactions by which six molecules of CO2 are converted into glucose.
cAMP phosphodiesterase An enzyme that degrades cyclic AMP.
cAMP-dependent protein kinase See protein kinase A.
cancer A malignant tumor.
carbohydrate A molecule with the formula (CH2O)n. Carbohydrates include both simple sugars and
polysaccharides.
carcinogen A cancer-inducing agent.
carcinoma A cancer of epithelial cells.
cardiolipin A phospholipid containing four hydrocarbon chains.
carrier proteins Proteins that selectively bind and transport small molecules across a membrane.
caspases A family of proteases that bring about programmed cell death.
catalase An enzyme that decomposes hydrogen peroxide.
caveolae Small invaginations of the plasma membrane that may be involved in endocytosis.
Cdc2 A protein-serine/threonine kinase that is a key regulator of mitosis in eukaryotic cells.
Cdk inhibitor (CKI) A family of proteins that bind Cdks and inhibit their activity.
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Cell
Biology
Glossary
Cdks Cyclin dependent protein kinases that control the cell cycle of eukaryotes. See also Cdc2.
cDNA library A collection of recombinant cDNA clones.
cell adhesion molecules Transmembrane proteins that mediate cell-cell interactions.
cell cortex The actin network underlying the plasma membrane.
cell wall A rigid, porous structure forming an external layer that provides structural support to bacteria, fungi,
and plant cells.
cellulose The principal structural component of the plant cell wall, a linear polymer of glucose residues linked
by b(1
4) glycosidic bonds.
central dogma The concept that genetic information flows from DNA to RNA to proteins.
centriole A cylindrical structure consisting of nine triplets of microtubules in the centrosomes of most animal
cells.
centromere A specialized chromosomal region that connects sister chromatids and attaches them to the mitotic
spindle.
centrosome The microtubule-organizing center in animal cells.
cGMP phosphodiesterase An enzyme that degrades cGMP.
channel proteins Proteins that form pores through a membrane.
chaperone A protein that facilitates the correct folding or assembly of other proteins.
chaperonin A family of heat-shock proteins within which protein folding takes place.
chemiosmotic coupling The generation of ATP from energy stored in a proton gradient across a membrane.
chiasmata Sites of recombination that link homologous chromosomes during meiosis.
chitin A polymer of N-acetylglucosamine residues that is the principal component of fungal cell walls.
chlorophyll The major photosynthetic pigment of plant cells.
chloroplast The organelle responsible for photosynthesis in the cells of plants and green algae.
cholesterol A lipid consisting of four hydrocarbon rings. Cholesterol is a major constituent of animal cell plasma
membranes and the precursor of steroid hormones.
chromatin The fibrous complex of eukaryotic DNA and histone proteins. See histones, nucleosome, and
chromatosome.
chromatosome A chromatin subunit consisting of 166 base pairs of DNA wrapped around a histone core and
held in place by a linker histone.
chromosomes The carriers of genes, consisting of long DNA molecules and associated proteins.
cilium A microtubule-based projection of the plasma membrane that moves a cell through fluid or fluid over a
cell.
cis-acting control element A regulatory DNA sequence that serves as a protein binding site and controls the
transcription of adjacent genes.
citric acid cycle A series of reactions in which acetyl CoA is oxidized to CO2. The central pathway of oxidative
metabolism.
clathrin A protein that coats the cytoplasmic surface of cell membranes and assembles into basketlike lattices
that drive vesicle budding.
clone See recombinant molecule.
codon The basic unit of the genetic code; one of the 64 nucleotide triplets that code for an amino acid or stop
sequence.
coenzyme A (CoA) A coenzyme that functions as a carrier of acyl groups in metabolic reactions.
coenzyme Q A small lipid-soluble molecule that carries electrons between protein complexes in the
mitochondrial electron transport chain.
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Cell
Biology
Glossary
coenzymes Low-molecular-weight organic molecules that work together with enzymes to catalyze biological
reactions.
cohesins A complex of proteins that maintain the connection between sister chromatids.
colcemid A drug that inhibits the polymerization of microtubules.
colchicine A drug that inhibits the polymerization of microtubules.
collagen The major structural protein of the extracellular matrix.
collenchyma Plant cells characterized by thick cell walls; they provide structural support to the plant.
complementary DNA (cDNA) A DNA molecule that is complementary to an mRNA molecule, synthesized in
vitro by reverse transcriptase.
contact inhibition The inhibition of movement or proliferation of normal cells that results from cell-cell contact.
contractile ring A structure of actin and myosin II that forms beneath the plasma membrane during mitosis and
mediates cytokinesis.
corticosteroids Steroid hormones produced by the adrenal gland.
cosmid
replication. It can accommodate large DNA inserts of up to 45 kb.
crista A fold in the inner mitochondrial membrane extending into the matrix.
cyanobacteria The largest and most complex prokaryotes in which photosynthesis is believed to have evolved.
cyclic AMP (cAMP) Adenosine monophosphate in which the phosphate group is covalently bound to both the
3´ and 5´ carbon atoms, forming a cyclic structure; an important second messenger in the response of cells to a
variety of hormones.
cyclic electron flow An electron transport pathway associated with photosystem I that produces ATP without
the synthesis of NADPH.
cyclic GMP (cGMP) Guanosine monophosphate in which the phosphate group is covalently bound to both the
3´ and 5´ carbon atoms, forming a cyclic structure; an important second messenger in the response of cells to a
variety of hormones and in vision.
cyclins A family of proteins that regulate the activity of Cdks and control progression through the cell cycle.
cytochalasin A drug that blocks the elongation of actin filaments.
cytochrome oxidase A protein complex in the electron transport chain that accepts electrons from cytochrome
c and transfers them to O2.
cytokines Growth factors that regulate blood cells and lymphocytes.
cytokinesis Division of a cell following mitosis or meiosis.
cytosine A pyrimidine that base-pairs with guanine.
cytoskeleton A network of protein filaments that extends throughout the cytoplasm of eukaryotic cells. It
provides the structural framework of the cell and is responsible for cell movements.
density gradient centrifugation A method of separating particles by centrifugation through a gradient of a
dense substance, such as sucrose or cesium chloride.
deoxyribonucleic acid (DNA) The genetic material of the cell.
desmosome A region of contact between epithelial cells at which keratin filaments are anchored to the plasma
membrane. See also hemidesmosome.
diacylglycerol A secondary messenger formed from the hydrolysis of PIP2 that activates protein kinase C.
diakinesis The final stage of the prophase of meiosis I during which the chromosomes fully condense and the
cell progresses to metaphase.
Dictyostelium discoideum A unicellular eukaryote used for studies of cell movement and cell-cell signaling.
diploid An organism or cell that carries two copies of each chromosome.
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Cell
Biology
Glossary
diplotene The stage of mieosis I during which homologous chromosomes separate along their length but
remain associated at chiasmata.
DNA glycosylase A DNA repair enzyme that cleaves the bond linking a purine or pyrimidine to the deoxyribose
of the backbone of a DNA molecule.
DNA ligase An enzyme that seals breaks in DNA strands.
DNA polymerase An enzyme catalyzing the synthesis of DNA.
dolichol phosphate A lipid molecule in the endoplasmic reticulum upon which oligosaccharides are assembled
for the glycosylation of proteins.
domains Compact, globular regions of proteins that are the basic units of tertiary structure.
dominant The allele that determines the phenotype of an organism when more than one allele is present.
dominant inhibitory mutant A mutant that interferes with the function of the normal allele of the gene.
Drosophila melanogaster A species of fruit fly commonly used for studies of animal genetics and development.
dynein A motor protein that moves along microtubules towards the minus end.
E. coli (Escherichia coli) A species of bacteria used as a model for biochemistry and molecular biology.
E2F A family of transcription factors that regulate the expression of genes involved in cell cycle progression and
DNA replication.
ecdysone An insect steroid hormone that triggers metamorphosis.
ectoderm The outer germ layer; gives rise to tissues that include the skin and nervous system.
eicosanoid A class of lipids, including prostaglandins, prostacyclins, thromboxanes, and leukotrienes, that act
in autocrine and paracrine signaling.
electrochemical gradient A difference in chemical concentration and electric potential across a membrane.
electron microscopy A type of microscopy that uses an electron beam to form an image. In transmission
electron microscopy, a beam of electrons is passed through a specimen stained with heavy metals. In scanning
electron microscopy, electrons scattered from the surface of a specimen are analyzed to generate a
three-dimensional image.
electron transport chain A series of carriers through which electrons are transported from a higher to a lower
energy state.
embryonal stem (ES) cells Cells cultured from early embryos.
endocrine signaling A type of cell-cell signaling in which endocrine cells secrete hormones that are carried by
the circulation to distant target cells.
endocytosis The uptake of extracellular material in vesicles formed from the plasma membrane.
endoderm The inner germ layer; gives rise to internal organs.
endoplasmic reticulum (ER) An extensive network of membrane-enclosed tubules and sacs involved in
protein sorting and processing as well as in lipid synthesis.
endosome A vesicular compartment involved in the sorting and transport to lysosomes of material taken up by
endocytosis.
endosymbiosis A symbiotic relationship in which one cell resides within a larger cell.
enhancer A transcriptional regulatory sequence that can be located at a site distant from the promoter.
enzymes Proteins or RNAs that catalyze biological reactions.
epidermal cells Cells forming a protective layer on the surfaces of plants and animals.
epidermal growth factor (EGF) A growth factor that stimulates cell proliferation.
epithelial cells Cells forming sheets (epithelial tissue) that cover the surface of the body and line internal
organs.
equilibrium centrifugation The separation of particles on the basis of density by centrifugation to equilibrium in
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Cell
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Glossary
a gradient of a dense substance.
erythrocytes Red blood cells.
estrogen A steroid hormone produced by the ovaries.
eubacteria One of two major groups of prokaryotes, including most common species of bacteria.
euchromatin Decondensed, transcriptionally active interphase chromatin.
eukaryotic cells Cells that have a nuclear envelope, cytoplasmic organelles, and a cytoskeleton.
exon A segment of a gene that contains a coding sequence.
exonuclease An enzyme that hydrolyzes DNA molecules in either the 5´ to 3´ or 3´ to 5´ direction.
exportin A receptor protein that recognizes nuclear export signals and directs transport from the nucleus to the
cytosol.
expression vector A vector used to direct expression of a cloned DNA fragment in a host cell.
extracellular matrix Secreted proteins and polysaccharides that fill spaces between cells and bind cells and
tissues together.
facilitated diffusion The transport of molecules across a membrane by carrier or channel proteins.
FAK (focal adhesion kinase) A nonreceptor protein-tyrosine kinase that plays a key role in integrin signaling.
fats See triacylglycerols.
fatty acids Long hydrocarbon chains usually linked to a carboxyl group (COO -).
feedback inhibition A type of allosteric regulation in which the product of a metabolic pathway inhibits the
activity of an enzyme involved in its synthesis.
fibroblast A cell type found in connective tissue.
fibronectin The principal adhesion protein of the extracellular matrix.
filopodium A thin projection of the plasma membrane supported by actin bundles.
flagellum A microtubule-based projection of the plasma membrane that is responsible for cell movement.
flavin adenine dinucleotide (FADH2) A coenzyme that functions as an electron carrier in oxidation/reduction
reactions.
flow cytometer An instrument that measures the fluoresence intensity of individual cells.
fluid mosaic model A model of membrane structure in which proteins are inserted in a fluid phospholipid bilayer.
fluid-phase endocytosis The nonselective uptake of extracellular fluids during endocytosis.
fluorescence in situ hybridization (FISH) A method used to localize genes on chromosomes using fluorescent
probes.
fluorescence microscopy Type of microscopy in which molecules are detected based on the emission of
fluorescent light.
focal adhesion A site of attachment of cells to the extracellular matrix at which integrins are linked to bundles of
actin filaments.
freeze fracture Method of electron microscopy in which specimens are frozen in liquid nitrogen and then
fractured to split the lipid bilayer, revealing the interior faces of cell membranes.
G-protein A family of cell signaling proteins regulated by guanine nucleotide binding.
G protein-coupled receptor A receptor characterized by seven membrane-spanning a helices. Ligand binding
causes a conformational change that activates a G protein.
G0 A quiescent state in which cells remain metabolically active but do not proliferate.
G1 phase The phase of the cell cycle between the end of mitosis and the begining of DNA synthesis.
G2 phase The phase of the cell cycle between the end of S phase and the begining of mitosis.
gap junction A plasma membrane channel forming a direct cytoplasmic connection between adjacent cells.
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Cell
Biology
Glossary
gene A segment of DNA that encodes a polypeptide chain or an RNA molecule.
gene amplification An increase in the number of copies of a gene resulting from the repeated replication of a
region of DNA.
gene transfer The introduction of foreign DNA into a cell.
general homologous recombination The exchange of segments between DNA molecules that share
extensive sequence homology.
general transcription factors Transcription factors that are part of the general transcription machinery.
genetic code The correspondence between nucleotide triplets and amino acids in proteins.
genomic imprinting The regulation of genes whose expression depends on whether they are maternally or
paternally inherited, apparently controlled by DNA methylation.
genomic library A collection of recombinant DNA clones that collectively contain the genome of an organism.
genotype The genetic composition of an organism.
Gibbs free energy (G) The thermodynamic function that combines the effects of enthalpy and entropy to predict
the energetically favorable direction of a chemical reaction.
gluconeogenesis The synthesis of glucose.
glycerol phospholipids Phospholipids consisting of two fatty acids bound to a glycerol molecule.
glycocalyx A carbohydrate coat covering the cell surface.
glycogen A polymer of glucose residues that is the principal storage form of carbohydrates in animals.
glycolipid A lipid consisting of two hydrocarbon chains linked to a polar head group containing carbohydrates.
glycolysis The anaerobic breakdown of glucose.
glycoprotein A protein linked to oligosaccharides.
glycosaminoglycan (GAG) A gel-forming polysaccharide of the extracellular matrix.
glycosidic bond The bond formed between sugar residues in oligosaccharides or polysaccharides.
glycosylation The addition of carbohydrates to proteins.
glycosylphosphatidylinositol (GPI) anchor Glycolipids containing phosphatidylinositol that anchor proteins to
the external face of the plasma membrane.
glyoxylate cycle The conversion of fatty acids to carbohydrates in plants.
Golgi apparatus A cytoplasmic organelle involved in the processing and sorting of proteins and lipids. In plant
cells, it is also the site of the synthesis of cell wall polysaccharides.
green fluorescent protein (GFP) A protein from jellyfish that is commonly used as a marker for fluorescence
microscopy.
growth factors Polypeptides that control animal cell growth and differentiation.
guanine A purine that base-pairs with cytosine.
guanylyl cyclase An enzyme that catalyzes the formation of cyclic GMP from GTP.
haploid An organism or cell that has one copy of each chromosome.
heat-shock proteins A highly conserved group of chaperone proteins expressed in cells exposed to elevated
temperatures or other forms of environmental stress.
helicase An enzyme that catalyzes the unwinding of DNA.
hemicellulose A polysaccharide that crosslinks cellulose microfibrils in plant cell walls.
hemidesmosome A region of contact between cells and the extracellular matrix at which keratin filaments are
attached to integrin.
heterochromatin Condensed, transcriptionally inactive chromatin.
high-energy bonds Chemical bonds that release a large amount of free energy when they are hydrolyzed.
histones Proteins that package DNA in eukaryotic chromosomes.
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Cell
HMG-14 and HMG-17
Biology
Glossary
Nonhistone chromosomal proteins associated with decondensed transcriptionally
active chromatin.
Holliday junction The central intermediate in recombination, consisting of a crossed-strand structure formed by
homologous base pairing between strands of two DNA moleucles.
homeobox Conserved DNA sequences of 180 base pairs that encode homeodomains.
homeodomain A type of DNA binding domain found in transcription factors that regulate gene expression
during embryonic development.
homologous recombination Recombination between segments of DNA with homologous nucleotide
sequences.
hormones Signaling molecules produced by endocrine glands that act on cells at distant body sites.
hydrophilic Soluble in water.
hydrophobic Not soluble in water.
immunoblotting A method that uses antibodies to detect proteins separated by SDS-polyacrylamide gel
electrophoresis.
immunoglobulin See antibody.
immunoprecipitation The use of antibodies to isolate proteins.
importin A receptor protein that recognizes nuclear localization signals and directs nuclear import.
in situ hybridization The use of radioactive or fluorescent probes to detect RNA or DNA sequences in cell
extracts, chromosomes, or intact cells.
in vitro mutagenesis The introduction of mutations into cloned DNA in vitro.
in vitro translation Protein synthesis in a cell-free extract.
inositol 1,4,5-trisphosphate (IP3) A second messenger, formed from the hydrolysis of PIP2, that signals the
release of calcium ions from the endoplasmic reticulum.
integral membrane proteins Proteins embedded within the lipid bilayer of cell membranes.
integrin A transmembrane protein that mediates the adhesion of cells to the extracellular matrix.
intermediate filament A cytoskeletal filament about 10 nm in diameter that provides mechanical strength to
cells in tissues. See also keratins and neurofilaments.
interphase The period of the cell cycle between mitoses that includes G 1, S, and G2 phases.
intracellular signal transduction A chain of reactions that transmits chemical signals from the cell surface to
their intracellular targets.
intron A noncoding sequence that interrupts exons in a gene.
ion channel A protein that mediates the rapid passage of ions across a membrane by forming open pores
through the phospholipid bilayer.
ion pump A protein that couples ATP hydrolysis to the transport of ions across a membrane.
Janus kinase (JAK) A family of nonreceptor protein-tyrosine kinases associated with cytokine receptors.
keratin A type of intermediate filament protein of epithelial cells.
kilobase (kb) One thousand nucleotides or nucleotide base pairs.
kinesin A motor protein that moves along microtubules towards the plus end.
kinetochore A specialized structure consisting of proteins attached to a centromere that mediates the
attachment and movement of chromosomes along the mitotic spindle.
kinetochore microtubules Microtubules of the mito-tic spindle that attach to condensed chromosomes at their
centromeres.
Krebs cycle See citric acid cycle.
lagging strand The strand of DNA synthesized opposite to the direction of movement of the replication fork by
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Cell
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Glossary
ligation of Okazaki fragments.
lamellipodium A broad, actin-based extension of the plasma membrane involved in the movement of fibroblasts.
laminin The principal adhesion protein of basal laminae.
lamins Intermediate filament proteins that form the nuclear lamina.
leading strand The strand of DNA synthesized continuously in the direction of movement of the replication fork.
leptotene The initial stage of the extended prophase of meiosis I during which homologous chromosomes pair
before condensation.
leucine zipper A protein dimerization domain containing repeated leucine residues; found in many transcription
factors.
leukemia Cancer arising from the precursors of circulating blood cells.
ligand A molecule that binds to a receptor.
light reactions The reactions of photosynthesis in which solar energy drives the synthesis of ATP and NADPH.
lignin A polymer of phenolic residues that strengthens secondary cell walls.
lipids Hydrophobic molecules that function as energy storage molecules, signaling molecules, and the major
components of cell membranes.
long terminal repeat (LTR) DNA sequences found at the ends of retroviral and retrotransposon DNA that are
direct repeats of several hundred nucleotides resulting from reverse transcriptase activity.
lymphocyte A blood cell that functions in the immune response. B lymphocytes produce antibodies and T
lymphocytes are responsible for cell mediated immunity.
lymphoma A cancer of lymphoid cells.
lysogeny A viral infection leading to integration of an inactive copy of viral DNA into the cell genome.
lysosome A cytoplasmic organelle containing enzymes that break down biological polymers.
M phase The mitotic phase of the cell cycle.
macrophage A type of white blood cell specialized for phagocytosis.
malignant tumor A tumor that invades normal tissue and spreads throughout the body.
MAP kinases A family of mitogen-activated protein-serine/threonine kinases that are ubiquitous regulators of
cell growth and differentiation.
matrix The inner mitochondrial space.
maturation promoting factor (MPF) A complex of Cdc2 and cyclin B that promotes entry into the M phase of
either mitosis or meiosis.
megabase (Mb) One million nucleotides or nucleotide base pairs.
meiosis The division of diploid cells to haploid progeny, consisting of two sequential rounds of nuclear and
cellular division.
membrane-anchored growth factors Growth factors associated with the plasma membrane that function as
signaling molecules during cell-cell contact.
mesoderm The middle germ layer; gives rise to connective tissues and the hematopoietic system.
messenger RNA (mRNA) An RNA molecule that serves as a template for protein synthesis.
metaphase The phase of mitosis during which the chromosomes are aligned on a metaphase plate in the
center of the cell.
metastasis Spread of cancer cells through the blood or lymphatic system to other organ sites.
5´ methylguanosine cap A structure consisting of GTP and methylated sugars that is added to the 5´ ends of
eukaryotic mRNAs.
microfilament A cytoskeleton filament composed of actin.
microspike See filopodium.
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Cell
Biology
Glossary
microtubule A cytoskeletal component formed by the polymerization of tubulin into rigid, hollow rods about 25
nm in diameter.
microtubule-organizing center An anchoring point near the center of the cell from which most microtubules
extend outward.
microvillus An actin-based protrusion of the plasma membrane, abundant on the surfaces of cells involved in
absorption.
mismatch repair A repair system that removes mismatched bases from newly synthesized DNA strands.
mitochondria Cytoplasmic organelles responsible for synthesis of most of the ATP in eukaryotic cells by
oxidative phosphorylation.
mitosis Nuclear division.
mitotic spindle An array of microtubules extending from the spindle poles that is responsible for separating
daughter chromosomes during mitosis. See also kinetochore microtubules, polar microtubules, and astral
microtubules.
molecular motor A protein that generates force and movement by converting chemical energy to mechanical
energy.
monocistronic Messenger RNAs that encode a single polypeptide chain.
monoclonal antibody An antibody produced by a clonal line of B lymphocytes.
monosaccharides Simple sugars with the basic formula of (CH2O)n.
mutagen A chemical that induces a high frequency of mutations.
mutation A genetic alteration.
myosin A protein that interacts with actin as a molecular motor.
N-myristoylation The addition of myristic acid (a 14-carbon fatty acid) to the N-terminal glycine residue of a
polypeptide chain.
Na+-K+ pump An ion pump that transports Na+ out of the cell and K+ into the cell.
Nernst equation The relationship between ion concentration and membrane potential.
neurofilament A type of intermediate filament that supports the axons of nerve cells.
neuron A nerve cell specialized to receive and transmit signals throughout the body.
neurotransmitter A small, hydrophilic molecule that carries a signal from a stimulated neuron to a target cell at
a synapse.
nexin A protein that links microtubule doublets to each other in the axoneme
nicotinamide-adenine dinucleotide (NAD+) A coenzyme that functions as an electron carrier in
oxidation/reduction reactions.
nitrogen fixation The reduction of atmospheric nitrogen (N2) to NH3.
nonreceptor protein-tyrosine kinase An intracellular protein-tyrosine kinase.
nuclear envelope The barrier separating the nucleus from the cytoplasm, composed of an inner and outer
membrane, a nuclear lamina, and nuclear pore complexes.
nuclear export signal An amino acid sequence that targets proteins for transport from the nucleus to the
cytosol.
nuclear lamina A meshwork of lamin filaments providing structural support to the nucleus.
nuclear localization signal An amino acid sequence that targets proteins for transportation from the cytoplasm
to the nucleus.
nuclear membranes Membranes forming the nuclear envelope; the outer nuclear membrane is continuous with
the endoplasmic reticulum and the inner nuclear membrane is adjacent to the nuclear lamina.
nuclear pore complex A large structure forming a transport channel through the nuclear envelope.
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Glossary
nucleic acid hybridization The formation of double stranded DNA and/or RNA molecules by complementary
base pairing.
nucleolus The nuclear site of rRNA transcription, processing, and ribosome assembly.
nucleoside A purine or pyrimidine base linked to a sugar (ribose or deoxyribose).
nucleosome The basic structural unit of chromatin consisting of DNA wrapped around a histone core.
nucleosome remodeling factors Proteins that disrupt chromatin structure, allowing transcription factors to
bind nucleosomal DNA
nucleotide A phosphorylated nucleoside.
nucleotide excision repair A mechanism of DNA repair in which oligonucleotides containing damaged bases
are removed from a DNA molecule.
nucleus The most prominent organelle of eukaryotic cells; contains the genetic material.
Okazaki fragments Short DNA fragments that are joined to form the lagging strand of DNA.
oligonucleotide A short polymer of only a few nucleotides.
oligosaccharide A short polymer of only a few sugars.
oncogene A gene capable of inducing one or more characteristics of cancer cells.
operator A regulatory sequence of DNA that controls transcription of an operon.
operon A group of adjacent genes transcribed as a single mRNA.
origin of replication A specific DNA sequence that serves as a binding site for proteins that initiate replication.
origin replication complex (ORC) A protein complex that initiates DNA replication at yeast origins.
oxidative phosphorylation The synthesis of ATP from ADP coupled to the energetically favorable transfer of
electrons to molecular oxygen as the final acceptor in an electron transport chain.
p53 A transcription factor (encoded by the p53 tumor suppressor gene) that arrests the cell cycle in G 1 in
response to damaged DNA and is required for apoptosis induced by a variety of stimuli.
pachytene The stage of meiosis I during which recombination takes place between homologous chromosomes.
palmitoylation The addition of palmitic acid (a 16-carbon fatty acid) to cysteine residues of a polypeptide chain.
paracrine signaling Local cell-cell signaling in which a molecule released by one cell acts on a neighboring
target cell.
parenchyma cell A type of plant cell responsible for most metabolic activities.
passive diffusion The diffusion of small hydrophobic molecules through a phospholipid bilayer.
passive transport The transport of molecules across a membrane in the energetically favorable direction.
pectin A gel-forming polysaccharide in plant cell walls.
peptide bond The bond joining amino acids in polypeptide chains.
peptidoglycan The principal component of bacterial cell walls consisting of linear polysaccharide chains
crosslinked by short peptides.
peptidyl prolyl isomerase An enzyme that facilitates protein folding by catalyzing the cis-trans isomerization of
prolyl peptide bonds.
pericentriolar material The material in the centrosome that initiates microtubule assembly.
peripheral membrane proteins Proteins indirectly associated with cell membranes by protein-protein
interactions.
peroxisome A cytoplasmic organelle specialized for carrying out oxidative reactions.
phagocytosis The uptake of large particles, such as bacteria, by a cell.
phalloidin A drug that binds to actin filaments and prevents their disassembly.
phenotype The physical appearance of an organism.
phorbol esters A class of tumor promoters that stimulate protein kinase C by acting as analogs of diacylglycerol.
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Glossary
phosphatidylinositide 3-kinase (PI 3-kinase) An enzyme that phosphorylates PIP2, yielding the second
messenger phosphatidylinositol 3,4,5-trisphosphate (PIP3).
phosphatidylinositol 4,5-bisphosphate (PIP2) A minor phospholipid component of the inner leaflet of the
plasma membrane. Hormones and growth factors stimulate its hydrolysis by phospholipase C, yielding the
second messengers diacylglycerol and inositol trisphosphate.
phosphodiester bond A bond between the 5´-phosphate of one nucleotide and the 3´-hydroxyl of another.
phospholipase C An enzyme that hydrolyzes PIP2 to form the second messengers diacylglycerol and inositol
trisphosphate.
phospholipid bilayer The basic structure of biological membranes, in which the hydrophobic tails of
phospholipids are buried in the interior of the membrane and their polar head groups are exposed to the
aqueous solution on either side.
phospholipid transfer protein A protein that transports phospholipid molecules between cell membranes.
phospholipids The principal components of cell membranes, consisting of two hydrocarbon chains (usually
fatty acids) joined to a polar head group containing phosphate.
phosphorylation The addition of a phosphate group to a molecule.
photoreactivation A mechanism of DNA repair in which solar energy is used to split pyrimidine dimers.
photosynthesis The process by which cells harness energy from sunlight and synthesize glucose from CO 2
and water.
pinocytosis The uptake of fluids or molecules into a cell by small vesicles.
plant hormones A group of small molecules that coordinate the responses of plant tissues to environmental
signals.
plasma membrane A phospholipid bilayer with associated proteins that surrounds the cell.
plasmalogens A family of phospholipids that have an ether bond and an ester bond.
plasmid A small, circular DNA molecule capable of independent replication in a host cell.
plasmodesma A cytoplasmic connection between adjacent plant cells formed by a continuous region of the
plasma membrane.
plastids A family of plant organelles including chloroplasts, chromoplasts, leucoplasts, amyloplasts, and
elaioplasts.
platelet-derived growth factor (PDGF) A growth factor released by platelets during blood clotting to stimulate
the proliferation of fibroblasts.
polar microtubules Microtubules of the mitotic spindle that overlap in the center of the cell and push the spindle
poles apart.
poly-A tail A tract of about 200 adenine nucleotides added to the 3´ ends of eukaryotic mRNAs.
polyadenylation The process of adding a poly-A tail to a pre-mRNA.
polycistronic Messenger RNAs that encode multiple polypeptide chains.
polymerase chain reaction (PCR) A method for amplifying a region of DNA by repeated cycles of DNA
synthesis in vitro.
polynucleotide A polymer containing up to millions of nucleotides.
polyp A benign tumor projecting from an epithelial surface.
polypeptide A polymer of amino acids.
polysaccharide A polymer containing hundreds or thousands of sugars.
polysome A series of ribosomes translating a messenger RNA.
pre-mRNA The primary transcripts that are processed to form messenger RNAs in eukaryotic cells.
pre-rRNA The primary transcript, which is cleaved to form individual ribosomal RNAs (the 28S, 18S, and 5.8S
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Glossary
rRNAs of eukaryotic cells).
pre-tRNA The primary transcript, which is cleaved to form transfer RNAs.
prenylation The addition of specific types of lipids (prenyl groups) to C terminal cysteine residues of a
polypeptide chain.
primary structure The sequence of amino acids in a polypeptide chain.
primase An RNA polymerase used to initiate DNA synthesis.
processed psuedogene A psuedogene that has arisen by reverse transcription of mRNA.
progesterone A steroid hormone produced by the ovaries.
programmed cell death A normal physiological form of cell death characterized by apoptosis.
prokaryotic cells Cells lacking a nuclear envelope, cytoplasmic organelles, and a cytoskeleton (primarily
bacteria).
prometaphase A transition period between prophase and metaphase during which the microtubules of the
mitotic spindle attach to the kinetochores and the chromosomes shuffle until they align in the center of the cell.
promoter A DNA sequence to which RNA polymerase binds to initiate transcription.
pronuclei Two haploid nuclei in a newly fertilized egg.
proofreading The selective removal of mismatched bases by DNA polymerase.
prophase The beginning phase of mitosis, marked by the appearance of condensed chromosomes and the
development of the mitotic spindle.
prosthetic groups Small molecules bound to proteins.
proteasome A large protease complex that degrades proteins tagged by ubiquitin.
protein disulfide isomerase An enzyme that catalyzes the formation and breakage of disulfide (S S) linkages.
protein kinase An enzyme that phosphorylates proteins by transferring a phosphate group from ATP.
protein kinase A A protein kinase regulated by cyclic AMP.
protein kinase C A family of protein-serine/threonine kinases that are activated by diacylglycerol and Ca 2+ and
function in intracellular signal transduction.
protein phosphatase An enzyme that reverses the action of protein kinases by removing phosphate groups
from phosphorylated amino acid residues.
protein-serine/threonine kinase A protein kinase that phosphorylates serine and threonine residues.
protein-tyrosine kinase A protein kinase that phosphorylates tyrosine residues.
protein-tyrosine phosphatase An enzyme that removes the phosphate groups from phosphotyrosine residues.
proteins Polypeptides with a unique amino acid sequence.
proteoglycan A protein linked to glycosaminoglycans.
proteolysis Degradation of polypeptide chains.
proto-oncogene A normal cell gene that can be converted into an oncogene.
pseudogene A nonfunctional gene copy.
pseudopodium An actin-based extension of the plasma membrane responsible for phagocytosis and amoeboid
movement.
PTEN A lipid phosphatase that dephosphorylates PIP3 and acts as a tumor suppressor.
pyrimidine dimer A common form of DNA damage caused by UV light in which adjacent pyrimidines are joined
to form a dimer.
quaternary structure The interactions between polypeptide chains in proteins consisting of more than one
polypeptide.
Raf A protein-serine/threonine kinase (encoded by the raf oncogene) that is activated by Ras and leads to
activation of MAP kinase.
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Glossary
Ran A small GTP-binding protein involved in nuclear import and export.
Ras A family of small GTP binding proteins (encoded by the ras oncogenes) that couple growth factor receptors
to intracellular targets, including the Raf protein-serine/threonine kinase and the MAP kinase pathway.
Rb A transcriptional regulatory protein encoded by a tumor suppressor gene that was identified by the genetic
analysis of retinoblastoma.
receptor protein-tyrosine kinase Membrane-spanning protein-tyrosine kinases that are receptors for
extracellular ligands.
receptor-mediated endocytosis The selective uptake of macromolecules that bind to cell surface receptors
that concentrate in clathrin-coated pits.
recessive An allele that is masked by a dominant allele.
recombinant DNA library A collection of genomic or cDNA clones.
recombinant molecule A DNA insert joined to a vector.
recombination The exchange of genetic material.
replication fork The region of DNA synthesis where the parental strands separate and two new daughter
strands elongate.
repressor A regulatory molecule that blocks transcription.
restriction endonuclease An enzyme that cleaves DNA at a specific sequence.
restriction map The locations of restriction endonuclease cleavage sites on a DNA molecule.
restriction point A regulatory point in animal cell cycles that occurs late in G 1. After this point, a cell is
committed to entering S and undergoing one cell division cycle.
retroposon A transposable element that moves via reverse transcription of an RNA intermediate.
retrovirus A virus that replicates by making a DNA copy of its RNA genome by reverse transcription.
reverse transcriptase A DNA polymerase that uses an RNA template.
rhodopsin A G protein-coupled photoreceptor in retinal rod cells that activates transducin in response to light
absorption.
ribonucleic acid (RNA) A polymer of ribonucleotides.
ribosomal RNA (rRNA) The RNA component of ribosomes.
ribosomes Particles composed of RNA and proteins that are the sites of protein synthesis.
ribozyme An RNA enzyme.
RNA editing RNA processing events other than splicing that alter the protein coding sequences of mRNAs.
RNA polymerase An enzyme that catalyzes the synthesis of RNA.
RNA splicing The joining of exons in a precursor RNA molecule.
RNase H An enzyme that degrades the RNA strand of RNA-DNA hybrid molecules.
RNase P A ribozyme that cleaves the 5´ end of pre-tRNAs.
rough endoplasmic reticulum The region of the endoplasmic reticulum covered with ribosomes and involved
in protein metabolism.
Rous sarcoma virus (RSV) An acutely transforming retrovirus, in which the first oncogene was identified.
ryanodine receptors Calcium channels in muscle and nerve cells that open in response to changes in
membrane potential.
S phase The phase of the cell cycle during which DNA replication occurs.
sarcoma A cancer of cells of connective tissue.
sarcomere The contractile unit of muscle cells composed of interacting myosin and actin filaments.
sarcoplasmic reticulum A specialized network of membranes in muscle cells that stores a high concentration
of Ca2+.
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Glossary
scanning electron microscopy See electron microscopy.
sclerenchyma cells Plant cells characterized by thick cell walls that provide structural support to the plant.
SDS-polyacrylamide gel electrophoresis (SDS-PAGE) A commonly used method to separate proteins by gel
electrophoresis on the basis of size.
second messenger A compound whose metabolism is modified as a result of a ligand-receptor interaction; it
functions as a signal transducer by regulating other intracellular processes.
secondary structure The regular arrangement of amino acids within localized regions of a polypeptide chain.
See a helix and b sheet.
secretory vesicles Membrane-enclosed sacs that transport proteins from the Golgi apparatus to the cell
surface.
selectins Cell adhesion molecules that recognize oligosaccharides exposed on the cell surface.
self-splicing The ability of some RNAs to catalyze the removal of their own introns.
SH2 domain A protein domain of approximately 100 amino acids that binds phosphotyrosine-containing
peptides.
Shine-Delgarno sequence The sequence prior to the initiation site that correctly aligns bacterial mRNAs on
ribosomes.
signal patch A recognition determinant formed by the three-dimensional folding of a polypeptide chain.
signal peptidase An enzyme that removes the signal sequence of a polypeptide chain by proteolysis.
signal recognition particle (SRP) A particle composed of proteins and 7SL RNA that binds to signal sequences
and targets polypeptide chains to the endoplasmic reticulum.
signal sequence A hydrophobic sequence at the amino terminus of a polypeptide chain that targets it for
secretion in bacteria or incorporation into the endoplasmic reticulum in eukaryotic cells.
site-specific recombination Recombination mediated by proteins that recognize specific DNA sequences.
smooth endoplasmic reticulum The major site of lipid synthesis in eukaryotic cells.
Southern blotting A method in which radioactive probes are used to detect specific DNA fragments that have
been separated by gel electrophoresis.
spectrin A major actin-binding protein of the cell cortex.
sphingomyelin A phospholipid consisting of two hydrocarbon chains bound to a polar head group containing
serine.
spliceosomes Large complexes of snRNAs and proteins that catalyze the splicing of pre-mRNAs.
Src A nonreceptor protein-tyrosine kinase encoded by the oncogene (src ) of Rous sarcoma virus.
starch A polymer of glucose residues that is the principal storage form of carbohydrates in plants.
START A regulatory point in the yeast cell cycle that occurs late in G 1. After this point a cell is committed to
entering S and undergoing one cell division cycle.
STAT proteins Trancription factors that have an SH2 domain and are activated by tyrosine phosphorylation,
which promotes their translocation from the cytoplasm to the nucleus.
stem cell A cell that divides to produce daughter cells that can either differentiate or remain as stem cells.
stereocilium A specialized microvillus of auditory hair cells.
steroid hormones A group of hydrophobic hormones that are derivatives of cholesterol.
steroid receptor superfamily A family of transcription factors that regulate gene expression in response to
steroids and related hormones.
substrate A molecule acted upon by an enzyme.
symport The transport of two molecules in the same direction across a membrane.
synapse The junction between a neuron and another cell, across which information is carried by
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Glossary
neurotransmitters.
synapsis The association of homologous chromosomes during meiosis.
synaptic vesicle A secretory vesicle that releases neurotransmitters at a synapse.
synaptonemal complex A zipperlike protein structure that forms along the length of paired homologous
chromosomes during meiosis.
T cell receptor A T lymphocyte surface protein that recognizes antigens expressed on the surface of other cells.
TATA box A regulatory DNA sequence found in the promoters of many eukaryotic genes transcribed by RNA
polymerase II.
TATA-binding protein (TBP) A basal transcription factor that binds directly to the TATA box.
taxol A drug that binds to and stabilizes microtubules.
telomerase A reverse transcriptase that synthesizes telomeric repeat sequences at the ends of chromosomes
from its own RNA template.
telomeres Repeats of simple-sequence DNA that maintain the ends of linear chromosomes.
telophase The final phase of mitosis, during which the nuclei re-form and chromosomes decondense.
temperature-sensitive mutant A cell expressing a protein that is functional at one temperature but not at
another, whereas the normal protein is functional at both temperatures.
tertiary structure The three-dimensional folding of a polypeptide chain that gives the protein its functional form.
testosterone A steroid hormone produced by the testis.
thylakoid membrane The innermost membrane of chloroplasts that is the site of electron transport and ATP
synthesis.
thymine A pyrimidine found in DNA that base-pairs with adenine.
thyroid hormone A hormone synthesized from tyrosine in the thyroid gland.
tight junction A continuous network of protein strands around the circumference of epithelial cells, sealing the
space between cells and forming a barrier between the apical and basolateral domains.
topoisomerase An enzyme that catalyzes the reversible breakage and rejoining of DNA strands.
transcription factor A protein that regulates the activity of RNA polymerase.
transcription The synthesis of an RNA molecule from a DNA template.
transcriptional attenuation A regulatory mechanism that controls elongation of an mRNA molecule past a
certain site on the DNA template.
transcytosis The sorting and transport of proteins to different domains of the plasma membrane following
endocytosis.
transducin A G protein that stimulates cGMP phosphodiesterase when it is activated by rhodopsin.
transfection The introduction of a foreign gene into eukaryotic cells.
transfer RNA (tRNA) RNA molecules that function as adaptors between amino acids and mRNA during protein
synthesis.
transforming growth factor (TGF-) A polypeptide growth factor that generally inhibits animal cell proliferation.
transgenic mouse A mouse that carries foreign genes incorporated into the germ line.
translation The synthesis of a polypeptide chain from an mRNA template.
transmembrane proteins Integral membrane proteins that span the lipid bilayer and have portions exposed on
both sides of the membrane.
transmission electron microscopy See electron microscopy.
transposition The movement of DNA sequences throughout the genome.
transposon A DNA sequence that can move to different positions in the genome.
triacylglycerol Three fatty acids linked to a glycerol molecule.
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Glossary
tropomyosin A fibrous protein that binds actin filaments and regulates contraction by blocking the interaction of
actin and myosin.
tubulin A cytoskeletal protein that polymerizes to form microtubules.
tumor Any abnormal proliferation of cells.
tumor necrosis factor (TNF) A polypeptide growth factor that induces programmed cell death.
tumor promoter A compound that leads to tumor development by stimulating cell proliferation.
tumor suppressor gene A gene whose inactivation leads to tumor development.
tumor virus A virus capable of causing cancer in animals or humans.
turgor pressure The internal hydrostatic pressure within plant cells.
ubiquinone See coenzyme Q.
ubiquitin A highly conserved protein that acts as a marker to target other cellular proteins for rapid degradation.
uniport The transport of a single molecule across a membrane.
uracil A pyrimidine found in RNA that base-pairs with adenine.
vacuole A large membrane-enclosed sac in the cytoplasm of eukaryotic cells. In plant cells, vacuoles function to
store nutrients and waste products, to degrade macromolecules, and to maintain turgor pressure.
vector A DNA molecule used to direct the replication of a cloned DNA fragment in a host cell.
vinblastine A drug that inhibits microtubule polymerization.
vincristine A drug that inhibits microtubule polymerization.
Western blotting See immunoblotting.
X-ray crystallography A method in which the diffraction pattern of X rays is used to determine the arrangement
of individual atoms within a molecule.
Xenopus laevis An African clawed frog used as a model system for developmental biology.
yeast artificial chromosome (YAC) A vector that can replicate as a chromosome in yeast cells and can
accommodate very large DNA inserts (hundreds of kb).
yeasts The simplest unicellular eukaryotes. Yeasts are important models for studies of eukaryotic cells.
zebrafish A species of small fish used for genetic studies of vertebrate development.
zinc finger domain A type of DNA binding domain consisting of loops containing cysteine and histidine residues
that bind zinc ions.
zygote A fertilized egg.
Molecular Cell Biology
Harvey Lodish
Arnold Berk
Glossary
Lawrence S. Zipursky
Paul Matsudaira
David Baltimore
James Darnell
Fourth EditionW. H. FREEMAN,
Boldfaced
terms within a definition are also defined in this glossary. Alternative names for main entries are in
italics. Figures and tables that illustrate defined terms are noted in parentheses.
acetyl CoA A. Small, water-soluble metabolite comprising an acetyl group linked to coenzyme A (CoA); formed
during oxidation of pyruvate, fatty acids, and amino acids. Its acetyl group is transferred to citrate in the citric
acid cycle.
acetylcholine (ACh) Neurotransmitter that functions at vertebrate neuromuscular junctions and at various
neuron-neuron synapses in the brain and peripheral nervous system.
acid A compound that can donate a proton (H+). The carboxyl and phosphate groups are the primary acidic
groups in biological molecules.
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Glossary
actin Abundant structural protein in eukaryotic cells that interacts with many other proteins. The monomeric
globular form (G actin) polymerizes to form actin filaments (F actin). In muscle cells, F actin interacts with
myosin during contraction. See also microfilaments.
action potential Rapid, transient, all-or-none electrical activity that is propagated in the plasma membrane of
excitable cells such as neurons and muscle cells. Action potentials, or nerve impulses, allow long-distance
signaling in the nervous system.
activation energy The input of energy required to (overcome the barrier to) initiate a chemical reaction. By
reducing the activation energy, an enzyme increases the rate of a reaction.
active site Region of an enzyme molecule where the substrate binds and undergoes a catalyzed reaction.
active transport Energy-requiring movement of an ion or small molecule across a membrane against its
concentration gradient or electrochemical gradient. Energy is provided by the coupled hydrolysis of ATP or the
cotransport of another molecule down its electrochemical gradient.
adenosine triphosphate See ATP.
adenylyl cyclase Membrane-bound enzyme that catalyzes formation of cyclic AMP (cAMP) from ATP; also
called adenylate cyclase. Binding of certain ligands to their cell-surface receptors leads to activation of adenylyl
cyclase and a rise in intracellular cAMP.
aerobic Referring to a cell, organism,or metabolic process that utilizes O 2 or that can grow in the presence of O2.
aerobic oxidation Oxygen-requiring metabolism of sugars and fatty acids to CO2 and H2O coupled to the
synthesis of ATP.
allele One of two or more alternative forms of a gene located at the corresponding site (locus) on homologous
chromosomes.
allosteric transition Change in the tertiary and/or quaternary structure of a protein induced by binding of a
small molecule to a specific regulatory site, causing a change in the protein's activity. Allosteric regulation is
particularly prevalent in multisubunit enzymes.
Common secondary structure of proteins in which the linear sequence of amino acids is folded
into a right-handed spiral stabilized by hydrogen bonds between carboxyl and amide groups in the backbone.
amino acid An organic compound containing at least one amino group and one carboxyl group. In the 20
different amino acids that compose proteins, an amino group and carboxyl group are linked to a central carbon
atom, the
carbon, to which a variable side chain is bound.
aminoacyl-tRNA Activated form of an amino acid, used in protein synthesis, consisting of an amino acid linked
via a highenergy ester bond to the 3 -hydroxyl group of a tRNA molecule.
amphipathic Referring to a molecule or structure that has both a hydrophobic and a hydrophilic part.
anabolism Cellular processes whereby energy is used to synthesize complex molecules from simpler ones.
See also catabolism.
anaerobic Referring to a cell, organism, or metabolic process that functions in the absence of O2.
anaphase Mitotic stage during which the sister chromatids (or paired homologs in meiosis I) separate and move
apart (segregate) toward the spindle poles.
antibody A protein that interacts with a particular site (epitope) on an antigen and facilitates clearance of that
antigen by various mechanisms. See also immunoglobulin.
anticodon Sequence of three nucleotides in a tRNA that is complementary to a codon in an mRNA. During
protein synthesis, base pairing between a codon and anticodon aligns the tRNA carrying the corresponding
amino acid for addition to the growing peptide chain.
antigen Any material (usually foreign) that elicits production of and is specifically bound by an antibody.
antiport A type of cotransport in which a membrane protein (antiporter) transports two different molecules or
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Glossary
ions across a cell membrane in opposite directions. See also symport.
antisense RNA An RNA, with sequence complementary to a specific RNA transcript or mRNA, whose binding
prevents processing of the transcript or translation of the mRNA.
apoptosis Regulated process leading to cell death via a series of well-defined morphological changes; also
called programmed cell death. archaea Class of prokaryotes that constitutes one of the three distinct
evolutionary lineages of modern-day organisms; also called archaebacteria and archaeans. These prokaryotes
are in some respects more similar to eukaryotes than to the so-called true bacteria (eubacteria). association
constant (Ka) See equilibrium constant.
aster Star-shaped structure composed of microtubules (called astral fibers) that radiates outward from a
centrosome during mitosis.
asymmetric carbon atom A carbon atom bonded to four different atoms; also called chiral carbon atom. The
bonds can be arranged in two different ways producing stereoisomers that are mirror images of each other.
ATP (adenosine 5 -triphosphate) A nucleotide that is the most important molecule for capturing and
transferring free energy in cells. Hydrolysis of each of the two high-energy phosphoanhydride bonds in ATP is
accompanied by a large free-energy change
G) of
7 kcal/mole.
ATP synthase Multimeric protein complex bound to inner mitochondrial membranes, thylakoid membranes of
chloroplasts, and the bacterial plasma membrane that catalyzes synthesis of ATP during oxidative
phosphorylation and photosynthesis; also called F0F1 complex.
ATPase One of a large group of enzymes that catalyze hydrolysis of ATP to yield ADP and inorganic phosphate
with release of free energy.
autonomously replicating sequence (ARS) Sequence that permits a DNA molecule to replicate in yeast; a
yeast DNA replication origin.
autoradiography Technique for visualizing radioactive molecules in a sample (e.g., a tissue section or
electrophoretic gel) by exposing a photographic film or emulsion to the sample. The exposed film is called an
autoradiogram or autoradiograph.
autosome Any chromosome other than a sex chromosome.
auxotroph A mutant cell or microorganism that grows only when the medium contains a specific nutrient or
metabolite that is not required by the wild type.
axon Long process extending from the cell body of a neuron that is capable of conducting an electric impulse
(action potential) generated at the junction with the cell body (called the axon hillock) toward its distal, branching
end (called the axon terminal).
axoneme Bundle of microtubules and associated proteins present in cilia and flagella and responsible for their
movement.
bacteriophage (phage) Any virus that infects bacterial cells. Some bacteriophage are widely used as cloning vectors.
basal body Structure at the base of cilia and flagella from which microtubules forming the axoneme radiate;
structurally similar to a centriole.
basal lamina (pl. basal laminae) A thin sheetlike network of extracellular-matrix components that underlies most
animal epi-thelial layers and other organized groups of cells (e.g., muscle), separating them from connective tissue.
base A compound, usually containing nitrogen, that can accept a proton (H+). Commonly used to denote the purines
and pyrimidines in DNA and RNA.
base pair Association of two complementary nucleotides in a DNA or RNA molecule stabilized by hydrogen bonding
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Cell
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Glossary
between their base components. Adenine pairs with thymine or uracil (A·T, A·U) and guanine pairs with cytosine (G·C).
benign Referring to a tumor containing cells that closely resemble normal cells. Benign tumors stay in the tissue where
they originate. See also malignant.
beta (
) sheet A planar secondary structure of proteins that is created by hydrogen bonding between the backbone
atoms in two different polypeptide chains or segments of a single folded chain.
bilayer See phospholipid bilayer.
biomembrane Permeability barrier, surrounding cells or organelles, that consists of a phospholipid bilayer, associated
membrane proteins, and in some cases cholesterol and glycolipids.
blastula An early embryonic form produced by cleavage of a fertilized ovum and usually consisting of a single layer of
cells surrounding a fluid-filled spherical cavity.
buffer A solution of the acid (HA) and base (A ) form of a compound that undergoes little change in pH when small
quantities of strong acid or base are added.
cadherin Protein belonging to a family of Ca2-dependent cell-adhesion molecules that play roles in tissue
differentiation and structure.
calmodulin A small cytosolic protein that binds four Ca2+ ions; the Ca2+-calmodulin complex binds to and activates
many enzymes.
Calvin cycle The major metabolic pathway that fixes CO2 into carbohydrates during photosynthesis; also called
carbon fixation. It is indirectly dependent on light but can occur both in the dark and light.
cAMP-dependent protein kinase (cAPK) Type of cytosolic enzyme that is activated by cAMP and functions to
regulate the activity of numerous cellular proteins; also called protein kinase A. Generally is activated in response to a
rise in cAMP level resulting from stimulation of G protein coupled receptors.
capsid The outer proteinaceous coat of a virus, formed by multiple copies of one or more protein subunits and
enclosing the viral nucleic acid.
carbohydrate General term for certain polyhydroxyaldehydes, polyhydroxyketones, or compounds derived from these
usually having the formula (CH2O)n. Primary type of compound used for storing and supplying energy in animal cells.
carbon fixation See Calvin cycle.
carcinogen Any chemical or physical agent that can cause cancer when cells or organisms are exposed to it.
carcinoma A malignant tumor derived from epithelial cells.
catabolism Cellular processes whereby complex molecules are degraded to simpler ones and energy is released.
See also anabolism.
catalyst A substance that increases the rate of a chemical reaction without undergoing a permanent change in its
structure. Enzymes are protein catalysts.
catecholamines Group of compounds derived from tyrosine that function as neurotransmitters; include epinephrine,
norepinephrine, and dopamine.
cDNA (complementary DNA) DNA molecule copied from an mRNA molecule by reverse transcriptase and therefore
lacking the introns present in genomic DNA. Sequencing of a cDNA permits the amino acid sequence of the encoded
protein to be deduced; expression of cDNAs in recombinant cells can be used to produce large quantities of their
encoded proteins in vitro.
cell cycle Ordered sequence of events in which a cell duplicates its chromosomes and divides into two. Most
eukaryotic cell cycles can be commonly divided into four phases: G1 before DNA synthesis occurs; S when DNA
replication occurs; G2 after DNA synthesis; and M when cell division occurs, yielding two daughter cells. Under certain
conditions, cells exit the cell cycle during G1 and remain in the G0 state as nongrowing, nondividing (quiescent) cells.
Appropriate stimulation of such cells induces them to return to G1 and resume growth and division.
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Glossary
cell division Separation of a cell into two daughter cells. In higher eukaryotes, it involves division of the nucleus
(mitosis) and of the cytoplasm (cytokinesis); mitosis often is used to refer to both nuclear and cytoplasmic division.
cell fusion Production of a hybrid cell containing two or more nuclei by various techniques that stimulate the fusion of
the plasma membranes of two cells at the point of contact and intermingling of their cytoplasms. See also hybridoma.
cell junctions Specialized regions on the cell surface through which cells are joined to each other or to the
extracellular matrix.
cell line A population of cultured cells, of plant or animal origin, that has undergone a change allowing the cells to grow
indefinitely, in contrast to a cell strain. Cell lines can result from chemical or viral transformation and are said to be
immortal.
cell strain A population of cultured cells, of plant or animal origin, that has a finite life span, in contrast to a cell line.
cell wall A specialized, rigid extracellular matrix that lies next to the plasma membrane, protecting a cell and
maintaining its shape. It is prominent in most fungi, plants, and prokaryotes, but is not present in most multicellular
animals.
cell-adhesion molecules (CAMs) Integral membrane proteins that mediate cell-cell binding. The five major classes
are the integrins, cadherins, selectins, immunoglobulin (Ig) superfamily, and mucins.
cellulose A structural polysaccharide made of glucose units linked together by
(1n4) glycosidic bonds. It forms long
microfibrils, which are the major component of plant cell walls.
central nervous system (CNS) The part of the vertebrate nervous system comprising the brain and spinal cord; the
main information-processing organ.
centriole Either of two cylindrical structures within the centrosome of animal cells and containing nine sets of triplet
microtubules; structurally similar to a basal body.
centromere Constricted portion of a mitotic chromosome where sister chromatids are attached and from which
kinetochore fibers extend toward a spindle pole; required for proper chromosome segregation during mitosis and
meiosis.
centrosome (cell center) Organelle located near the nucleus of animal cells that is the primary
microtubule-organizing center (MTOC) and contains a pair of centrioles. It divides during mitosis, forming the spindle
poles.
chaperone Collective term for two types of proteins that prevent misfolding of a target protein (molecular chaperones)
or actively facilitate its proper folding (chaperonins).
checkpoint Any of several points in the eukaryotic cell cycle at which progression of a cell to the next stage can be
halted until conditions are suitable.
chemical equilibrium The state of a chemical reaction in which the concentration of all products and reactants is
constant and the rates of the forward and reverse reactions are equal.
chemiosmosis Process whereby an electrochemical proton gradient (pH plus electric potential) across a membrane
is used to drive an energy-requiring process such as ATP synthesis or transport of molecules across a membrane
against their concentration gradient; also called chemiosmotic coupling. chimera An animal or tissue composed of
elements derived from genetically distinct individuals; also a protein molecule containing segments derived from
different proteins.
chlorophylls A group of light-absorbing porphyrin pigments that are critical in photosynthesis.
chloroplast A specialized organelle in plant cells that is surrounded by a double membrane and contains internal
chlorophyll-containing membranes (thylakoids) where the light-absorbing reactions of photosynthesis occur.
cholesterol An amphipathic lipid containing the four-ring steroid structure with a hydroxyl group on one ring; a major
component of many eukaryotic membranes and precursor of steroid hormones.
chromatid One copy of a duplicated chromosome, formed during the S phase of the cell cycle, that is still joined at the
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Glossary
centromere to the other copy; also called sister chromatid. During mitosis, the two chromatids separate, each
becoming a chromosome of one of the two daughter cells.
chromatin Complex of DNA, histones, and nonhistone proteins from which eukaryotic chromosomes are formed.
Condensation of chromatin during mitosis yields the visible metaphase chromosomes.
chromatography, liquid Group of biochemical techniques for separating mixtures of molecules based on their mass
(gel-filtration chromatography), charge (ion-exchange chromatography), or ability to bind specifically to other
molecules (affinity chromatography). Commonly used technique for separating and purifying proteins.
chromosome In eukaryotes, the structural unit of the genetic material consisting of a single, linear double-stranded
DNA molecule and associated proteins. During mitosis, chromosomes condense into compact structures visible in the
light microscope. In prokaryotes, a single, circular double-stranded DNA molecule constitutes the bulk of the genetic
material. See also karyotype.
cilium (pl. cilia) Membrane-enclosed motile structure extending from the surface of eukaryotic cells. Cilia usually
occur in groups and beat rhythmically to move a cell (e.g., single-celled organism) or to move small particles or fluid
along the surface (e.g., trachea cells). See also axoneme and flagellum.
cis-acting Referring to a regulatory sequence in DNA (e.g., enhancer, promoter) that can control a gene only on the
same chromosome. In bacteria, cis-acting elements are adjacent or proximal to the gene(s) they control, whereas in
eukaryotes they may also be far away. See also trans-acting.
cisterna (pl. cisternae) Flattened membrane-bounded compartment, as found in the Golgi complex and endoplasmic
reticulum.
cistron A genetic unit that encodes a single polypeptide.
citric acid cycle A set of nine coupled reactions occurring in the matrix of the mitochondrion in which acetyl groups
derived from food molecules are oxidized, generating CO2 and reduced intermediates used to produce ATP; also
called Krebs cycle and tricarboxylic acid cycle (TCA).
clathrin A fibrous protein that with the aid of assembly proteins polymerizes into a lattice-like network at specific
regions on the cytosolic side of a membrane, thereby forming a clathrin-coated pit, which buds off to form a vesicle.
clone A population of identical cells or DNA molecules descended from a single progenitor. Also viruses or organisms
that are genetically identical and descended from a single progenitor.
cloning vector An autonomously replicating genetic element used to carry a cDNA or fragment of genomic DNA into a
host cell for the purpose of gene cloning. Commonly used vectors are bacterial plasmids and modified bacteriophage
genomes.
codon Sequence of three nucleotides in DNA or mRNA that specifies a particular amino acid during protein synthesis;
also called triplet. Of the 64 possible codons, three are stop codons, which do not specify amino acids.
coenzyme Small organic molecule that associates with an enzyme and participates in the reaction catalyzed by the
enzyme; also called cofactor. Some coenzymes form a transient covalent bond to the substrate; others function as
carriers of electrons, acyl groups, or other activated groups. Generally, a coenzyme is bound less firmly to a protein
than a prosthetic group.
coenzyme A (CoA) See acetyl CoA.
coiled-coil Stable rodlike quaternary protein structure formed by two or three
helices interacting with each other
along their length; commonly found in fibrous proteins and certain transcription factors.
collagen A triple-helical protein that forms fibrils of great tensile strength; a major component of the extracellular
matrix and connective tissues. The numerous collagen subtypes differ in their tissue distribution and the extracellular
components and cell-surface proteins with which they associate.
complementary Referring to two nucleic acid sequences or strands that can form a perfect base-paired double helix
with each other; also describing regions on two interacting molecules (e.g., an enzyme and its substrate) that fit
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Glossary
together in a lock-and-key fashion.
complementary DNA (cDNA) See cDNA.
complementation In genetics, the restoration of a wild-type function (e.g., ability to grow on galactose) in diploid
heterozygotes generated from haploids, each of which carries a mutation in a different gene whose encoded protein is
required for the same biochemical pathway. If two mutants with the same mutant phenotype (e.g., inability to grow on
galactose) can complement each other, then their mutations are in different genes.
conformation The precise shape of a protein or other macromolecule in three dimensions resulting from the spatial
location of the atoms in the molecule. A small change in the conformations of some proteins affects their activity
considerably.
consensus sequence The nucleotides or amino acids most commonly found at each position in the sequences of
related DNAs, RNAs, or proteins. See also homology.
constitutive Referring to cellular production of a molecule at a constant rate, which is not regulated by internal or
external stimuli.
constitutive mutant (1) A mutant in which a protein is produced at a constant level, as if continuously induced; (2) a
bacterial regulatory mutant in which an operon is transcribed in the absence of inducer; (3) a mutant in which a
regulated enzyme is in a continuously active form.
cooperativity Property exhibited by some proteins with multiple ligand-binding sites whereby binding of one ligand
molecule increases (positive cooperativity) or decreases (negative cooperativity) the binding affinity of successive
ligand molecules.
cosmid A type of vector used to clone large DNA fragments.
cotransport Protein-mediated transport of an ion or small molecule across a membrane against a concentration
gradient driven by coupling to movement of a second molecule down its concentration gradient. See also antiport and
symport.
covalent bond Stable chemical force that holds the atoms in molecules together by sharing of one or more pairs of
electrons. Such a bond has a strength of 50
200 kcal/mol.
crossing over Exchange of genetic material between maternal and paternal chromatids during meiosis to produce
recombined chromosomes. See also recombination.
cyclic AMP (cAMP) A second messenger, produced in response to hormonal stimulation of certain G protein
coupled receptors, that activates cAMP-dependent protein kinases.
cyclin Any of several related proteins whose concentrations rise and fall during the course of the eukaryotic cell cycle.
Cyclins form complexes with cyclin-dependent kinases, thereby activating and determining the substrate specificity of
these enzymes.
cyclin-dependent kinase (Cdk) A protein kinase that is catalytically active only when bound to a cyclin. Various
Cdk-cyclin complexes trigger progression through different stages of the eukaryotic cell cycle by phosphorylating
specific target proteins.
cytochromes A group of colored, heme-containing proteins that transfer electrons during cellular respiration and
photosynthesis.
cytokine Any of numerous secreted, small proteins (e.g., interferons, interleukins) that bind to cell-surface receptors
on certain cells to trigger their differentiation or proliferation.
cytokinesis The last stage of mitosis, where the two daughter cells separate, each with a nucleus and cytoplasmic
organelles.
cytoplasm Viscous contents of a cell that are contained within the plasma membrane but, in eukaryotic cells, outside
the nucleus.
cytoskeleton Network of fibrous elements, consisting primarily of microtubules, actin microfilaments, and intermediate
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Glossary
filaments, found in the cytoplasm of eukaryotic cells. The cytoskeleton provides structural support for the cell and
permits directed movement of organelles, chromosomes, and the cell itself.
cytosol Unstructured aqueous phase of the cytoplasm excluding organelles, membranes, and insoluble cytoskeletal
components.
cytosolic face The face of a cell membrane directed toward the cytoplasm.
dalton Unit of molecular mass approximately equal to the mass of a hydrogen atom (1.66 × 10 24 g).
degenerate In reference to the genetic code, having more than one codon specifying a particular amino acid.
denaturation Drastic alteration in the conformation of a protein or nucleic acid due to disruption of various noncovalent
bonds caused by heating or exposure to certain chemicals; usually results in loss of biological function.
dendrite Process extending from the cell body of a neuron that is relatively short and typically branched and receives
signals from axons of other neurons.
deoxyribonucleic acid See DNA.
depolarization Change in the potential that normally exists across the plasma membrane of a cell at rest, resulting in
a less negative membrane potential.
desmosomes Specialized regions of the plasma membrane, consisting of dense protein plaques connected to
intermediate filaments, that mediate adhesion between adjacent cells (especially epithelial cells) and between cells
and the extracellular matrix.
determination In embryogenesis, a change in a cell that commits the cell to a particular developmental pathway.
development Overall process involving growth and differentiation by which a fertilized egg gives rise to an adult plant
or animal, including the formation of individual cell types, tissues, and organs.
diacylglycerol (DAG) Intracellular signaling molecule produced by cleavage of phosphoinositides in response to
stimulation of certain cell-surface receptors; functions as a membrane-bound second messenger in inositol-lipid
signaling pathways.
differentiation Process usually involving changes in gene expression by which a precursor cell becomes a distinct
specialized cell type.
diploid Referring to an organism or cell having two full sets of homologous chromosomes and hence two copies
(alleles) of each gene or genetic locus. Somatic cells contain the diploid number of chromosomes (2n) characteristic of
a species. See also haploid.
disaccharide A small carbohydrate (sugar) composed of two monosaccharides covalently joined by a glycosidic bond.
Common examples are lactose (milk sugar) and sucrose, a major photosynthetic product in higher plants.
dissociation constant (KD) See equilibrium constant.
disulfide bond (-S S) A common covalent linkage between the sulfhydryl groups on two cysteine residues in different
proteins or in different parts of the same protein; generally found only in extracellular proteins or protein domains.
DNA (deoxyribonucleic acid) Long linear polymer, composed of four kinds of deoxyribose nucleotides, that is the
carrier of genetic information. In its native state, DNA is a double helix of two antiparallel strands held together by
hydrogen bonds between complementary purine and pyramidine bases.
DNA cloning Recombinant DNA technique in which specific cDNAs or fragments of genomic DNA are inserted into a
cloning vector, which then is incorporated into cultured host cells (e.g., E. coli cells) and maintained during growth of
the host cells; also called gene cloning.
DNA library Collection of cloned DNA molecules consisting of fragments of the entire genome (genomic library) or of
DNA copies of all the mRNAs produced by a cell type (cDNA library) inserted into a suitable cloning vector.
DNA polymerase An enzyme that copies one strand of DNA (the template strand) to make the complementary strand,
forming a new double-stranded DNA molecule. All DNA polymerases add deoxyribonucleotides one at a time in the 5
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Glossary
n3 direction to a short pre-existing primer strand of DNA or RNA.
domain Region of a protein with a distinct tertiary structure (e.g., globular or rodlike) and characteristic activity;
homologous domains may occur in different proteins.
dominant In genetics, referring to that allele of a gene expressed in the phenotype of a heterozygote; the
nonexpressed allele is recessive. Also referring to the phenotype associated with a dominant allele.
dorsal Relating to the back of an animal or the upper surface of a structure (e.g., leaf, wing).
double helix, DNA The most common three-dimensional structure for cellular DNA in which the two polynucleotide
strands are anti-parallel and wound around each other with complementary bases hydrogen-bonded.
downstream For a gene, the direction RNA polymerase moves during transcription, which is toward the end of the
template DNA strand with a 3 hydroxyl group. By convention, the +1 position of a gene is the first transcribed
nucleotide; nucleotides downstream from the +1 position are designated +2, +3, etc. Also, events that occur later in a
cascade of steps. See also upstream.
dynein Member of a family of ATP-powered motor proteins that move toward the ( ) end of microtubules by
sequentially breaking and forming new bonds with microtubule proteins. Dyneins can transport vesicles and are
responsible for the movement of cilia and flagella.
ectoderm Outermost of the three primary cell layers of the animal embryo; gives rise to epidermal tissues, the nervous
system, and external sense organs. See also endoderm and mesoderm.
electron carrier Any molecule or atom that accepts electrons from donor molecules and transfers them to acceptor
molecules. Most are prosthetic groups (e.g, heme, copper, iron-sulfur clusters) associated with membrane-bound
proteins.
electron transport Flow of electrons via a series of electron carriers from reduced electron donors (e.g., NADH) to O2
in the inner mitochondrial membrane, or from H2O to NADP in the thylakoid membrane of plant chloroplasts.
electrophoresis Any of several techniques for separating macromolecules based on their migration in a gel or other
medium subjected to a strong electric field.
electrophoretogram An autoradiogram of a gel in which molecules have been separated by gel electrophoresis.
elongation factor One of a group of nonribosomal proteins required for continued translation of mRNA following
initiation.
embryogenesis Early development of an individual from a fertilized egg (zygote). Following cleavage of the zygote,
the major axes are established during the blastula stage; in the subsequent gastrula stage, the early embryo
invaginates and acquires three cell layers.
endocytosis Uptake of extracellular materials by invagination of the plasma membrane to form a small
membrane-bounded vesicle (early endosome). endoderm Innermost of the three primary cell layers of the animal
embryo; gives rise to the gut and most of the respiratory tract. See ectoderm and mesoderm.
endoplasmic reticulum (ER) Network of interconnected membranous structures within the cytoplasm of eukaryotic
cells. The rough ER, which is associated with ribosomes, functions in the synthesis and processing of secretory and
membrane proteins; the smooth ER, which lacks ribosomes, functions in lipid synthesis.
endosome, late A sorting vesicle with an acidic internal pH in which bound ligands dissociate from their
membrane-bound receptor proteins. Late endosomes participate in sorting of lysosomal enzymes and in recycling of
receptors endocytosed from the plasma membrane.
endothelium Layer of highly flattened cells that forms the lining of all blood vessels and regulates exchange of
materials between the bloodstream and surrounding tissues; it usually is underlain by a basal lamina.
endothermic Referring to a chemical reaction that absorbs heat (i.e., has a positive change in enthalpy).
enhancer A regulatory sequence in eukaryotic DNA (rarely in prokaryotic DNA) that may be located at a great
distance from the gene it controls. Binding of specific proteins to an enhancer modulates the rate of transcription of the
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Glossary
associated gene.
enthalpy (H) Heat; in a chemical reaction, the enthalpy of the reactants or products is equal to their total bond
energies.
entropy (S) A measure of the degree of disorder or randomness in a system; the higher the entropy, the greater the
disorder.
enzyme A biological macromolecule that acts as a catalyst. Most enzymes are proteins, but certain RNAs, called
ribozymes, also have catalytic activity.
epinephrine A catecholamine secreted by the adrenal gland and some neurons in response to stress; also called
adrenaline. It functions as both a hormone and neurotransmitter, mediating "fight or flight" responses including
increased blood glucose levels and heart rate.
epithelium Coherent sheet comprising one or more layers of cells that covers an external body surface or lines an
internal cavity.
epitope The part of an antigen molecule that binds to an antibody; also called antigenic determinant.
equilibrium constant (K) Ratio of forward and reverse rate constants for a reaction. For a binding reaction, A + B 1 2
AB, it equals the association constant, Ka; the higher the Ka, the tighter the binding between A and B. The reciprocal of
the Ka is the dissociation constant, KD; the higher the KD, the weaker the binding between A and B.
eubacteria Class of prokaryotes that constitutes one of the three distinct evolutionary lineages of modern-day
organisms; also called the true bacteria or simply bacteria. Phylogenetically distinct from archaea and eukaryotes.
euchromatin Less condensed portions of chromatin, including most transcribed regions, present in interphase
chromosomes. See also heterochromatin.
eukaryotes Class of organisms, composed of one or more cells containing a membrane-enclosed nucleus and
organelles, that constitutes one of the three distinct evolutionary lineages of modern-day organisms; also called
eukarya. Includes all organisms except viruses and prokaryotes.
exocytosis Release of intracellular molecules (e.g., hormones, matrix proteins) contained within a
membrane-bounded vesicle by fusion of the vesicle with the plasma membrane of a cell. This is the process whereby
most molecules are secreted from eukaryotic cells.
exon Segments of a eukaryotic gene (or of its primary transcript) that reaches the cytoplasm as part of a mature
mRNA, rRNA, or tRNA molecule. See also intron.
exoplasmic face The face of a cell membrane directed away from the cytoplasm. The exoplasmic face of the plasma
membrane faces the cell exterior, whereas the exoplasmic face of organelles (e.g., mitochondria, chloroplasts, and the
endoplasmic reticulum) face their lumen.
exothermic Referring to a chemical reaction that releases heat (i.e., has a negative change in enthalpy).
expression See gene expression.
expression cloning Recombinant DNA techniques for isolating a cDNA or genomic DNA segment based on
functional properties of the encoded protein and without prior purification of the protein. Also refers to techniques for
producing high levels of a full-length protein once its cDNA or gene has been cloned.
expression vector A modified plasmid or virus that carries a gene or cDNA into a suitable host cell and there directs
synthesis of the encoded protein. Some expression vectors are designed for screening DNA libraries for a gene of
interest; others, for producing large amounts of a protein from its cloned gene
extracellular matrix A usually insoluble network consisting of polysaccharides, fibrous proteins, and adhesive
proteins that are secreted by animal cells. It provides structural support in tissues and can affect the development and
biochemical functions of cells.
extrinsic protein See peripheral membrane protein.
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Glossary
F0F1 complex See ATP synthase.
facilitated transport Protein-aided transport of an ion or molecule across a cell membrane down its concentration
gradient at a rate greater than that obtained by passive diffusion; also called facilitated diffusion. Such transport
exhibits ligand specificity and saturation kinetics. The glucose transporter GLUT1 is a wellstudied example of a protein
that mediates facilitated diffusion.
FAD (flavin adenine dinucleotide) A coenzyme that participates in oxidation reactions by accepting two electrons
from a donor molecule and two H+ from the solution. The reduced form, FADH2, transfers electrons to carriers that
function in oxidative phosphorylation.
fatty acid Any hydrocarbon chain that has a carboxyl group at one end; a major source of energy during metabolism
and precursors for synthesis of phospholipids.
fertilization Fusion of a female and male gamete (both haploid) to form a diploid zygote, which develops into a new
individual.
fibroblast A common type of connective-tissue cell that secretes collagen and other components of the extracellular
matrix. It migrates and proliferates during wound healing and in tissue culture.
fibronectin An extracellular multiadhesive protein that binds to other matrix components, fibrin, and cell-surface
receptors of the integrin family. It functions to attach cells to the extracellular matrix and is important in wound healing.
flagellum (pl. flagella) Long locomotory structure, extending from the surface of a eukaryotic cell, whose whiplike
bending propels the cell forward or backward. Usually there is only one flagellum per cell (as in sperm cells). Bacterial
flagella are smaller and much simpler structures. See also axoneme and cilium.
fluorescein See fluorescent staining.
fluorescent staining General technique for visualizing cellular components by treating cells with a fluorescent-labeled
agent that binds specifically to a component of interest and then observing the cells by fluorescence microscopy. For
instance, an antibody specific for a protein of interest can be chemically linked to a fluorescent dye such as fluorescein,
which emits green light, or rhodamine, which emits red light. Various fluorescent dyes that bind specifically to DNA are
used to detect chromosomes or specific chromosomal regions.
footprinting Technique for identifying protein-binding regions of DNA or RNA. A radiolabeled nucleic acid sample is
digested with a nuclease in the presence and absence of a specific binding protein. Because regions of DNA or RNA
with bound protein are protected from digestion, the patterns of fragment bands separated by gel electrophoresis
obtained from protected and unprotected samples differ, permitting identification of the protein-binding regions.
free energy (G) A measure of the potential energy of a system, which is a function of the enthalpy (H) and entropy (S).
free-
G) The difference in the free energy of the product molecules and of the starting molecules
G indicates that a reaction has a strong tendency to
occur; that is, at chemical equilibrium the concentration of products will be much greater than the concentration of
reactants.
G protein Any of numerous heterotrimeric GTP-binding proteins that function in intracellular signaling pathways;
usually activated by ligand binding to a coupled seven-spanning receptor on the cell surface. See also GTPase
superfamily.
G protein coupled receptor (GPCR) Member of an important class of cell-surface receptors that have seven
transmembrane
helices and are directly coupled to a trimeric G protein.
G0, G1, G2phase See cell cycle.
gamete Specialized haploid cell (in animals either a sperm or an egg) produced by meiosis of germ cells; in sexual
reproduction, union of a sperm and an egg initiates the development of a new individual.
ganglion (pl. ganglia) Collection of neuron cell bodies located outside of the central nervous system.
ganglioside Any glycolipid containing one or more N-acetylneuraminic acid (sialic acid) residues in its structure.
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Glossary
Gangliosides are found in the plasma membrane of eukaryotic cells and confer a net negative charge on most animal
cells.
gap junction Protein-lined channel between adjacent cells that allows passage of ions and small molecules between
the cells.
gastrula An early embryonic form subsequent to the blastula characterized by invagination of the cells to form a
rudimentary gut cavity and development of three cell layers.
gene Physical and functional unit of heredity, which carries information from one generation to the next. In molecular
terms, it is the entire DNA sequence
including exons, introns, and noncoding transcription-control regions
necessary for production of a functional protein or RNA. See also cistron and transcription unit.
gene cloning See DNA cloning.
gene control All of the mechanisms involved in regulating gene expression. Most common is regulation of
transcription, although mechanisms influencing the processing, stabilization, and translation of mRNAs help control
expression of some genes.
gene conversion Phenomenon in which one allele of a gene is converted to another during meiotic recombination.
gene expression Overall process by which the information encoded in a gene is converted into an observable
phenotype (most commonly production of a protein).
genetic code The set of rules whereby nucleotide triplets (codons) in DNA or RNA specify amino acids in proteins.
genome Total genetic information carried by a cell or organism.
genomic DNA All the DNA sequences composing the genome of a cell or organism. See also cDNA.
genomics Comparative analysis of the complete genomic sequences from different organisms; used to assess
evolutionary relations between species and to predict the number and general types of proteins produced by an
organism.
genotype Entire genetic constitution of an individual cell or organism; also, the alleles at one or more specific loci.
germ cell Any precursor cell that can give rise to gametes. See also somatic cell.
germ line Lineage of germ cells, which give rise to gametes and thus participate in formation of the next generation of
organisms; also the genetic material transmitted from one generation to the next through the gametes.
glial cells Nonexcitable supportive cells in the nervous system; also called neuroglial cells. Include astrocytes and
oligodendrocytes in the vertebrate central nervous system and Schwann cells in the peripheral nervous system.
glucagon A peptide hormone produced in the
cells of the pancreas that triggers the conversion of glycogen to
glucose by the liver; acts with insulin to control blood glucose levels.
glucose Six-carbon monosaccharide (sugar) that is the primary metabolic fuel in most cells. The large glucose
polymers, glycogen and starch, are used to store energy in animal cells and plant cells, respectively.
glycocalyx Carbohydrate-rich layer covering the outer surface of the plasma membrane of eukaryotic cells; composed
of membrane glycolipids, the oligosaccharide side chains of integral membrane proteins, and absorbed peripheral
membrane proteins.
glycogen A very long, branched polysaccharide, composed exclusively of glucose units, that is the primary storage
carbohydrate in animal cells. It is found primarily in liver and muscle cells.
glycogenolysis Breakdown of glycogen to glucose 6-phosphate; stimulated by a rise in cAMP following epinephrine
stimulation of cells and, in muscle, by a rise in Ca2+ following neuronal stimulation.
glycolipid Any lipid to which a short carbohydrate chain is covalently linked; commonly found in the plasma
membrane.
glycolysis Metabolic pathway whereby sugars are degraded anaerobically to lactate or pyruvate in the cytosol with
the production of ATP; also called Embden-Meyerhof pathway.
glycoprotein Any protein to which one or more oligosaccharide chains are covalently linked. Most secreted proteins
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Cell
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Glossary
and many membrane proteins are glycoproteins.
glycosaminoglycan (GAG) A long, linear, highly charged polymer of a repeating disaccharide in which one member
of the pair usually is a sugar acid (uronic acid) and the other is an amino sugar and many residues are sulfated.
Generally are covalently bound to core proteins forming proteoglycans, which are major components of the
extracellular matrix. glycosidic bond The covalent linkage between two monosaccharide residues formed by a
condensation reaction in which one carbon, usually carbon #1, of one sugar reacts with a hydroxyl group on a second
sugar with the loss of a water molecule.
glycosyl transferase An enzyme that forms a glycosidic bond between a sugar residue (monosaccharide) and an
amino acid side chain of a protein or a residue in an existing carbohydrate chain.
Golgi complex Stacks of membranous structures in eukaryotic cells that function in processing and sorting of proteins
and lipids destined for other cellular compartments or for secretion; also called Golgi apparatus.
growing fork Site in double-stranded DNA at which the template strands are separated and addition of
deoxyribonucleotides to each newly formed chain occurs; also called replication fork.
growth factor An extracellular polypeptide molecule that binds to a cell-surface receptor triggering an intracellular
signaling pathway leading to proliferation, differentiation, or other cellular response.
GTP (guanosine 5 -triphosphate) A nucleotide that is a precursor in RNA synthesis and also plays a special role in
protein synthesis, signal-transduction pathways, and microtubule assembly.
GTPase superfamily Group of GTP-binding proteins that cycle between an inactive state with bound GDP and an
active state with bound GTP. These proteins
elongation factors
including G proteins, Ras proteins, and certain polypeptide
function as intracellular switch proteins.
haploid Referring to an organism or cell having only one member of each pair of homologous chromosomes and
hence only one copy (allele) of each gene or genetic locus. Gametes and bacterial cells are haploid. See also diploid.
HeLa cell Line of human epithelial cells, derived from a human cervical carcinoma, that grows readily in culture and is
widely used in research.
helicase Any enzyme that moves along a DNA duplex using the energy released by ATP hydrolysis to separate
(unwind) the two strands. Required for the replication and transcription of DNA.
helix-loop-helix A conserved structural motif found in many monomeric Ca2+-binding proteins and dimeric eukaryotic
transcription factors.
helix-turn-helix A DNA-binding motif found in most bacterial DNA-binding proteins.
heterochromatin Regions of chromatin that remain highly condensed and transcriptionally inactive during interphase.
heteroduplex A double-stranded DNA molecule containing one or more mispaired bases.
heterokaryon Cell with more than one functional nucleus produced by the fusion of two or more different cells.
heterozygous Referring to a diploid cell or organism having two different alleles of a particular gene.
hexose A six-carbon monosaccharide.
high-energy bond Covalent bond that releases a large amount of energy when hydrolyzed under the usual
intracellular conditions. Examples include the phosphoanhydride bonds in ATP, thioester bond in acetyl CoA, and
various phosphate ester bonds.
histones A family of small, highly conserved basic proteins, found in the chromatin of all eukaryotic cells, that
associate with DNA in the nucleosome.
Holliday structure Intermediate structure in recombination between homologous chromosomes.
homeobox Conserved DNA sequence that encodes a DNAbinding domain (homeodomain) in a class of transcription
factors encoded by certain homeotic genes.
homeodomain A conserved DNA-binding motif found in many developmentally important transcription factors. See
also homeobox.
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Glossary
homeosis Transformation of one body part into another arising from mutation in or misexpression of certain
developmentally critical genes.
homeotic gene A gene in which mutations cause cells in one region of the body to act as though they were located in
another, giving rise to conversions of one cell, tissue, or body region into another.
homologous chromosome One of the two copies of each morphologic type of chromosome present in a diploid cell;
also called homologue. Each homologue is derived from a different parent.
homologue See homologous chromosome.
homology Similarity in the sequence of a protein or nucleic acid or in the structure of an organ that reflects a common
evolutionary origin. Molecules or sequences that exhibit homology are referred to as homologs. In contrast, analogy is
a similarity in structure or function that does not reflect a common evolutionary origin.
homozygous Referring to a diploid cell or organism having two identical alleles of a particular gene.
hormone General term for any extracellular substance that induces specific responses in target cells. Hormones
coordinate the growth, differentiation, and metabolic activities of various cells, tissues, and organs in multicellular
organisms.
Hox complex Clusters of homologous selector genes, which help determine the body plan in animals.
hyaluronan A large, highly hydrated polysaccharide that is a major component of the extracellular matrix; also called
hyaluronic acid and hyaluronate. It imparts stiffness and resilience as well as a lubricating quality to many types of
connective tissue.
hybridization Association of two complementary nucleic acid strands to form double-stranded molecules, which can
contain two DNA strands, two RNA strands, or one DNA and one RNA strand. Used experimentally in various ways to
detect specific DNA or RNA sequences.
hybridoma A clone of hybrid cells that are immortal and produce monoclonal antibodies; formed by fusion of normal
antibody-producing B lymphocytes with myeloma cells.
hydrogen bond A noncovalent bond between an electronegative atom (commonly oxygen or nitrogen) and a
hydrogen atom covalently bonded to another electronegative atom. Particularly important in stabilizing the
three-dimensional structure of proteins and formation of base pairs in nucleic acids.
hydrolysis Reaction in which a covalent bond is cleaved with addition of an H from water to one product of the
cleavage and of an OH from water to the other.
hydrophilic Interacting effectively with water. See also polar.
hydrophobic Not interacting effectively with water; in general, poorly soluble or insoluble in water. See also nonpolar.
hydrophobic bond The force that drives nonpolar molecules or parts of molecules to associate with each other in
aqueous solution. A type of noncovalent bond that is particularly important in stabilization of the phospholipid bilayer.
hypertonic Referring to an external solution whose solute concentration is high enough to cause water to move out of
cells due to osmosis.
hypotonic Referring to an external solution whose solute concentration is low enough to cause water to move into
cells due to osmosis.
immunoglobulin (Ig) Any protein that functions as an antibody. The five major classes of vertebrate immunoglobulins
(IgA, IgD, IgE, IgG, and IgM) differ in their specific functions in the immune response.
in vitro Denoting a reaction or process taking place in an isolated cell-free extract; sometimes used to distinguish cells
growing in culture from those in an organism.
in vivo In an intact cell or organism.
induction In embryogenesis, a change in the developmental fate of one cell or tissue caused by direct interaction with
another cell or tissue or with an extracellular signaling molecule; in metabolism, an increase in the synthesis of an
enzyme or series of enzymes mediated by a specific molecule (inducer).
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Glossary
initiation factor One of a group of proteins that promote the proper association of ribosomes and mRNA and are
required for initiation of protein synthesis.
initiator A eukaryotic promoter sequence for RNA polymerase II that specifies transcription initiation within the
sequence.
insulin A protein hormone produced in the
cells of the pancreas that stimulates uptake of glucose into muscle and
fat cells and with glucagon helps to regulate blood glucose levels. Insulin also functions as a growth factor for many
cells.
integral membrane protein Any membrane-bound protein all or part of which interacts with the hydrophobic core of
the phospholipid bilayer and can be removed from the membrane only by extraction with detergent; also called intrinsic
membrane protein.
integrins A large family of heterodimeric transmembrane proteins that promote adhesion of cells to the extracellular
matrix or to the surface of other cells.
interferons (IFNs) Small group of cytokines that bind to cell- surface receptors on target cells inducing changes in
gene expression leading to an antiviral state or other cellular responses important in the immune response.
intermediate filaments Cytoskeletal fibers (10 nm in diameter) formed by polymerization of several classes of
cell-specific subunit proteins including keratins, lamins, and vimentin. They constitute the major structural proteins of
skin and hair; form the scaffold that holds Z disks and myofibrils in place in muscle; and generally function as important
structural components of many animal cells and tissues.
interphase Long period of the cell cycle, including the G1, S, and G2 phases, between one M (mitotic) phase and the
next.
intrinsic protein See integral membrane protein.
intron Part of a primary transcript (or the DNA encoding it) that is removed by splicing during RNA processing and is
not included in the mature, functional mRNA, rRNA, or tRNA; also called intervening sequence.
ion channel Any transmembrane protein complex that forms a water-filled channel across the phospholipid bilayer
allowing selective ion transport down its electrochemical gradient. See also ion pump.
ion pump Any transmembrane ATPase that couples hydrolysis of ATP to the transport of a specific ion across the
phospholipid bilayer against its electrochemical gradient.
ionic bond A noncovalent bond between a positively charged ion (cation) and negatively charged ion (anion).
isoelectric focusing Technique for separating molecules by gel electrophoresis in a pH gradient subjected to an
electric field. A protein migrates to the pH at which its overall net charge is zero.
isoelectric point (pI) The pH of a solution at which a dissolved protein or other potentially charged molecule has a net
charge of zero and therefore does not move in an electric field.
isoform One of several forms of the same protein whose amino acid sequences differ slightly but whose general
activity is identical.
isotonic Referring to a solution whose solute concentration is such that it causes no net movement of water in or out
of cells.
karyotype Number, sizes, and shapes of the entire set of metaphase chromosomes of a eukaryotic cell.
kinase An enzyme that transfers the term
phosphorylate specific serine, threonine, or tyrosine residues in target proteins, play a critical role in regulating the
activity of many cellular proteins. See also phosphatases.
kinesin Member of a family of motor proteins that use energy released by ATP hydrolysis to move toward the (+) end
of a microtubule, transporting vesicles or particles in the process.
kinetochore A three-layer protein structure located at or near the centromere of each mitotic chromosome from which
microtubules (kinetochore fibers) extend toward the spindle poles of the cell; plays an active role in movement of
32
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Glossary
chromosomes toward the poles during anaphase.
Km A parameter that describes the affinity of an enzyme for its substrate and equals the substrate concentration that
yields the half-maximal reaction rate; also called the Michaelis constant. A similar parameter describes the affinity of a
transport protein for the transported molecule or the affinity of a receptor for its ligand.
knockin, gene Technique in which the coding sequences of one gene are replaced by those of another.
knockout, gene Technique for selectively inactivating a gene by replacing it with a mutant allele in an otherwise
normal organism.
Krebs cycle See citric acid cycle.
label A fluorescent chemical group or radioactive atom incorporated into a molecule in order to spatially locate the
molecule or follow it through a reaction or purification scheme. As a verb, to add such a group or atom to a cell or
molecule.
lagging strand Newly synthesized DNA strand formed at the growing fork as short, discontinuous segments, called
Okazaki fragments, which are later joined by DNA ligase. Although overall lagging-strand synthesis occurs in the 3
5 direction, each Okazaki fragment is synthesized in the 5
3 direction. See also leading strand.
laminin A component of the extracellular matrix that is found in all basal laminae and has binding sites for cell-surface
receptors, collagen, and heparan sulfate proteoglycans.
lamins A group of intermediate filament proteins that form the fibrous network (nuclear lamina) on the inner surface of
the nuclear envelope.
leading strand Newly synthesized DNA strand formed by continuous synthesis in the 5 n3 direction at the growing
fork. The direction of leading-strand synthesis is the same as movement of the growing fork. See also lagging strand.
lectin Any protein that binds tightly to specific sugars. Lectins can be used in affinity chromatography to purify
glycoproteins or as reagents to detect them in situ.
leucine zipper Common structural motif in some dimeric eukaryotic transcription factors characterized by a C-terminal
coiled-coil dimerization domain and N-terminal DNA-binding domain.
leukemia Cancer of white blood cells and their precursors.
library See DNA library.
ligand Any molecule, other than an enzyme substrate, that binds tightly and specifically to a macromolecule, usually a
protein, forming a macromolecule-ligand complex.
ligase An enzyme that links together the 3 end of one nucleic acid strand with the 5 end of another, forming a
continuous strand.
linkage In genetics, the tendency of two different loci on the same chromosome to be inherited together. The closer
two loci are, the greater their linkage and the lower the frequency of recombination between them.
lipid Any organic molecule that is insoluble in water but is soluble in nonpolar organic solvents. Lipids contain
covalently linked fatty acids and are found in fat droplets and, as phospholipids, in biomembranes.
lipophilic See hydrophobic.
liposome Spherical phospholipid bilayer structure with an aqueous interior that forms in vitro from phospholipids and
may contain protein.
locus In genetics, the specific site of a gene on a chromosome. All the alleles of a particular gene occupy the same
locus.
lymphocytes Two classes of white blood cells that can recognize foreign molecules (antigens) and mediate immune
responses. B lymphocytes are responsible for production of antibodies; T lymphocytes are responsible for destroying
virus- and bacteria-infected cells, foreign cells, and cancer cells.
lysogenic cycle Series of events in which a bacterial virus (bacteriophage) enters a host cell and its DNA is
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Cell
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Glossary
incorporated into the host-cell genome in such a way that the virus (the prophage) lays dormant. The association of a
prophage with the host-cell genome is called lysogeny. By various mechanisms, the prophage can be activated so that
it enters the lytic cycle.
lysogeny See lysogenic cycle.
lysosome Small organelle having an internal pH of 4 5 and containing hydrolytic enzymes.
lytic cycle Series of events in which a virus enters and replicates within a host cell to produce new viral particles
eventually causing lysis of the cell. See also lysogenic cycle.
M (mitotic) phase
See cell cycle.
macromolecule Any large, usually polymeric molecule (e.g., a protein, nucleic acid, polysaccharide) with a molecular
mass greater than a few thousand daltons.
malignant Referring to a tumor or tumor cells that can invade surrounding normal tissue and/or undergo metastasis.
See also benign.
MAP kinase Protein kinase that is activated in response to cell stimulation by many different growth factors and that
mediates cellular responses by phosphorylating specific target proteins.
mapping Various techniques for determining the relative order of genes on a chromosome (genetic map), the absolute
position of genes (physical map), or the relative position of restriction sites (restriction map).
meiosis In eukaryotes, a special type of cell division that occurs during maturation of germ cells; comprises two
successive nuclear and cellular divisions with only one round of DNA replication resulting in production of four
genetically nonequivalent haploid cells (gametes) from an initial diploid cell.
membrane See biomembrane.
membrane potential Voltage difference across a membrane due to the slight excess of positive ions (cations) on one
side and negative ions (anions) on the other.
mesenchyme Embryonic mesoderm tissue in animals from which are formed the connective tissues, blood vessels,
and lymphatic vessels.
mesoderm The middle of the three primary cell layers of the animal embryo, lying between the ectoderm and
endoderm; gives rise to the notochord, connective tissue, muscle, blood, and other tissues.
messenger RNA See mRNA.
metabolism The sum of the chemical processes that occur in living cells; includes anabolism and catabolism.
metaphase Mitotic stage at which chromosomes are fully condensed and attached to the mitotic spindle at its equator
but have not yet started to segregate toward the opposite spindle poles.
metastasis Spread of tumor cells from their site of origin and establishment of areas of secondary growth.
Michaelis constant See Km.
microfilaments Cytoskeletal fibers (
7 nm in diameter) that are formed by polymerization of monomeric globular (G)
actin; also called actin filaments. Microfilaments play an important role in muscle contraction, cytokinesis, cell
movement, and other cellular functions and structures.
microtubule-associated protein (MAP) Any protein, including motor proteins, that binds to microtubules in a
constant ratio and determines the unique properties of different types of microtubules.
microtubules Cytoskeletal fibers (24 nm in diameter) that are formed by polymerization of
,
-tubulin monomers
and exhibit structural and functional polarity. They are important components of cilia, flagella, the mitotic spindle, and
other cellular structures.
microvillus (pl. microvilli) Small, membrane-covered projection on the surface of an animal cell containing a core of
actin filaments. Numerous microvilli are present on the absorptive surface of intestinal epithelial cells, increasing the
surface area for transport of nutrients.
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Glossary
mitochondrion (pl. mitochondria) Large organelle that is surrounded by two phospholipid bilayer membranes,
contains DNA, and carries out oxidative phosphorylation, thereby producing most of the ATP in eukaryotic cells.
mitogen Any extracellular substance, such as a growth factor, that promotes cell proliferation.
mitosis In eukaryotic cells, the process whereby the nucleus is divided to produce two genetically equivalent daughter
nuclei with the diploid number of chromosomes. See also cytokinesis and meiosis.
mitotic apparatus A specialized temporary structure, present in eukaryotic cells only during mitosis, that captures the
chromosomes and then pushes and pulls them to opposite sides of the dividing cell. Consists of a central bilaterally
symmetric bundle of microtubules with the overall shape of a football (the mitotic spindle) and two star-shaped tufts of
microtubules (the asters), one at each pole of the spindle.
mitotic spindle See mitotic apparatus.
mobile DNA element Any DNA sequence that is not present in the same chromosomal location in all individuals of a
species.
monoclonal antibody Antibody produced by the progeny of a single B cell and thus a homogeneous protein exhibiting
a single antigen specificity. Experimentally, it is produced by use of a hybridoma.
monomer Any small molecule that can be linked with others of the same type to form a polymer. Examples include
amino acids, nucleotides, and monosaccharides.
monomeric For proteins, consisting of a single polypeptide chain.
monosaccharide Any simple sugar with the formula (CH2O)n where n = 3
7.
morphogen A molecule that specifies cell identity during development as a function of its concentration.
motif In proteins, a structural unit exhibiting a particular three-dimensional architecture that is found in a variety of
proteins and usually is associated with a particular function.
motor protein Any member of a special class of enzymes that use energy from ATP hydrolysis to walk or slide along a
microfilament (myosin) or a microtubule (dynein and kinesin).
MPF (mitosis-promoting factor) A heterodimeric protein, composed of a cyclin and cyclin-dependent kinase (Cdk),
that triggers entrance of a cell into mitosis by inducing chromatin condensation and nuclear-envelope breakdown;
originally called maturation-promoting factor.
mRNA (messenger RNA) Any RNA that specifies the order of amino acids in a protein. It is produced by transcription
of DNA by RNA polymerase and, in RNA viruses, by transcription of viral RNA. In eukaryotes, the initial RNA product
(primary transcript) undergoes processing to yield functional mRNA, which is transported to the cytoplasm. See also
translation.
MTOC (microtubule-organizing center) General term for any structure (e.g., the centrosome) that organizes
microtubules in nonmitotic (interphase) cells.
multiadhesive matrix proteins Group of long flexible proteins that bind to other components of the extracellular
matrix (collagen, polysaccharides) and to cell-surface receptors, thereby cross-linking the matrix to the cell membrane.
multimeric For proteins, containing several polypeptide chains (or subunits).
mutagen A chemical or physical agent that induces mutations.
mutation In genetics, a permanent, heritable change in the nucleotide sequence of a chromosome, usually in a single
gene; commonly leads to a change in or loss of the normal function of the gene product.
myelin sheath Stacked specialized cell membrane that forms an insulating layer around vertebrate axons and
increases the speed of impulse conduction.
myofibril Long, highly organized bundle of actin and myosin filaments and other proteins that constitute the basic
structural unit of muscle cells (myofibers)
myosin One of a family of motor proteins with a globular head region and coiled-coil tail region that has
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Cell
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Glossary
actin-stimulated ATPase activity; drives movement along actin filaments during muscle contraction and cytokinesis
(myosin II) and mediates vesicle translocation (myosins I and V).
NAD+ (nicotinic adenine dinucleotide) A widely used coenzyme that participates in oxidation reactions by accepting
two electrons from a donor molecule and one H+ from the solution. The reduced form, NADH, transfers electrons to
carriers that function in oxidative phosphorylation.
NADP+ (nicotinic adenine dinucleotide phosphate) Phosphorylated form of NAD+, which is used extensively as an
electron carrier in biosynthetic pathways and during photosynthesis.
Nernst equation Mathematical expression that defines the electric potential E across a membrane as directly
proportional to the logarithm of the ratio of the ion concentrations on either side of the membrane and inversely
proportional to the valency of the ions.
neuron (nerve cell) Any of the impulse-conducting cells of the ner-vous system. A typical neuron contains a cell body;
several short, branched processes (dendrites); and one long process (axon).
neuropeptide A peptide secreted by neurons that functions as a signaling molecule either at a synapse or elsewhere.
These molecules have diverse, often long-lived effects in contrast to neurotransmitters.
neurotransmitter Extracellular signaling molecule that is released by the presynaptic neuron at a chemical synapse
and relays the signal to the postsynaptic cell. The response elicited by a neurotransmitter, either excitatory or inhibitory,
is determined by its receptor on the postsynaptic cell. Examples include acetylcholine, dopamine, GABA (
-aminobutyric acid), and serotonin.
noncovalent bond Any relatively weak chemical bond that does not involve an intimate sharing of electrons. Multiple
noncovalent bonds often stabilize the conformation of macromolecules and mediate highly specific interactions
between molecules.
nonpolar Referring to a molecule or structure that lacks any net electric charge or asymmetric distribution of positive
and negative charges. Nonpolar molecules generally are insoluble in water.
Northern blotting Technique for detecting specific RNAs separated by electrophoresis by hybridization to a labeled
DNA probe. See also Southern blotting.
nuclear envelope Double-membrane structure surrounding the nucleus; the outer membrane is continuous with the
endoplasmic reticulum and the two membranes are perforated by nuclear pores.
nuclear lamina Fibrous network on the inner surface of the inner nuclear membrane composed of lamin filaments.
nuclear pore complex (NPC) Large, multiprotein structure in the nuclear envelope through which ions and small
molecules can diffuse and which mediates the active transport of ribonucleoproteins and large proteins between the
nucleus and cytoplasm.
nuclear receptor General term for intracellular receptors that bind lipid-soluble hormones (e.g., steroid hormones);
also called steroid receptor superfamily. Following ligand binding, the hormonereceptor complex translocates to the
nucleus and functions as a transcription factor.
nucleic acid A polymer of nucleotides linked by phosphodiester bonds. DNA and RNA are the primary nucleic acids in
cells.
nucleocapsid A viral capsid plus the enclosed nucleic acid.
nucleolus Large structure in the nucleus of eukaryotic cells where rRNA synthesis and processing occurs and
ribosome subunits are assembled.
nucleoside A small molecule composed of a purine or pyrimidine base linked to a pentose (either ribose or
deoxyribose).
nucleosome Small structural unit of chromatin consisting of a disk-shaped core of histone proteins around which a
146-bp segment of DNA is wrapped.
nucleotide A nucleoside with one or more phosphate groups linked via an ester bond to the sugar moiety. DNA and
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Cell
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Glossary
RNA are polymers of nucleotides.
nucleus Large membrane-bounded organelle in eukaryotic cells that contains DNA organized into chromosomes;
synthesis and processing of RNA and ribosome assembly occur in the nucleus.
Okazaki fragments Short (<1000 bases), single-stranded DNA fragments that are formed during synthesis of the
lagging strand in DNA replication and are rapidly joined by DNA ligase to form a continuous DNA strand.
oncogene A gene whose product is involved either in transforming cells in culture or in inducing cancer in animals.
Most oncogenes are mutant forms of normal genes (proto-oncogenes) involved in the control of cell growth or division.
oocyte Developing egg cell.
operator Short DNA sequence in a bacterial or viral genome that binds a repressor protein and controls transcription
of an adjacent gene.
operon In bacterial DNA, a cluster of contiguous genes transcribed from one promoter that gives rise to a
polycistronic mRNA.
organelle Any membrane-limited structure found in the cytoplasm of eukaryotic cells.
osmosis Net movement of water across a semipermeable membrane from a solution of lesser to one of greater solute
concentration. The membrane must be permeable to water but not to solute molecules.
osmotic pressure Hydrostatic pressure that must be applied to the more concentrated solution to stop the net flow of
water across a semipermeable membrane separating solutions of different concentrations.
oxidation Loss of electrons from an atom or molecule as occurs when hydrogen is removed from a molecule or
oxygen is added. The opposite of reduction.
oxidation potential The voltage change when an atom or molecule loses an electron.
oxidative phosphorylation The phosphorylation of ADP to form ATP driven by the transfer of electrons to oxygen (O2)
in bacteria and mitochondria. This process involves generation of a proton-motive force during electron transport, and
its subsequent use to power ATP synthesis.
passive (simple) diffusion
Net movement of a molecule across a membrane down its concentration gradient at a
rate proportional to the gradient and the permeability of the membrane.
patch clamping Technique for determining ion flow through a single ion channel or across the membrane of an entire
cell by use of a micropipette whose tip is applied to a small patch of the cell membrane.
PCR (polymerase chain reaction) Technique for amplifying a specific DNA segment in a complex mixture by multiple
cycles of DNA synthesis from short oligonucleotide primers followed by brief heat treatment to separate the
complementary strands.
pentose A five-carbon monosaccharide. The pentoses ribose and deoxyribose are present in RNA and DNA,
respectively.
peptide A small polymer usually containing fewer than 30 amino acids connected by peptide bonds.
peptide bond Covalent bond that links adjacent amino acid residues in proteins; formed by a condensation reaction
between the amino group of one amino acid and the carboxyl group of another with release of a water molecule.
peripheral membrane protein Any protein that associates with the cytosolic or exoplasmic face of a membrane but
does not enter the hydrophobic core of the phospholipid bilayer; also called extrinsic protein. See also integral
membrane protein.
peroxisome Small organelle in eukaryotic cells whose functions include degradation of fatty acids and amino acids by
means of reactions that generate hydrogen peroxide, which is converted to water and oxygen by catalase.
pH A measure of the acidity or alkalinity of a solution defined as the negative logarithm of the hydrogen ion
concentration in moles per liter: pH= log [H+]. Neutrality is equivalent to a pH of 7; values below this are acidic and
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Cell
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Glossary
those above are alkaline.
phage See bacteriophage.
phagocytosis Process by which relatively large particles (e.g., bacterial cells) are internalized by certain eukaryotic
cells.
phenotype The observable characteristics of a cell or organism as distinct from its genotype.
pheromone A signaling molecule released by an individual that can alter the behavior or gene expression of other
individuals of the same species. The yeast
and a mating-type factors are well-studied examples.
phosphatase An enzyme that removes a phosphate group from a substrate by hydrolysis. Phosphoprotein
phosphatases act with protein kinases to control the activity of many cellular proteins.
phosphoanhydride bond A type of high-energy bond formed between two phosphate groups, such as the
phosphates and the
and
and
phosphates in ATP.
phosphodiester bond A covalent bond in which two hydroxyl groups form ester linkages to the same phosphate
group; joins adjacent nucleotides in DNA and RNA.
phosphoinositides A family of membrane-bound lipids containing phosphorylated inositol derivatives that are
important in signal-transduction pathways in eukaryotic cells.
phospholipid bilayer A symmetrical two-layer structure, found in all biomembranes, in which the polar head groups of
phospholipids are exposed to the aqueous medium, while the nonpolar hydrocarbon chains of the fatty acids are in the
center.
phospholipids The major class of lipids present in biomembranes, usually composed of two fatty acid chains
esterified to two of the carbons of glycerol phosphate, with the phosphate esterified to one of various polar groups.
photosynthesis Complex series of reactions occurring in some bacteria and plant chloroplasts whereby light energy
is used to generate carbohydrates from CO2, usually with the consumption of H2O and evolution of O2.
pI See isoelectric point.
pinocytosis The nonspecific uptake of small droplets of extracellular fluid into endocytic vesicles.
plaque assay Technique for determining the number of infectious viral particles in a sample by culturing a diluted
sample on a layer of susceptible host cells and then counting the clear areas of lysed cells (plaques) that develop.
plasma membrane The membrane surrounding a cell that separates the cell from its external environment, consisting
of a phospholipid bilayer and associated proteins.
plasmid Small, circular extrachromosomal DNA molecule capable of autonomous replication in a cell. Commonly
used as a cloning vector.
plasmodesmata (sing. plasmodesma) Tubelike cell junctions that interconnect the cytoplasm of adjacent plant cells
and are functionally analogous to gap junctions in animal cells.
point mutation Change of a single nucleotide in DNA, especially in a region coding for protein; can result in formation
of a codon specifying a different amino acid or a stop codon, or a shift in the reading frame.
polar Referring to a molecule or structure with a net electric charge or asymmetric distribution of positive and negative
charges. Polar molecules are usually soluble in water.
polarity Presence of functional and/or structural differences in distinct regions of a cell or cellular component.
polymer Any large molecule composed of multiple identical or similar units (monomers) linked by covalent bonds.
polymerase chain reaction See PCR.
polypeptide Linear polymer of amino acids connected by peptide bonds. Proteins are large polypeptides, and the two
terms commonly are used interchangeably.
polyribosome A complex containing several ribosomes all translating a single messenger RNA; also called polysome.
polysaccharide Linear or branched polymer of monosaccharides, linked by glycosidic bonds, usually containing more
than 15 residues. Examples include glycogen, cellulose, and glycosaminoglycans.
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Glossary
positional cloning Isolation and cloning of the normal form of a mutation-defined gene (i.e., a gene identified by
genetic analysis of mutants).
pre-mRNA Precursor messenger RNA; the primary transcript and intermediates in RNA processing.
pre-rRNA Large precursor ribosomal RNA that is synthesized in the nucleolus of eukaryotic cells and processed to
yield three of the four RNAs present in ribosomes.
primary structure In proteins, the linear arrangement (sequence) of amino acids and the location of covalent (mostly
disulfide) bonds within a polypeptide chain.
primary transcript Initial RNA product, containing introns and exons, produced by transcription of DNA. Many primary
transcripts must undergo RNA processing to form the physiologically active RNA species.
primase A specialized RNA polymerase that synthesizes short stretches of RNA used as primers for DNA synthesis.
primer A short nucleic acid sequence containing a free 3 hydroxyl group that forms base pairs with a complementary
template strand and functions as the starting point for addition of nucleotides to copy the template strand.
probe Defined RNA or DNA fragment, radioactively or chemically labeled, that is used to detect specific nucleic acid
sequences by hybridization.
prokaryotes Class of organisms, including the eubacteria and archaea, that lack a true membrane-limited nucleus
and other organelles. See also eukaryotes.
promoter DNA sequence that determines the site of transcription initiation for an RNA polymerase.
promoter-proximal element Any regulatory sequence in eukaryotic DNA that is located within
200 base pairs of
the transcription start site. Transcription of many genes is controlled by multiple promoter-proximal elements.
prophase Earliest stage in mitosis during which the chromosomes condense and the centrioles begin moving toward
the spindle poles.
prosthetic group A nonpeptide organic molecule or metal ion that binds tightly and specifically with a protein and is
required for its activity, such as heme in hemoglobin. See also coenzyme.
proteasome Large multifunctional protease complex in the cytosol that degrades intracellular proteins marked for
destruction by attachment of multiple ubiquitin molecules.
protein A linear polymer of amino acids linked together in a specific sequence and usually containing more than 50
residues. Proteins form the key structural elements in cells and participate in nearly all cellular activities.
proteoglycans A group of glycoproteins that contain a core protein to which is attached one or more
glycosaminoglycans. They are found in nearly all extracellular matrices, and some are attached to the plasma
membrane.
proton-motive force The energy equivalent of the proton (H+) concentration gradient and electric potential gradient
across a membrane; used to drive ATP synthesis by ATP synthase, transport of molecules against their concentration
gradient, and movement of bacterial flagella.
proto-oncogene A normal cellular gene that encodes a protein usually involved in regulation of cell growth or
proliferation and that can be mutated into a cancer-promoting oncogene, either by changing the protein-coding
segment or by altering its expression.
pulse-chase A type of experiment in which a radioactive small molecule is added to a cell for a brief period (the pulse)
and then is replaced with an excess of the unlabeled form of same small molecule (the chase). Used to detect changes
in the cellular location of a molecule or its metabolic fate over time.
pump Any transmembrane protein that mediates the active transport of an ion or small molecule across a
biomembrane.
purines A class of nitrogenous compounds containing two fused heterocyclic rings. Two purines, adenine and guanine,
commonly are found in DNA and RNA.
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Cell
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Glossary
pyrimidines A class of nitrogenous compounds containing one heterocyclic ring. Two pyrimidines, cytosine and
thymine, commonly are found in DNA; in RNA, uracil replaces thymine.
quaternary structure The number and relative positions of the polypeptide chains in multisubunit proteins.
quiescent Referring to a cell that has exited the cell cycle and is in the G0 state.
radioisotope Unstable form of an atom that emits radiation as it decays. Several radioisotopes are commonly used
experimentally as labels in biological molecules.
Ras protein A monomeric GTP-binding protein that functions in intracellular signaling pathways and is activated by
ligand binding to receptor tyrosine kinases
and other cell-surface receptors. See also GTPase superfamily.
reading frame The sequence of nucleotide triplets (codons) that runs from a specific translation start codon in a
mRNA to a stop codon. Some mRNAs can be translated into different polypeptides by reading in two different reading
frames.
receptor Any protein that binds a specific extracellular signaling molecule (ligand) and then initiates a cellular
response. Receptors for steroid hormones, which diffuse across the plasma membrane, are located within the cell;
receptors for water-soluble hormones, peptide growth factors, and neurotransmitters are located in the plasma
membrane with their ligand-binding domain exposed to the external medium.
receptor tyrosine kinase (RTK) Member of an important class of cell-surface receptors whose cytosolic domain has
tyrosine-specific protein kinase activity. Ligand binding activates this kinase activity and initiates intracellular signaling
pathways.
recessive In genetics, referring to that allele of a gene that is not expressed in the phenotype when the dominant allele
is present. Also refers to the phenotype of an individual (homozygote) carrying two recessive alleles.
recombinant DNA Any DNA molecule formed by joining DNA fragments from different sources. Commonly produced
by cutting DNA molecules with restriction enzymes and then joining the resulting fragments from different sources with
DNA ligase.
recombination Any process in which chromosomes or DNA molecules are cleaved and the fragments are rejoined to
give new combinations. Occurs naturally in cells as the result of the exchange (crossing over) of DNA sequences on
maternal and paternal chromatids during meiosis; also is carried out in vitro with purified DNA and enzymes.
reduction Gain of electrons by an atom or molecule as occurs when hydrogen is added to a molecule or oxygen is
removed. The opposite of oxidation.
reduction potential The voltage change when an atom or molecule gains an electron.
replication fork See growing fork.
replication origin Unique DNA segments present in an organism's genome at which DNA replication begins.
Eukaryotic chromosomes contain multiple origins, whereas bacterial chromosomes and plasmids often contain just
one.
replicon Region of DNA served by one replication origin.
resolution The minimum distance that can be distinguished by an optical apparatus; also called resolving power.
respiration General term for any cellular process involving the uptake of O2 coupled to production of CO2.
restriction enzyme (endonuclease) Any enzyme that recognizes and cleaves a specific short sequence, the
restriction site, in double-stranded DNA molecules. These enzymes are widespread in bacteria and are used
extensively in recombinant DNA technology.
restriction fragment A defined DNA fragment resulting from cleavage with a particular restriction enzyme. These
fragments are used in the production of recombinant DNA molecules and DNA cloning.
restriction point The point in late G1 of the cell cycle at which mammalian cells become committed to entering the S
phase and completing the cycle even in the absence of growth factors.
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Cell
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Glossary
retrotransposon Type of eukaryotic mobile DNA element whose movement in the genome is mediated by an RNA
intermediate and involves a reverse transcription step. See also transposon.
retrovirus A type of eukaryotic virus containing an RNA genome that replicates in cells by first making a DNA copy of
the RNA. This proviral DNA is inserted into cellular chromosomal DNA, and gives rise to further genomic RNA as well
as the mRNAs for viral proteins.
reverse transcriptase Enzyme found in retroviruses that catalyzes synthesis of a double-stranded DNA from a
single-stranded RNA template.
ribosomal RNA See rRNA.
ribosome A large complex comprising several different rRNA molecules and more than 50 proteins, organized into a
large subunit and small subunit; the site of protein synthesis.
ribozyme An RNA molecule or segment with catalytic activity.
RNA (ribonucleic acid) Linear, single-stranded polymer, composed of ribose nucleotides, that is synthesized by
transcription of DNA or by copying of RNA. The three types of cellular RNA
mRNA, rRNA, and tRNA
play
different roles in protein synthesis.
RNA editing Unusual type of RNA processing in which the sequence of a pre-mRNA is altered.
RNA polymerase An enzyme that copies one strand of DNA or RNA (the template strand) to make the
complementary RNA strand using as substrates ribonucleoside triphosphates.
RNA processing Various modifications that occur to many but not all primary transcripts to yield functional RNA
molecules.
RNA splicing A process that results in removal of introns and joining of exons in RNAs. See also spliceosome.
rRNA (ribosomal RNA) Any one of several large RNA molecules that are structural and functional components of
ribosomes. Often designated by their sedimentation coefficient: 28S, 18S, 5.8S, and 5S rRNA in higher eukaryotes
S (synthesis) phase See cell cycle.
sarcoma A malignant tumor derived from connective tissue.
sarcomere Repeating unit of a myofibril in striated muscle that extends from one Z disk to an adjacent one and
shortens during contraction.
sarcoplasmic reticulum Network of membranes that surrounds each myofibril in a muscle cell and sequesters Ca2
ions. Stimulation of a muscle cell induces release of Ca2 ions into the cytosol, triggering coordinated contraction along
the length of the cell.
Schwann cell Type of glial cell that forms the myelin sheath around axons in the peripheral nervous system.
second messenger An intracellular signaling molecule whose concentration increases (or decreases) in response to
binding of an extracellular ligand to a cell-surface receptor. Examples include cAMP, Ca2, diacylglycerol (DAG), and
inositol 1,4,5-trisphosphate (IP3).
secondary structure In proteins, local folding of a polypeptide chain into regular structures including the
helix,
sheet, and U-shaped turns and loops.
secretory vesicle Small membrane-bound organelle containing molecules destined to be released from the cell.
segregation In genetics, the process that distributes an equal complement of chromosomes to daughter cells during
mitosis and meiosis.
seven-spanning receptor See G protein coupled receptor (GPCR).
signal sequence A relatively short amino acid sequence that directs a protein to a specific location within the cell; also
called signal peptide and targeting sequence.
signal transduction Conversion of a signal from one physical or chemical form into another. In cell biology commonly
refers to the sequential process initiated by binding of an extracellular signal to a receptor and culminating in one or
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Cell
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Glossary
more specific cellular responses.
signaling molecule General term for any extracellular or intracellular molecule involved in mediating the response of
a cell to its external environment or other cells.
silencer sequence A sequence in eukaryotic DNA that promotes formation of condensed chromatin structures in a
localized region, thereby blocking access of proteins required for transcription of genes within several hundred base
pairs of the silencer sequence.
simple-sequence DNA Short, tandemly repeated sequences that are found in centromeres and telomeres as well as
at other chromosomal locations and are not transcribed.
somatic cell Any plant or animal cell other than a germ cell or germ-cell precursor.
Southern blotting Technique for detecting specific DNA sequences separated by electrophoresis by hybridization to
a labeled nucleic acid probe.
SPF (S phase
promoting factor) A heterodimeric protein, composed of a cyclin and cyclin-dependent kinase
(Cdk), that triggers entrance of a cell into the S phase of the cell cycle by inducing expression of proteins required for
DNA replication and passage through the S phase.
spliceosome Large ribonucleoprotein complex that assembles on a pre-mRNA and carries out RNA splicing.
starch A very long, branched polysaccharide, composed exclusively of glucose units, that is the primary storage
carbohydrate in plant cells.
start A point in the G1 stage of the yeast cell cycle that controls entry of cells into the S phase. Passage of a cell
through start commits a cell to proceed through the remainder of the cell cycle.
stem cell A self-renewing cell that divides to give rise to a cell with an identical developmental potential and/or one
with a more restricted developmental potential.
stereoisomers Two compounds with identical molecular formulas whose atoms are linked in the same order but in
different spatial arrangements. In optical isomers, designated d and l, the atoms bonded to an asymmetric carbon
atom are arranged in a mirror-image fashion. Geometric isomers include the cis and trans forms of molecules
containing a double bond.
steroid A group of four-ring hydrocarbons including cholesterol and related compounds. Many important hormones
(e.g., estrogen and progesterone) are steroids.
substrate Molecule that undergoes a change in a reaction catalyzed by an enzyme.
substrate-level phosphorylation Formation of ATP from ADP and Pi catalyzed by cytosolic enzymes in reactions
that do not depend on a proton-motive force.
sulfhydryl group (
SH) A hydrogen atom covalently bonded to a sulfur atom; also called a thiol group. A substituent
group present in the amino acid cysteine and other molecules.
supercoils, DNA Regions of DNA in which the double helix is twisted on itself.
suppressor mutation A mutation that reverses the phenotypic effect of a second mutation. Suppressor mutations are
frequently used to identify genes encoding interacting proteins.
symport A type of cotransport in which a membrane protein (symporter) transports two different molecules or ions
across a cell membrane in the same direction. See also antiport.
synapse Specialized region between an axon terminus of a neuron and an adjacent neuron or other excitable cell
(e.g., muscle cell) across which impulses are transmitted. At a chemical synapse, the impulse is conducted by a
neurotransmitter; at an electric synapse, impulse transmission occurs via gap junctions connecting the cyto-plasms of
the pre- and postsynaptic cells.
syncytium A multinucleated mass of cytoplasm enclosed by a single plasma membrane.
TATA box A conserved sequence in the promoter of many eukaryotic protein-coding genes where the
transcription-initiation complex assembles
42
Cell
Biology
Glossary
telomere End region of a eukaryotic chromosome containing characteristic telomeric (TEL) sequences that are
replicated by a special process, thereby counteracting the tendency of a chromosome to be shortened during each
round of replication.
telophase Final mitotic stage during which the nuclear-envelope re-forms around the two sets of separated
chromosomes; the chromosomes decondense; and division of the cytoplasm (cytokinesis) is completed.
temperature-sensitive (ts) mutant A cell or organism with a mutant gene encoding an altered protein that functions
normally at one temperature (the permissive temperature) but is nonfunctional at another temperature (the
nonpermissive temperature).
template A molecular "mold" that dictates the structure of another molecule; most commonly, one strand of DNA that
directs synthesis of a complementary DNA strand during DNA replication or of an RNA during transcription.
termination factor One of several proteins that acts to terminate protein synthesis by recognizing a stop codon in
mRNA and causing release of the ribosomal subunits.
tertiary structure In proteins, overall three-dimensional form of a polypeptide chain, which is stabilized by multiple
noncovalent interactions between side chains.
thylakoids Flattened membranous sacs in a chloroplast that are arranged in stacks forming the grana and contain the
photosynthetic pigments.
tight junction Ribbon-like bands connecting adjacent epithelial cells that prevent leakage of fluid across the cell layer.
topoisomerase Class of enzymes that control the number and topology of supercoils in DNA. Type I enzymes cut
one DNA strand, rotate it about the other, and reseal the ends. Type II enzymes cut and reseal both DNA strands.
trans-acting Referring to DNA sequences encoding diffusible proteins (e.g., transcription activators and repressors)
that control genes on the same or different chromosomes. See also cis-acting.
transcript See primary transcript.
transcription Process whereby one strand of a DNA molecule is used as a template for synthesis of a complementary
RNA by RNA polymerase.
transcription factor (TF) General term for any protein, other than RNA polymerase, required to initiate or regulate
transcription in eukaryotic cells. General factors, required for transcription of all genes, participate in formation of the
transcription-initiation complex near the start site. Specific factors stimulate (or repress) transcription of particular
genes by binding to their regulatory sequences.
transcription unit A region in DNA, bounded by an initiation (start) site and termination site, that is transcribed into a
single primary transcript.
transcription-control region Collective term for all the cis-acting DNA regulatory sequences that regulate
transcription of a particular gene.
transfection Experimental introduction of foreign DNA into cells in culture, usually followed by expression of genes in
the introduced DNA.
transfer RNA See tRNA.
transformation Permanent, heritable alteration in a cell resulting from the uptake and incorporation of a foreign DNA.
Also, conversion of a "normal" mammalian cell into a cell with cancer-like properties usually induced by treatment with
a virus or other cancer-causing agent.
transgene A cloned gene that is introduced and stably incorporated into a plant or animal and is passed on to
successive generations.
transgenic Referring to any plant or animal carrying a transgene.
translation The ribosome-mediated production of a polypeptide whose amino acid sequence is specified by the
nucleotide sequence in an mRNA.
translocon Multiprotein complex in the membrane of the rough endoplasmic reticulum through which a nascent
43
Cell
Biology
Glossary
secretory protein enters the ER lumen as it is being synthesized.
transport vesicles Small membrane-bounded organelles that carry secretory and membrane proteins in both
directions between the rough endoplasmic reticulum (ER) and the Golgi complex, and from the Golgi to the cell surface
or other destination. Form by budding off from the donor organelle and release their contents by fusion with the target
membrane.
transposon A relatively long mobile DNA element, in prokaryotes and eukaryotes, that moves in the genome by a
mechanism involving DNA synthesis and transposition. See also retrotransposon.
tricarboxylic acid cycle See citric acid cycle.
tRNA (transfer RNA) A group of small RNA molecules that function as amino acid donors during protein synthesis.
Each tRNA becomes covalently linked to a particular amino acid, forming an aminoacyl-tRNA. (Figure 4-26)
tubulin A family of globular cytoskeletal proteins that polymerize to form microtubules.
tumor A mass of cells, generally derived from a single cell, that is not controlled by normal regulators of cell growth.
tumor-suppressor gene Any gene whose encoded protein directly or indirectly inhibits progression through the cell
cycle and in which a loss-of-function mutation is oncogenic. Inheritance of a single mutant allele of many
tumor-suppressor genes (e.g., RB, APC, and BRCA1) greatly increases the risk for developing certain types of cancer.
ubiquitin A small, highly conserved protein that becomes covalently linked to lysine residues in other intracellular
proteins. Proteins to which a chain of ubiquitin molecules is added usually are degraded in a proteasome.
uncoupler An agent that dissipates the proton-motive force across the inner mitochondrial membrane and thylakoid
membrane of chloroplasts, thereby inhibiting ATP synthesis.
upstream The direction on a DNA opposite to the direction RNA polymerase moves during transcription. By
convention, the +1 position in a gene is the first transcribed base; nucleotides upstream from the +1 position are
designated
1,
2, etc. See also downstream.
upstream activating sequence (UAS) Any protein-binding regulatory sequence in the DNA of yeast and other simple
eukaryotes that is necessary for maximal gene expression; equivalent to an enhancer or promoter-proximal element in
higher eukaryotes.
van der Waals interaction A weak noncovalent attraction due to small, transient asymmetric electron distributions
around atoms (dipoles).
vector In cell biology, an agent that can carry DNA into a cell or organism. See also cloning vector and expression
vector.
ventral Relating to the front of an animal or lower surface of a structure (e.g., wing or leaf).
virion An individual viral particle.
virus A small parasite consisting of nucleic acid (RNA or DNA) enclosed in a protein coat that can replicate only in a
susceptible host cell; widely used in cell biology research.
Vmax Parameter that describes the maximal velocity of an enzyme-catalyzed reaction or other process such as
protein-mediated transport of molecules across a membrane. (Figure 3-26)
Western blotting Technique for detecting specific proteins separated by electrophoresis by use of labeled antibodies.
wild type Normal, nonmutant form of a macromolecule, cell, or organism.
x-ray crystallography Most commonly used technique for determining the three-dimensional structure of
macromolecules (particularly proteins and nucleic acids) by passing x-rays through a crystal of the purified molecules
and analyzing the diffraction pattern of discrete spots that results.
44
Cell
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Glossary
zinc finger Several types of conserved DNA-binding motifs composed of protein domains folded around a zinc ion;
present in several types of eukaryotic transcription factors.
zygote A fertilized egg; diploid cell resulting from fusion of a male and female gamete.
Molecular Biology of the Cell
Bruce Alberts
Alexander Johnson
Julian Lewis
Martin Raff
Keith Roberts
Peter Walter New York:
Garland Publishing; c2002
helix see alpha helix
ABC transporter proteins Large superfamily of membrane transport proteins that use the energy of hydrolysis
of ATP to transfer peptides and a variety of small molecules across membranes.
acetyl Chemical group derived from acetic acid. Acetyl groups are important in metabolism and are added
covalently to some proteins as a posttranslational modification.
acetyl CoA Small water-soluble molecule that carries acetyl groups in cells. It consists of an acetyl group linked
to coenzyme A (CoA) by an easily hydrolyzable thioester bond. (See Figure 2 62.)
acetylcholine receptor Ion channel that opens in response to binding of acetylcholine, thereby converting a
chemical signal into an electrical one. Best understood example of a transmitter-gated channel. Sometimes
called the nicotinic acetylcholine receptor to distinguish it from a muscarinic acetylcholine receptor, which is a
G-protein-linked cell-surface receptor.
acetylcholine Neurotransmitter that functions at a class of chemical synapses known as cholinergic synapses.
Found both in the brain and in the peripheral nervous system. It is the neurotransmitter at vertebrate
neuromuscular junctions. (See Figure 15 9.)
acid Substance that releases protons when dissolved in water, forming a hydronium ion (H 3O+).
acid hydrolase Any of a group of diverse hydrolytic enzymes (including proteases, nucleases, glycosidases,
etc.) that have their optimal activity at acid pH (around 5.0) and are found in lysosomes.
acquired immunological tolerance Unresponsiveness of the immune system to a given foreign antigen that
can develop in some circumstances.
acrosomal vesicle Region at the head end of a sperm cell that contains a sac of hydrolytic enzymes used to
digest the protective coating of the egg.
acrosome reaction Reaction that occurs when a sperm starts to enter an egg, in which the contents of the
acrosomal vesicle are released, helping the sperm to penetrate the zona pellucida.
actin Abundant protein that forms actin filaments in all eucaryotic cells. The monomeric form is sometimes
called globular or G-actin; the polymeric form is filamentous or F-actin.
actin-binding protein Protein that associates with either actin monomers or actin filaments in cells and
-actinin, and profilin.
actin filament (microfilament) Helical protein filament formed by the polymerization of globular actin molecules.
A major constituent of the cytoskeleton of all eucaryotic cells and part of the contractile apparatus of skeletal
muscle. (See Panel 16 1, p. 909.)
action potential Rapid, transient, self-propagating electrical excitation in the plasma membrane of a cell such
as a neuron or muscle cell. Action potentials, or nerve impulses, make possible long-distance signaling in the
nervous system.
activated carrier Small diffusible molecule in cells that stores easily-exchangeable energy in the form of one or
more energy-rich covalent bonds. Examples are ATP and NADPH. Also called a coenzyme.
activation energy Extra energy that must be possessed by atoms or molecules in addition to their ground-state
45
Cell
Biology
Glossary
energy in order to undergo a particular chemical reaction. (See Figure 9 1.)
active site Region of an enzyme surface to which a substrate molecule binds in order to undergo a catalyzed
reaction.
active transport Movement of a molecule across a membrane or other barrier driven by energy other than that
stored in the electrochemical gradient of the transported molecule.
acyl group Functional group derived from a carboxylic acid (
). (R represents an alkyl group, such as
methyl.)
adaptation Adjustment of sensitivity following repeated stimulation. This is the mechanism that allows a neuron,
a photodetector, or a bacterium to react to small changes in stimuli even against a high background level of
stimulation.
adaptin Protein that binds clathrin to the membrane surface in clathrin-coated vesicles.
adaptive immune response Response of the vertebrate immune system to a specific antigen that typically
generates immunological memory.
adaptor protein General term for proteins in intracellular signaling pathways that link different proteins in the
pathway directly together.
adenomatous polyposis coli (APC) Tumor suppressor protein that forms part of a protein complex that
-catenin and degrades it.
adenosine triphosphate see ATP
adenylyl cyclase (adenylate cyclase) Membrane-bound enzyme that catalyzes the formation of cyclic AMP
from ATP. An important component of some intracellular signaling pathways.
adherens junction Cell junction in which the cytoplasmic face of the plasma membrane is attached to actin
filaments. Examples include the adhesion belts linking adjacent epithelial cells and the focal contacts on the
lower surface of cultured fibroblasts.
adhesion belt Beltlike adherens junction that encircles the apical end of an epithelial cell and attaches it to the
adjoining cell. Also known as the zonula adherens.
adhesion plaque see focal adhesion.
adipocyte A fat cell.
ADP (adenosine 5 -diphosphate) Nucleotide that is produced by hydrolysis of the terminal phosphate of ATP.
It regenerates ATP when phosphorylated by an energy-generating process such as oxidative phosphorylation.
(See Figure 2 57.)
adrenaline (epinephrine) Hormone released by chromaffin cells (in the adrenal gland) and by some neurons in
response to stress. Produces "fight or flight" responses, including increased heart rate and blood sugar levels.
aerobic Describes a process that requires, or occurs in the presence of, gaseous oxygen (O 2).
affinity chromatography Type of chromatography in which the protein mixture to be purified is passed over a
matrix to which specific ligands for the required protein are attached, so that the protein is retained on the matrix.
affinity constant (association constant) (Ka) Measure of the strength of binding of the components in a
complex. For components A and B and a binding equilibrium A + B
AB, the association constant is given by
[AB]/[A][B], and is larger the tighter the binding between A and B. (See also dissociation constant.)
affinity maturation Progressive increase in the affinity of antibodies for the immunizing antigen with the
passage of time after immunization.
Akt see protein kinase B
alcohol Polar organic molecule that contains a functional hydroxyl group ( OH) bound to a carbon atom that is
not in an aromatic ring. An example is ethyl alcohol (CH3CH2OH).
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Cell
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Glossary
aldehyde Organic compound that contains the group. An example is glyceraldehyde. Can be oxidized to an
acid or reduced to an alcohol.
alga (algae) Informal term used to describe a wide range of simple unicellular and multicellular eucaryotic
photosynthetic organisms. Examples include Nitella, Volvox, and Fucus.
alkaloid Small but chemically complex nitrogen-containing metabolite produced by plants as a defense against
herbivores. Examples include caffeine, morphine, and colchicine.
alkane (adjective aliphatic) Compound of carbon and hydrogen that has only single covalent bonds. An
example is ethane (CH3CH3).
alkene Hydrocarbon with one or more carbon-carbon double bonds. An example is ethylene (CH2CH2).
alkyl group General term for a group of covalently linked carbon and hydrogen atoms such as methyl ( CH3) or
ethyl ( CH2CH3) groups. These groups can be formed by removing a hydrogen atom from an alkane.
allele One of a set of alternative forms of a gene. In a diploid cell each gene will have two alleles, each
occupying the same position (locus) on homologous chromosomes.
allelic exclusion The expression of an immunoglobulin chain (or T cell receptor chain) gene from only one of
the two homologous loci present for that gene in the lymphocyte.
allosteric protein Protein that changes from one conformation to another when it binds another molecule or
when it is covalently modified. The change in conformation alters the activity of the protein and can form the
basis of directed movement.
Common folding pattern in proteins in which a linear sequence of amino acids folds into a
right-handed helix stabilized by internal hydrogen bonding between backbone atoms.
alternative RNA splicing The production of different proteins from the same RNA transcript by splicing it in
different ways.
amide Molecule containing a carbonyl group linked to an amine.
amine Chemical group containing nitrogen and hydrogen. It becomes positively charged in water.
amino acid Organic molecule containing both an amino group and a carboxyl group. Those that serve as the
building blocks of proteins are alpha amino acids, having both the amino and carboxyl groups linked to the same
carbon atom. (See Panel 3 1, pp. 132 133.)
aminoacyl-tRNA synthetase Enzyme that attaches the correct amino acid to a tRNA molecule to form an
aminoacyl-tRNA. (See Figure 6 57.)
amino group Weakly basic functional group derived from ammonia (NH3) in which one or more hydrogen atoms
are replaced by another atom. In aqueous solution it can accept a proton and carry a positive charge.
amino terminus (N terminus) The end of a polypeptid
-amino group.
aminoacyl tRNA Activated form of amino acid used in protein synthesis. Consists of an amino acid linked
through a labile ester bond from its carboxyl group to a hydroxyl group on tRNA. (See Figure 6 57.)
AMP (adenosine 5 -monophosphate) One of the four nucleotides in an RNA molecule. Two phosphates are
added to AMP to form ATP. (See Panel 2 6, pp. 120 121.)
amphipathic Having both hydrophobic and hydrophilic regions, as in a phospholipid or a detergent molecule.
anabolism System of biosynthetic reactions in a cell by which large molecules are made from smaller ones.
anaerobic Describes a cell, organism, or metabolic process that functions in the absence of air or, more
precisely, in the absence of molecular oxygen (O2).
anaphase Stage of mitosis during which the two sets of chromosomes separate and move away from each
other. Composed of anaphase A (chromosomes move toward the two spindle poles) and anaphase B (spindle
poles move apart).
anaphase-promoting complex (APC) Ubiquitin ligase that promotes the destruction of a set of proteins, some
47
Cell
Biology
Glossary
of which initiate the separation of sister chromatids during the metaphase-to-anaphase transition during mitosis.
anchorage dependence Dependence of cell growth on attachment to a substratum.
anchoring junction Type of cell junction that attaches cells to neighboring cells or to the extracellular matrix.
angiogenesis Growth of new blood vessels by sprouting from existing ones.
Ångstrom (Å) Unit of length used to measure atoms and molecules. Equal to 10
10
meter or 0.1 nanometer
(nm).
animal pole In yolky eggs, that end free of yolk that cleaves more rapidly than the vegetal pole.
ankyrin Protein mainly responsible for attaching the spectrin cytoskeleton to the red blood cell plasma
membrane.
antenna complex Part of a photosystem that captures light energy and channels it into the photochemical
reaction center. It consists of protein complexes that bind large numbers of chlorophyll molecules and other
pigments.
anterior Situated toward the head end of the body.
anteroposterior Describes the axis running from the head to the tail of the animal body.
antibiotic Substance such as penicillin or streptomycin that is toxic to microorganisms. Usually a product of a
particular microorganism or plant.
antibody (immunoglobulin) Protein produced by B cells in response to a foreign molecule or invading
microorganism. Often binds to the foreign molecule or cell extremely tightly, thereby inactivating it or marking it
for destruction by phagocytosis or complement-induced lysis.
anticodon Sequence of three nucleotides in a transfer RNA molecule that is complementary to a
three-nucleotide codon in a messenger RNA molecule.
antigen Molecule that is able to provoke an immune response.
antigenic determinant (epitope) Specific region of an antigenic molecule that binds to an antibody or a T cell
receptor.
antigenic variation The ability to change the antigens displayed on the cell surface; a property of some
pathogenic microorganisms that enables them to evade attack by the immune system.
antigen-presenting cell Cell that displays foreign antigen complexed with MHC molecules on its surface.
antiparallel Describes the relative orientation of the two strands in a DNA double helix; the polarity of one
strand is oriented in the opposite direction to that of the other.
antiporter Carrier protein that transports two different ions or small molecules across a membrane in opposite
directions, either simultaneously or in sequence.
antisense RNA RNA complementary to a specific RNA transcript of a gene that can hybridize to the specific
RNA and block its function.
APC see adenomatous polyposis coli; anaphase-promoting complex
apical Describes the tip of a cell, a structure, or an organ. The apical surface of an epithelial cell is the exposed
free surface, opposite to the basal surface. The basal surface rests on the basal lamina that separates the
epithelium from other tissue.
apoptosis Form of cell death, also known as programmed cell death, in which a 'suicide' program is activated
within the cell, leading to fragmentation of the DNA, shrinkage of the cytoplasm, membrane changes and cell
death without lysis or damage to neighboring cells. It is a normal phenomenon, occurring frequently in a
multicellular organism.
aqueous Pertaining to water, as for example, in an aqueous solution.
archea (singular archeon) Members of one of the two major divisions of procaryotes (the Archea), the other
being the Bacteria.
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Cell
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Glossary
ARF protein Monomeric GTPase responsible for regulating both COPI coat assembly and clathrin coat
assembly at Golgi membranes.
aromatic Describes a molecule that contains carbon atoms in a ring, commonly drawn as linked through
alternating single and double bonds. Often a molecule related to benzene.
ARP complex (ARP2/3 complex) Complex of proteins that nucleates actin filament growth from the minus end.
asexual reproduction Any type of reproduction (such as budding in Hydra, binary fission in bacteria, or mitotic
division in eucaryotic microorganisms) that does not involve gamete formation and fusion. It produces an
individual genetically identical to the parent.
association constant see affinity constant
aster Star-shaped system of microtubules emanating from a centrosome or from a pole of a mitotic spindle.
astral microtubule In the mitotic spindle, any of the microtubules radiating from the aster which are not
attached to a kinetochore of a chromosome.
asymmetric cell division Cell division that produces two daughter cells that differ, for example in size or in the
presence or absence of some cytoplasmic constituent.
atomic weight Mass of an atom relative to the mass of a hydrogen atom. Essentially equal to the number of
protons plus neutrons.
ATP (adenosine 5 -triphosphate) Nucleoside triphosphate composed of adenine, ribose, and three phosphate
groups that is the principal carrier of chemical energy in cells. The terminal phosphate groups are highly reactive
in the sense that their hydrolysis, or transfer to another molecule, takes place with release of a large amount of
free energy. (See Figure 2 26.)
ATP synthase Enzyme complex in the inner membrane of a mitochon-drion and the thylakoid membrane of a
chloroplast that catalyzes the formation of ATP from ADP and inorganic phosphate during oxidative
phosphorylation and photosynthesis, respectively. Also present in the plasma membrane of bacteria.
ATPase Enzyme that catalyzes a process involving the hydrolysis of ATP. A large number of different proteins
have ATPase activity.
autoantibody Antibody produced by an individual against a protein, or other potential antigen, of its own cells
and tissues. Autoantibodies can cause autoimmune disease.
autocatalysis Reaction that is catalyzed by one of its products, creating a positive feedback (self-amplifying)
effect on the reaction rate.
autocrine signaling Type of cell signaling in which a cell secretes signal molecules that act on itself or on other
adjacent cells of the same type.
autoimmune disease A pathological state in which the body mounts an immune response against one or more
of its own potential antigens.
autophagy Digestion of worn-out organelles by the cell's own lysosomes.
autoradiography Technique in which a radioactive object produces an image of itself on a photographic film.
The image is called an autoradiograph or autoradiogram.
autosome Any chromosome other than a sex chromosome.
avidity Total binding strength of a polyvalent antibody with a polyvalent antigen.
Avogadro's number 6 × 1023. This is the number of atoms in 1 gram of hydrogen, and thus in the atomic or
molecular weight equivalent in grams of any element or molecule.
axon Long nerve cell process that is capable of rapidly conducting nerve impulses over long distances so as to
deliver signals to other cells.
axonal transport Directed transport of organelles and molecules along a nerve cell axon. It can be anterograde
(outward from the cell body) or retrograde (back toward the cell body).
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Cell
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Glossary
axoneme Bundle of microtubules and associated proteins that forms the core of a cilium or flagellum in a
eucaryotic cell and is responsible for their movements.
see beta sheet
B cell (B lymphocyte) Type of lymphocyte that makes antibodies.
bacteria (singular bacterium) Members of the Bacteria, one of the two major divisions of procaryotes, the
other being the Archea. Most exist as single cells and some cause disease.
bacterial artificial chromosome (BAC) Cloning vector that can accommodate large pieces of DNA up to 1
million base pairs.
bacteriophage (phage) Any virus that infects bacteria. Bacteriophages were the first entities used for the study
of molecular genetics and are now widely used as cloning vectors.
bacteriorhodopsin Pigmented protein found in the plasma membrane of a salt-loving bacterium,
Halobacterium halobium. It pumps protons out of the cell in response to light.
basal Situated near the base. The basal surface of a cell is opposite the apical surface.
basal body Short cylindrical array of microtubules plus their associated proteins found at the base of a
eucaryotic cell cilium or flagellum. Serves as a nucleation site for the growth of the axoneme. Closely similar in
structure to a centriole.
basal lamina (basal laminae) Thin mat of extracellular matrix that separates epithelial sheets, and many other
types of cells such as muscle or fat cells, from connective tissue.
base A substance that can accept a proton in solution. The purines and pyrimidines in DNA and RNA are
organic nitrogenous bases and are often referred to simply as bases.
base pair Two nucleotides in an RNA or DNA molecule that are held together by hydrogen bonds
for example,
G pairs with C, and A with T or U.
basic Having the properties of a base.
benign Describes tumors that are self-limiting in their growth and noninvasive.
benzene Molecule composed of a six-membered ring of carbon atoms, commonly drawn containing three
alternating double bonds. The benzene ring occurs as part of many biological molecules.
beta-
-catenin) Multifunctional cytoplasmic protein that is involved in cadherin-mediated cell cell
adhesion, linking cadherins to the actin cytoskeleton. Can also act independently as a gene regulatory protein.
Has an important role in animal development as part of a Wnt signaling pathway.
Common structural motif in proteins in which different sections of the polypeptide chain
run alongside each other, joined together by hydrogen bonding between atoms of the polypeptide backbone.
-pleated sheet.
binding site A region on the surface of one molecule (usually a protein or nucleic acid) that can interact with
another molecule through noncovalent bonding.
biosphere The world of living organisms.
biotin Low-molecular-weight compound used as a coenzyme. Useful technically as a covalent label for proteins,
allowing them to be detected by the egg protein avidin, which binds extremely tightly to biotin. (See Figure 2 63.)
bivalent A duplicated chromosome paired with its homologous duplicated chromosome at the beginning of
meiosis.
black membrane Artificial planar lipid bilayer membrane.
blastomere One of the cells formed by the cleavage of a fertilized egg.
blastula Early stage of an animal embryo, usually consisting of a hollow ball of cells, before gastrulation begins.
blotting Biochemical technique in which macromolecules separated on an agarose or polyacrylamide gel are
transferred to a nylon membrane or sheet of paper, thereby immobilizing them for further analysis. (See
50
Cell
Biology
Glossary
Northern blotting, Southern blotting, Western blotting.)
bond energy Strength of the chemical linkage between two atoms, measured by the energy in kilocalories or
kilojoules needed to break it.
bright-field microscope The normal light microscope in which the image is obtained by simple transmission of
light through the object being viewed.
brush border Dense covering of microvilli on the apical surface of epithelial cells in the intestine and kidney.
The microvilli aid absorption by increasing the surface area of the cell.
budding yeast Common name often given to the baker's yeast Saccharomyces cerevisiae, a common
experimental organism, which divides by budding off a smaller cell.
C terminus see carboxyl terminus
Ca2+/calmodulin-dependent protein kinase (CaM kinase) Protein kinase whose activity is regulated by the
binding of Ca2+-activated calmodulin (Ca2+/calmodulin), and which indirectly mediates the effects of Ca 2+ by
phosphorylation of other proteins.
cadherin A member of a family of proteins that mediates Ca2+-dependent cell cell adhesion in animal tissues.
caged molecule Organic molecule designed to change into an active form when irradiated with light of a
specific wavelength. An example is caged ATP.
calcium pump (Ca2+ ATPase) Transport protein in the membrane of the sarcoplasmic reticulum of muscle cells
(and elsewhere) that pumps Ca2+ out of the cytoplasm into the sarcoplasmic reticulum using the energy of ATP
hydrolysis.
calmodulin Ubiquitous calcium-binding protein whose binding to other proteins is governed by changes in
intracellular Ca2+ concentration. Its binding modifies the activity of many target enzymes and membrane
transport proteins.
calorie Unit of heat. One calorie (small "c") is the amount of heat needed to raise the temperature of 1 gram of
water by 1°C. A kilocalorie (1000 calories) is the unit used to describe the energy content of foods.
Calvin cycle (Calvin-Benson cycle) Major metabolic pathway by which CO2 is incorporated into carbohydrate
during the second stage of photosynthesis (carbon fixation) in plants. Also called the carbon-fixation cycle.
CAM see cell adhesion molecule
CaM kinase see Ca2+/calmodulin-dependent protein kinase
CaM-kinase II Multifunctional Ca2+/calmodulin-dependent protein kinase found in all animal cells that
undergoes autophosphorylation when activated. It is especially abundant in brain and is thought to have a role in
learning and memory in vertebrates.
cAMP see cyclic AMP
capacitation Poorly understood process that sperm must go through in the female reproductive tract before
they are competent for fertilization.
capsid Protein coat of a virus, formed by the self-assembly of one or more protein subunits into a geometrically
regular structure.
carbohydrate General term for sugars and related compounds containing carbon, hydrogen, and oxygen,
usually with the empirical formula (CH2O)n.
carbon fixation Process by which green plants incorporate carbon atoms from atmospheric carbon dioxide into
sugars. The second stage of photosynthesis.
carbon-fixation cycle see Calvin cycle
carbonyl group Pair of atoms consisting of a carbon atom linked to an oxygen atom by a double bond (C=O).
carboxyl group Carbon atom linked both to an oxygen atom by a double bond and to a hydroxyl group.
51
Cell
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Molecules containing a carboxyl group are weak acids
Glossary
carboxylic acids (
carboxyl terminus (C terminus)
).
-carbonyl group.
carcinogen Any agent, such as a chemical or a form of radiation, that causes cancer.
carcinogenesis Generation of a cancer.
carcinoma Cancer of epithelial cells. The most common form of human cancer.
cardiac muscle Specialized form of striated muscle found in the heart, consisting of individual heart muscle
cells linked together by cell junctions.
carrier protein Membrane transport protein that binds to a solute and transports it across the membrane by
undergoing a series of conformational changes.
cartilage Form of connective tissue composed of cells (chondrocytes) embedded in a matrix rich in type II
collagen and chondroitin sulfate.
caspase Any of a family of intracellular proteases that are involved in initiating the cellular events of apoptosis.
catabolism General term for the enzyme-catalyzed reactions in a cell by which complex molecules are
degraded to simpler ones with release of energy. Intermediates in these reactions are sometimes called
catabolites.
catalyst Substance that can lower the activation energy of a reaction, thus increasing its rate.
caveola (caveolae) Invaginations at the cell surface that bud off internally to form pinocytic vesicles. Thought to
form from lipid rafts, regions of membrane rich in certain lipids.
CD28 Cell-surface protein on T cells that binds the co-stimulatory B7 protein on "professional"
antigen-presenting cells, providing an additional signal required for the activation of a naïve T cell by antigen.
CD4 Co-receptor protein found on helper T cells that binds to class II MHC molecules outside the
antigen-binding site.
CD8 Co-receptor protein found on cytotoxic T cells that binds to class I MHC molecules outside the
antigen-binding site.
cdc gene see cell-division-cycle gene
Cdk inhibitor protein (CKI) Protein that binds to and inhibits cyclin-Cdk complexes, primarily involved in the
control of G1 and S phases.
Cdk-activating kinase (CAK) Protein kinase that phosphorylates Cdks in cyclin-Cdk complexes, activating the
Cdk.
Cdk see cyclin-dependent kinase
cDNA DNA molecule made as a copy of messenger RNA and therefore lacking the introns that are present in
genomic DNA. cDNA clones represent DNA cloned from cDNA and a collection of such clones, usually
representing the genes expressed in a particular cell type or tissue, is a cDNA library.
cell adhesion molecule (CAM) Protein on the surface of an animal cell that mediates cell cell binding or cell
matrix binding.
cell body Main part of a nerve cell that contains the nucleus. The other parts are the axons and dendrites.
cell coat see glycocalyx
cell cortex Specialized layer of cytoplasm on the inner face of the plasma membrane. In animal cells it is an
actin-rich layer responsible for movements of the cell surface.
cell cycle (cell-division cycle) Reproductive cycle of a cell: the orderly sequence of events by which a cell
duplicates its contents and divides into two.
cell division Separation of a cell into two daughter cells. In eucaryotic cells it entails division of the nucleus
(mitosis) closely followed by division of the cytoplasm (cytokinesis).
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Glossary
cell fate In developmental biology, describes what a particular cell at a given stage of development will normally
give rise to.
cell fusion Process in which the plasma membranes of two cells fuse down at the point of contact between
them, allowing the two cytoplasms to mingle.
cell junction Specialized region of connection between two cells or between a cell and the extracellular matrix.
cell line Population of cells of plant or animal origin capable of dividing indefinitely in culture.
cell plate Flattened membrane-bounded structure that forms by fusing vesicles in the cytoplasm of a dividing
plant cell and is the precursor of the new cell wall.
cell wall Mechanically strong extracellular matrix deposited by a cell outside its plasma membrane. It is
prominent in most plants, bacteria, algae, and fungi. Not present in most animal cells.
cell-cycle control system Network of regulatory proteins that governs progression of a eucaryotic cell through
the cell cycle.
cell-division-cycle gene (cdc gene) Gene that controls a specific step or set of steps in the eucaryotic cell
cycle. Originally identified in yeasts.
cell-free system Fractionated cell homogenate that retains a particular biological function of the intact cell, and
in which biochemical reactions and cell processes can be more easily studied.
cell-mediated immune response That part of an adaptive immune response in which antigen-specific T cells
are activated to perform various functions such as killing infected cells and activating macrophages.
cellularization The formation of cells around each nucleus in a multinucleate cytoplasm, transforming it into a
multicellular structure.
cellulose Structural polysaccharide consisting of long chains of covalently linked glucose units. It provides
tensile strength in plant cell walls.
centimorgan see genetic map distance
central lymphoid organ (primary lymphoid organ) Lymphoid organ in which lymphocytes develop. In adult
mammals these are the thymus and bone marrow.
central nervous system (CNS) Main information-processing organ of the nervous system. In vertebrates it
consists of the brain and spinal cord.
centriole Short cylindrical array of microtubules, closely similar in structure to a basal body. A pair of centrioles
is usually found at the center of a centrosome in animal cells.
centromere Constricted region of a mitotic chromosome that holds sister chromatids together. It is also the site
on the DNA where the kinetochore forms that captures microtubules from the mitotic spindle.
centrosome cycle Duplication of the centrosome (during interphase) and separation of the two new
centrosomes (at the beginning of mitosis), which provides two centrosomes to form the poles of the mitotic
spindle.
centrosome Centrally located organelle of animal cells that is the primary microtubule-organizing center and
acts as the spindle pole during mitosis. In most animal cells it contains a pair of centrioles.
CG island Region of DNA with a greater than average density of CG sequences; these regions generally
remain unmethylated.
CGN see cis Golgi network
channel protein Membrane transport protein that forms an aqueous pore in the membrane through which a
specific solute, usually an ion, can pass.
chaperone (molecular chaperone) Protein that helps other proteins avoid misfolding pathways that produce
inactive or aggregated polypeptides.
checkpoint Point in the eucaryotic cell-division cycle where progress through the cycle can be halted until
53
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Glossary
conditions are suitable for the cell to proceed to the next stage.
chelate Combine reversibly, usually with high affinity, with a metal ion such as iron, calcium, or magnesium.
chemical group Set of covalently linked atoms, such as a hydroxyl group ( OH) or an amino group ( NH2), the
chemical behavior of which is well characterized.
chemiosmotic coupling Mechanism in which a gradient of hydrogen ions (a pH gradient) across a membrane
is used to drive an energy-requiring process, such as ATP production or the rotation of bacterial flagella.
chemokine Small secreted protein that attracts cells, such as white blood cells, to move towards its source.
Important in the functioning of the immune system.
chemotaxis Directed movement of a cell or organism towards or away from a diffusible chemical.
chiasma (chiasmata) X-shaped connection visible between paired homologous chromosomes in division I of
meiosis, and which represents a site of crossing-over.
chlorophyll Light-absorbing green pigment that plays a central part in photosynthesis in bacteria, plants, and
algae.
chloroplast Organelle in green algae and plants that contains chlorophyll and carries out photosynthesis. It is a
specialized form of plastid.
cholesterol Lipid molecule with a characteristic four-ring steroid structure that is an important component of the
plasma membranes of animal cells. (See Figure 10 10.)
chondrocyte (cartilage cell) Connective-tissue cell that secretes the matrix of cartilage.
chromaffin cell Cell that stores adrenaline in secretory vesicles and secretes it in times of stress when
stimulated by the nervous system.
chromatid One copy of a chromosome formed by DNA replication that is still joined at the centromere to the
other copy. The two identical chromatids are called sister chromatids.
chromatin Complex of DNA, histones, and nonhistone proteins found in the nucleus of a eucaryotic cell. The
material of which chromosomes are made.
chromatography Biochemical technique in which a mixture of substances is separated by charge, size, or
some other property by allowing it to partition between a moving phase and a stationary phase. (See affinity
chromatography, DNA affinity chromatography, high-performance liquid chromatography.)
chromosomal crossing-over The exchange of DNA between paired homologous chromosomes in division I of
meiosis. It is a sign of genetic recombination and the crossovers (chiasmata) are visible in the light microscope.
(See Figure 20 10.)
chromosome Structure composed of a very long DNA molecule and associated proteins that carries part (or all)
of the hereditary information of an organism. Especially evident in plant and animal cells undergoing mitosis or
meiosis, where each chromosome becomes condensed into a compact rodlike structure visible under the light
microscope.
chromosome condensation Process by which a chromosome becomes packed up into a more compact
structure prior to M phase of the cell cycle.
cilium (cilia) Hairlike extension of a eucaryotic cell containing a core bundle of microtubules and capable of
performing repeated beating movements. Cilia are found in large numbers on the surface of many cells, and are
responsible for the swimming of many single-celled organisms.
circadian clock Internal cyclical process that produces a particular change in a cell or organism with a period of
around 24 hours, for example the sleep-wakefulness cycle in humans.
cis face Face of a Golgi stack at which material enters the organelle. It is adjacent to the cis Golgi network.
cis Golgi network (CGN) Network of interconnected cisternae and tubules which receives vesicles from the
endoplasmic reticulum and transfers material to the cis face of the Golgi apparatus.
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Glossary
cisterna (cisternae) Flattened membrane-bounded compartment, as found in the endoplasmic reticulum or
Golgi apparatus.
citric acid cycle (tricarboxylic acid (TCA) cycle, Krebs cycle) Central metabolic pathway found in aerobic
organisms. Oxidizes acetyl groups derived from food molecules to CO 2 and H2O. In eucaryotic cells it occurs in
the mitochondria.
CKI see Cdk inhibitor protein
class I MHC molecule One of the two classes of MHC molecule. It is present on almost all cell types and
presents viral peptides on the surface of virus-infected cells, where they are recognized by cytotoxic T cells.
(See Figure 24 49.)
class II MHC molecule One of the two classes of MHC molecule. It is present on "professional"
antigen-presenting cells and presents foreign peptides to helper T cells. (See Figure 24 49.)
class switching The change from making one class of immunoglobulin (for example IgM) to making another
class (for example IgG) that many B cells undergo during the course of an immune response.
classical pathway A pathway for activating the complement system that is initiated by IgG or IgM antibodies
bound to the surface of a microbe.
clathrin Protein that assembles into a polyhedral cage on the cytosolic side of a membrane so as to form a
clathrin-coated pit, which buds off by endocytosis to form an intracellular clathrin-coated vesicle.
clathrin-coated pit Region of plasma membrane of animal cells that is coated with the protein clathrin on its
cytosolic face. Such regions are continually forming and budding off by endocytosis to form intracellular
clathrin-coated vesicles containing extracellular fluid and the materials dissolved in it.
cleavage (1) Physical splitting of a cell into two. (2) Specialized type of cell division seen in many early embryos
whereby a large cell becomes subdivided into many smaller cells without growth.
clonal selection theory Theory that explains how the adaptive immune system can respond to millions of
different antigens in a highly specific way. From a population of lymphocytes with a vast repertoire of randomly
generated antigen specificities, a given foreign antigen activates (selects) only those cells with the
corresponding antigen specificity.
clone Population of cells or organisms formed by repeated (asexual) division from a common cell or organism.
Also used as a verb: "to clone a gene" means to produce many copies of a gene by repeated cycles of
replication.
cloning vector A small DNA molecule, usually derived from a bacteriophage or plasmid, which is used to carry
the fragment of DNA to be cloned into the recipient cell, and which enables the DNA fragment to be replicated.
coated vesicle Small membrane-bounded organelle with a cage of proteins (the coat) on its cytosolic surface. It
is formed by the pinching off of a coated region of membrane (coated pit). Some coats are made of clathrin,
whereas others are made from other proteins.
codon Sequence of three nucleotides in a DNA or messenger RNA molecule that represents the instruction for
incorporation of a specific amino acid into a growing polypeptide chain.
coenzyme Small molecule tightly associated with an enzyme that participates in the reaction that the enzyme
catalyzes, often by forming a covalent bond to the substrate. Examples include biotin, NAD +, and coenzyme A.
coenzyme A Small molecule used in the enzymatic transfer of acyl groups in the cell. (See also acetyl CoA.)
cofactor Inorganic ion or coenzyme that is required for an enzyme's activity.
cohesin, cohesin complex Complex of proteins that holds sister chomatids together along their length before
their separation.
coiled-coil
around each other.
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Glossary
collagen fibril Extracellular structure formed by self-assembly of secreted fibrillar collagen subunits. An
abundant constituent of the extracellular matrix in many animal tissues.
collagen Fibrous protein rich in glycine and proline that is a major component of the extracellular matrix and
connective tissues. Exists in many forms: type I, the most common, is found in skin, tendon, and bone; type II is
found in cartilage; type IV is present in basal laminae.
colony-stimulating factor (CSF) General name for the numerous signal molecules that control the
differentiation of blood cells.
colorectal tumor Common carcinoma of the epithelium lining the colon and rectum.
combinatorial control Describes the control of a step in a cellular process, such as the initiation of DNA
transcription, by a combination of proteins rather than by any individual one.
communicating junction Type of cell junction that allows the passage of chemical or electrical signals from
one cell to another.
compartment Regions in the embryo that are formed exclusively from the descendants of a few founder cells;
there is no cell movement beween compartments once delimited.
complement system System of serum proteins activated by antibody antigen complexes or by
microorganisms. Helps eliminate pathogenic microorganisms by directly causing their lysis or by promoting their
phagocytosis.
complementary DNA see cDNA
complementary Two nucleic acid sequences are said to be complementary if they can form a perfect
base-paired double helix with each other.
complex oligosaccharide Chain of sugars attached to a glycoprotein that is generated by trimming of the
original oligosaccharide attached in the endoplasmic reticulum and subsequent addition of further sugars. (See
Figure 13 25.)
complex Assembly of molecules that are held together by noncovalent bonds. Protein complexes perform most
cell functions.
condensation reaction Chemical reaction in which two molecules are covalently linked through
OH groups
with the removal of a molecule of water.
condensin, condensin complex Complex of proteins involved in chromosome condensation prior to mitosis.
Target for the M-Cdk.
conditional mutation A mutation that changes a protein or RNA molecule so that its function is altered only
under some conditions, such as at an unusually high or an unusually low temperature.
confocal microscope Type of light microscope that produces a clear image of a given plane within a solid
object. It uses a laser beam as a pinpoint source of illumination and scans across the plane to produce a
two-dimensional 'optical section.'
conformation The spatial arrangement of atoms in three dimensions in a macromolecule such as a protein or
nucleic acid.
connective tissue Any supporting tissue that lies between other tissues and consists of cells embedded in a
relatively large amount of extracellular matrix. Includes bone, cartilage, and loose connective tissue.
connective-tissue cell Any of the various cell types found in connective tissue, e.g. fibroblasts, cartilage cells
(chondrocytes), bone cells (osteoblasts and osteocytes), fat cells (adipocytes) and smooth muscle cells.
connexon Water-filled pore in the plasma membrane formed by a ring of six protein subunits. Part of a gap
junction: connexons from two adjoining cells join to form a continuous channel between the two cells.
consensus sequence Average or most typical form of a sequence that is reproduced with minor variations in a
group of related DNA, RNA, or protein sequences. The consensus sequence shows the nucleotide or amino
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Cell
Biology
Glossary
acid most often found at each position. The preservation of a consensus implies that the sequence is
functionally important. (See Figure 6 12.)
constitutive secretory pathway Pathway present in all cells by which molecules such as plasma membrane
proteins are continually delivered to the plasma membrane from the Golgi apparatus in vesicles that fuse with
the plasma membrane. (See also default pathway.)
constitutive Produced in constant amount; opposite of regulated.
contact-dependent signaling Cell cell communication in which the signal molecule remains bound to the
signaling cell and only influences cells that physically contact it.
contractile ring Ring containing actin and myosin that forms under the surface of animal cells undergoing cell
division and contracts to pinch the two daughter cells apart.
convergent extension Cellular rearrangement within a tissue that causes it to extend in one dimension (e.g.
length) and shrink in another (e.g. width).
cooperativity Phenomenon in which the binding of one ligand molecule to a target molecule promotes the
binding of successive ligand molecules. Seen in the assembly of large complexes, as well as in enzymes and
receptors composed of multiple allosteric subunits, where it sharpens the response to a ligand..
cortical granule Specialized secretory vesicle present under the plasma membrane of unfertilized eggs,
including those of mammals; after fertilization it is involved in preventing the entry of further sperm.
co-translational Describes import of a protein into the endoplasmic reticulum before the polypeptide chain is
completely synthesized.
co-transport (coupled transport) Membrane transport process in which the transfer of one molecule depends
on the simultaneous or sequential transfer of a second molecule.
coupled reaction Linked pair of chemical reactions in which the free energy released by one of the reactions
serves to drive the other.
covalent bond Stable chemical link between two atoms produced by sharing one or more pairs of electrons.
crista (cristae) (1) (2) One of the folds of the inner mitochondrial membrane. (3) A sensory structure in the inner
ear.
critical concentration Concentration of a protein monomer, such as actin or tubulin, that is in equilibrium with
the assembled form of the protein (i.e. assembled into actin filaments or microtubules respectively).
crossing-over see chromosomal crossing-over
cryoelectron microscopy Electron microscopy technique in which the objects to be viewed, such as
macromolecules and viruses, are rapidly frozen.
cryptochrome Flavoprotein responsive to blue light, found in both plants and animals. In animals it is involved
in circadian rhythms.
cut-and-paste transposition Type of movement of a transposable element in which it is cut out of the DNA and
inserted into a new site by a special transposase enzyme.
cyclic AMP (cAMP) Nucleotide that is generated from ATP by adenylyl cyclase in response to stimulation of
many types of cell-surface receptors. cAMP acts as an intracellular signaling molecule by activating
cyclic-AMP-dependent kinase (protein kinase A, PKA). It is hydrolyzed to AMP by a phosphodiesterase.
cyclic AMP-dependent protein kinase (protein kinase A, PKA) Enzyme that phosphorylates target proteins
in response to a rise in intracellular cyclic AMP.
cyclic GMP Small soluble intracellular signaling molecule formed from GTP by the enzyme guanylyl cyclase in
response to photoreceptor stimulation in the retina.
cyclin Protein that periodically rises and falls in concentration in step with the eucaryotic cell cycle. Cyclins
activate crucial protein kinases (called a cyclin-dependent protein kinase, or Cdk) and thereby help control
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Glossary
progression from one stage of the cell cycle to the next.
cyclin-Cdk complex Protein complexes that are formed periodically during the eucaryotic cell cycle as the level
of cyclin increases, and in which the cyclin-dependent kinase (Cdk) becomes partially activated.
cyclin-dependent kinase (Cdk) Protein kinase that has to be complexed with a cyclin protein in order to act.
Different Cdk-cyclin complexes trigger different steps in the cell-division cycle by phosphorylating specific target
proteins.
cytochrome b-c1 complex Second of the three electron-driven proton pumps in the respiratory chain. It
accepts electrons from ubiquinone.
cytochrome oxidase complex Third of the three electron-driven proton pumps in the respiratory chain. It
accepts electrons from cytochrome c and generates water using molecular oxygen as an electron acceptor.
cytochrome Colored, heme-containing protein that transfers electrons during cellular respiration and
photosynthesis.
cytokine Extracellular signal protein or peptide that acts as a local mediator in cell cell communication.
cytokine receptor Type of cell-surface receptor whose ligands are cytokines such as interferons, growth
hormone and prolactin, and which acts through the Jak-STAT pathway.
cytokinesis Division of the cytoplasm of a plant or animal cell into two, as distinct from the division of its nucleus
(which is mitosis)
cytoplasm Contents of a cell that are contained within its plasma membrane but, in the case of eucaryotic cells,
outside the nucleus.
cytoskeleton System of protein filaments in the cytoplasm of a eucaryotic cell that gives the cell shape and the
capacity for directed movement. Its most abundant components are actin filaments, microtubules, and
intermediate filaments.
cytosol Contents of the main compartment of the cytoplasm, excluding membrane-bounded organelles such as
endoplasmic reticulum and mitochondria. Originally defined operationally as the cell fraction remaining after
membranes, cytoskeletal components, and other organelles have been removed by low-speed centrifugation.
cytotoxic T cell Type of T cell responsible for killing infected cells.
G° see standard free-energy change
G see free-energy change
dalton Unit of molecular mass. Approximately equal to the mass of a hydrogen atom (1.66 × 10
24
g).
default pathway Constitutive secretory pathway that automatically delivers material from the Golgi apparatus to
the plasma membrane if no other sorting signals are present.
degenerate Not a moral judgment but an adjective that describes multiple states that amount to the same thing:
different triplet combinations of nucleotide bases (codons) that code for the same amino acid, for example.
deletion Type of mutation in which a single nucleotide or sequence of nucleotides has been removed from the
DNA.
denaturation Dramatic change in conformation of a protein or nucleic acid caused by heating or by exposure to
chemicals and usually resulting in the loss of biological function.
dendrite Extension of a nerve cell, typically branched and relatively short, that receives stimuli from other nerve
cells.
dendritic cell Cell derived from bone marrow and present in lymphoid and other tissues that is specialized for
the uptake of particulate material by phagocytosis and which acts as a "professional" antigen-presenting cell in
immune responses.
deoxyribonucleic acid see DNA
desensitization see adaptation
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Glossary
desmosome Type of anchoring cell cell junction, usually formed between two epithelial cells, characterized by
dense plaques of protein into which intermediate filaments in the two adjoining cells insert.
detergent Type of small amphipathic molecule that tends to coalesce in water, with its hydrophobic tails buried
and its hydrophilic heads exposed. It is widely used to solubilize membrane proteins.
determined In developmental biology, an embryonic cell is said to be determined if it has become committed to
a particular specialized path of development. This determination reflects a change in the internal character of
the cell, and it precedes the much more readily detected process of cell differentiation.
development Succession of changes that take place in an organism as a fertilized egg gives rise to an adult
plant or animal.
diacylglycerol Lipid produced by the cleavage of inositol phospholipids in response to extracellular signals.
Composed of two fatty acid chains linked to glycerol, it serves as a signaling molecule to help activate protein
kinase C.
dideoxy method The standard method of DNA sequencing.
differentiation Process by which a cell undergoes a change to an overtly specialized cell type.
diffraction pattern Pattern set up by wave interference between radiation transmitted or scattered by different
parts of an object.
diffusion Net drift of molecules in the direction of lower concentration due to random thermal movement.
diploid Containing two sets of homologous chromosomes and hence two copies of each gene or genetic locus.
diplotene Fourth stage of division I of meiosis, in which chiasmata are first seen.
disaccharide Carbohydrate molecule consisting of two covalently joined monosaccharide units.
dissociation constant (Kd) Measure of the tendency of a complex to dissociate. For components A and B and
the binding equilibrium A + B
AB, the dissociation constant is given by [A][B]/[AB], and it is smaller the tighter
the binding between A and B. (See also association constant.)
disulfide bond ( S S ) Covalent linkage formed between two sulfhydryl groups on cysteines. For
extracellular proteins, a common way of joining two proteins together or linking different parts of the same
protein. Formed in the endoplasmic reticulum of eucaryotic cells.
division I of meiosis The first cell division of meiosis, in which the members of each pair of (duplicated)
homologous chromosomes are segregated to opposite poles of the dividing cell.
division II of meiosis The second cell division of meiosis, in which the chromatids of each duplicated
chromosome are segregated to opposite poles of the dividing cell.
DNA (deoxyribonucleic acid) Polynucleotide formed from covalently linked deoxyribonucleotide units. It
serves as the store of hereditary information within a cell and the carrier of this information from generation to
generation.
DNA affinity chromatography Technique for purifying sequence-specific DNA-binding proteins by their binding
to a matrix to which the appropriate DNA fragments are attached.
DNA footprinting Technique for determining the DNA sequence to which a DNA-binding protein binds.
DNA helicase Enzyme that is involved in opening the DNA helix into its single strands for DNA replication.
DNA library Collection of cloned DNA molecules, representing either an entire genome (genomic library) or
DNA copies of the messenger RNA produced by a cell (cDNA library).
DNA ligase Enzyme that joins the ends of two strands of DNA together with a covalent bond to make a
continuous DNA strand.
DNA methylation Addition of a methyl group to DNA. Extensive methylation of the cytosine base in CG
sequences is used in vertebrates to keep genes in an inactive state.
DNA microarray Technique for analyzing the simultaneous expression of large numbers of genes in cells, in
59
Cell
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Glossary
which isolated cellular RNA is hybridized to a large array of short DNA probes immobilized on glass slides.
DNA polymerase Enzyme that synthesizes DNA by joining nucleotides together using a DNA template as a
guide.
DNA primase Enzyme that synthesizes a short strand of RNA on a DNA template, producing a primer for DNA
synthesis.
DNA repair Collective name for those biochemical processes that correct accidental changes in the DNA.
DNA sequencing Determination of the order of nucleotides in a DNA molecule. (See Figure 8 36.)
DNA supercoiling Additional twisting of the DNA helix that occurs in response to the superhelical tension
created when, for example, a circular DNA is partly unwound (See Figure 6 20.)
DNA topoisomerase Enzyme that binds to DNA and reversibly breaks a phosphodiester bond in one or both
strands, allowing the DNA to rotate at that point. It prevents DNA tangling during replication.
DNA transcription see transcription
DNA tumor virus A general term for a variety of different DNA viruses that can cause tumors.
DNA-only transposon Type of transposable element that exists as DNA throughout its life cycle. Many types
move by cut-and-paste transposition.
domain see protein domain
dominant negative mutation Mutation that dominantly affects the phenotype by means of a defective protein
or RNA molecule that interferes with the function of the normal gene product in the same cell.
dominant In genetics, refers to the member of a pair of alleles that is expressed in the phenotype of the
organism while the other allele is not, even though both alleles are present. Opposite of recessive.
dorsal Relating to the back of an animal. Also the upper surface of a leaf, wing, etc.
dorsoventral Describes the axis running from the back to the belly of an animal or from the upper side to the
underside of a structure.
double helix The three-dimensional structure of DNA, in which two DNA chains held together by hydrogen
bonding between the bases are wound into a helix.
Drosophila melanogaster Species of small fly, commonly called a fruit fly, much used in genetic studies of
development.
dynamic instability The property of sudden conversion from growth to shrinkage, and vice versa, in a protein
filament such as a microtubule or actin filament. (See Panel 16 2, pp. 912 913.)
dynamin Cytosolic GTPase that binds to the neck of a clathrin-coated vesicle in the process of budding from the
membrane, and which is involved in completing vesicle formation.
dynein Member of a family of large motor proteins that undergo ATP-dependent movement along microtubules.
In cilia, dynein forms the side arms in the axoneme that cause adjacent microtubule doublets to slide past one
another.
dysplasia A change in cell growth and behavior in a tissue in which the structure becomes disordered.
ectoderm Embryonic tissue that is the precursor of the epidermis and nervous system.
effector cell A cell that carries out the final response or function of a particular process. The main effector cells
of the immune system, for example, are activated lymphocytes and phagocytes
the cells involved in
destroying pathogens and removing them from the body.
egg The mature female gamete in sexually reproducing organisms. It is usually a large and immobile cell.
elastin Hydrophobic protein that forms extracellular extensible fibres (elastic fibres) that give tissues their
stretchability and resilience.
electrochemical gradient The combined influence of a difference in the concentration of an ion on the two
sides of the membrane and the electrical charge difference across the membrane (membrane potential). It
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Cell
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Glossary
produces a driving force that causes the ion to move across the membrane.
electrochemical proton gradient The result of a combined pH gradient (proton gradient) and the membrane
potential.
electron Negatively charged subatomic particle that generally occupies orbitals surrounding the nucleus in an
atom.
electron acceptor Atom or molecule that takes up electrons readily, thereby gaining an electron and becoming
reduced.
electron carrier Molecule such as cytochrome c, which transfers an electron from a donor molecule to an
acceptor molecule.
electron donor Molecule that easily gives up an electron, becoming oxidized in the process.
electron microscope Type of microscope that uses a beam of electrons to create the image.
electron-transport chain Series of electron carrier molecules along which electrons move from a higher to a
lower energy level to a final acceptor molecule. The energy released during electron movement can be used to
power various processes. Electron-transport chains present in the inner mitochondrial membrane and in the
thylakoid membrane of chloroplasts generate a proton gradient across the membrane that is used to drive ATP
synthesis.
elongation factor Protein required for the addition of amino acids to growing polypeptide chains on ribosomes.
embryogenesis Development of an embryo from a fertilized egg, or zygote.
embryonic stem cell (ES cell) Cell derived from the inner cell mass of the early mammalian embryo that can
give rise to all the cells in the body. It can be grown in culture, genetically modified and inserted into a blastocyst
to develop a transgenic animal.
endocrine cell Specialized animal cell that secretes a hormone into the blood. Usually part of a gland, such as
the thyroid or pituitary gland.
endocytic-exocytic cycle The processes of endocytosis and exocytosis that, respectively, add and remove
plasma membrane from the cell, resulting in no overall change in the cell's surface area and volume.
endocytosis Uptake of material into a cell by an invagination of the plasma membrane and its internalization in
a membrane-bounded vesicle. (See also pinocytosis and phagocytosis.)
endoderm Embryonic tissue that is the precursor of the gut and associated organs.
endoplasmic reticulum (ER) Labyrinthine membrane-bounded compartment in the cytoplasm of eucaryotic
cells, where lipids are synthesized and membrane-bound proteins and secretory proteins are made.
endosome Membrane-bounded organelle in animal cells that carries materials newly ingested by endocytosis
and passes many of them on to lysosomes for degradation.
endothelial cell Flattened cell type that forms a sheet (the endothelium) lining all blood vessels.
enhancer Regulatory DNA sequence to which gene regulatory proteins bind, influencing the rate of transcription
of a structural gene that can be many thousands of base pairs away.
entropy Thermodynamic quantity that measures the degree of disorder in a system; the higher the entropy, the
greater the disorder.
enveloped virus Virus with a capsid surrounded by a lipid membrane (the envelope), which is derived from the
host cell plasma membrane when the virus buds from the cell.
enzyme Protein that catalyzes a specific chemical reaction.
enzyme-linked receptor Major type of cell-surface receptor in which the cytoplasmic domain either has
enzymatic activity itself or is associated with an intracellular enzyme. In both cases enzymatic activity is
stimulated by ligand binding to the receptor.
epidermis Epithelial layer covering the outer surface of the body. It has different structures in different animal
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Glossary
groups. The outer layer of plant tissue is also called the epidermis.
epimerization Reaction that alters the steric arrangement around one atom, as in a sugar molecule.
epinephrine see adrenaline
epithelial tissue see epithelium
epithelium (epithelia) Coherent cell sheet formed from one or more layers of cells covering an external surface
or lining a cavity.
epitope see antigenic determinant
equilibrium constant (K) Ratio of forward and reverse rate constants for a reaction and equal to the
association constant.
equilibrium State where there is no net change in a system. For example, equilibrium is reached in a chemical
reaction when the forward and reverse rates are equal.
ER lumen The space enclosed by the membrane of the endoplasmic reticulum (ER).
ER resident protein Protein that remains in the endoplasmic reticulum (ER) or its membranes and carries out
its function there, as opposed to proteins that are present in the ER only in transit.
ER retention signal Short amino acid sequence on a protein that prevents it moving out of the endoplasmic
reticulum (ER). Found on proteins that are resident in the ER and function there.
ER signal sequence N-terminal signal sequence that directs proteins to enter the endoplasmic reticulum (ER).
It is cleaved off by signal peptidase after entry.
ER see endoplasmic reticulum
erythrocyte (red blood cell) Small, hemoglobin-containing blood cell of vertebrates that transports oxygen and
carbon dioxide to and from tissues.
erythropoietin Growth factor that stimulates the production of red blood cells. It is produced by the kidney and
acts on precursor cells in bone marrow.
ES cell see embryonic stem cell
Escherichia coli (E. coli) Rodlike bacterium normally found in the colon of humans and other mammals and
widely used in biomedical research.
ester Molecule formed by the condensation reaction of an alcohol group with an acidic group. Phosphate
groups usually form esters when linked to a second molecule. (See Panel 2 1, 110 111.)
ethyl ( CH2CH3) Hydrophobic chemical group derived from ethane (CH 3CH3).
eucaryote (eukaryote) Organism composed of one or more cells with a distinct nucleus and cytoplasm.
Includes all forms of life except viruses and procaryotes (bacteria and archea).
euchromatin Region of an interphase chromosome that stains diffusely; "normal" chromatin, as opposed to the
more condensed heterochromatin.
exocytosis Process by which most molecules are secreted from a eucaryotic cell. These molecules are
packaged in membrane-bounded vesicles that fuse with the plasma membrane, releasing their contents to the
outside.
exon Segment of a eucaryotic gene that consists of a sequence of nucleotides that will be represented in
messenger RNA or the final transfer RNA or ribosomal RNA. In protein-coding genes, exons encode amino
acids in the protein. An exon is usually adjacent to a noncoding DNA segment called an intron.
expression vector A virus or plasmid that carries a DNA sequence into a suitable host cell and there directs the
synthesis of the protein encoded by the sequence.
expression Production of an observable phenotype by a gene
usually by directing the synthesis of a protein.
extracellular matrix Complex network of polysaccharides (such as glycosaminoglycans or cellulose) and
proteins (such as collagen) secreted by cells. Serves as a structural element in tissues and also influences their
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development and physiology.
facilitated diffusion see passive transport
FADH2 (reduced flavin adenine dinucleotide) Activated carrier molecule that is produced by the citric acid
cycle.
FAK see focal adhesion kinase
Fas protein (Fas) Membrane-bound receptor that initiates apoptosis in the receptor-bearing cell after binding to
its ligand (Fas ligand).
fat Energy-storage lipid in cells. It is composed of triglycerides
fatty acids esterified with glycerol.
fat cell Connective-tissue cell that produces and stores fat in animals.
fatty acid Compound such as palmitic acid that has a carboxylic acid attached to a long hydrocarbon chain.
Used as a major source of energy during metabolism and as a starting point for the synthesis of phospholipids.
Fc receptorOne of a family of receptors specific for the invariant constant region (Fc region) of immunoglobulins
(other than IgM and IgD); different Fc receptors are specific for IgG, IgA, IgE and their subclasses.
feedback inhibition Type of regulation of metabolism in which an enzyme acting early in a reaction pathway is
inhibited by a late product of that pathway.
fermentation Anaerobic energy-yielding metabolic pathway in which pyruvate produced by glycolysis is
converted, for example, into lactate or ethanol, with the conversion of NADH to NAD +.
fertilization Fusion of a male and a female gamete (both haploid) to form a diploid zygote, which develops into
a new individual.
fibrillar collagen Type of collagen molecule which assembles into rope-like structures. Collagens type I
(common in skin), II, III, V and XI are of this type.
fibroblast Common cell type found in connective tissue. Secretes an extracellular matrix rich in collagen and
other extracellular matrix macromolecules. Migrates and proliferates readily in wounded tissue and in tissue
culture.
fibronectin Extracellular matrix protein that is involved in adhesion of cells to the matrix and the guidance of
migrating cells during embryogenesis. Integrins on the cell surface are receptors for fibronectin.
filopodium (filopodia) Thin, spike-like protrusion with an actin filament core, generated on the leading edge of
a crawling animal cell.
fission yeast Common name often given to the yeast Schizosaccharomyces pombe, a common experimental
organism. It divides to give two equal-sized cells.
fixative Chemical reagent such as formaldehyde or osmium tetroxide used to preserve cells for microscropy.
Samples treated with these reagents are said to be "fixed," and the process is called fixation.
flagellum (flagella) Long, whiplike protrusion whose undulations drive a cell through a fluid medium. Eucaryotic
flagella are longer versions of cilia. Bacterial flagella are smaller and completely different in construction and
mechanism of action.
fluid-phase endocytosis Type of endocytosis in which small vesicles bud off internally from the plasma
membrane, carrying extracellular fluid and dissolved material into the cell. (See also pinocytosis.)
fluorescein Fluorescent dye that fluoresces green when illuminated with blue light or ultraviolet light.
fluorescence microscope Microscope designed to view material stained with fluorescent dyes. Similar to a
light microscope but the illuminating light is passed through one set of filters before the specimen, to select
those wavelengths that excite the dye, and through another set of filters before it reaches the eye, to select only
those wavelengths emitted when the dye fluoresces.
fluorescent dye Molecule that absorbs light at one wavelength and responds by emitting light at another
wavelength. The emitted light is of longer wavelength (and hence of lower energy) than the light absorbed.
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fluorescent resonance energy transfer (FRET) Technique for monitoring the closeness of two fluorescently
labeled molecules (and thus their interaction) in cells.
focal adhesion kinase (FAK) Cytoplasmic tyrosine kinase present at cell-matrix junctions (focal adhesions) in
association with the cytoplasmic tails of integrins.
focal adhesion, focal contact (adhesion plaque) A type of anchoring cell junction, forming a small region on
the surface of a fibroblast or other cell that is anchored to the extracellular matrix. Attachment is mediated by
transmembrane proteins such as integrins, which are linked, through other proteins, to actin filaments in the
cytoplasm.
follicle cell One of the cell types that surround a developing oocyte or egg.
free energy (G) The energy that can be extracted from a system to drive reactions. Takes into account changes
in both energy and entropy.
free ribosome Ribosome that is free in the cytosol, unattached to any membrane. It is the site of synthesis of all
proteins encoded by the nuclear genome other than those destined to enter the endoplasmic reticulum.
free-
G) Change in the free energy during a reaction: the free energy of the product
G indicates that the
reaction has a strong tendency to occur. (See Panel 2 7, pp. 122 123.)
freeze-fracture electron microscopy Technique for studying membrane structure, in which the membrane of a
frozen cell is fractured along the interior of the bilayer, separating it into the two monolayers with the interior
faces exposed.
FRET see fluorescent resonance energy transfer
fungus (fungi) Kingdom of eucaryotic organisms that includes the yeasts, molds, and mushrooms. Many plant
diseases and a relatively small number of animal diseases are caused by fungi.
-
Protein complex contai
-tubulin and other proteins that is an efficient
nucleator of microtubules.
G see free energy
G0 G-"zero" phase. State of withdrawal from the eucaroytic cell-division cycle by entry into a quiescent G1 phase.
A common state for differentiated cells.
G1 phase Gap 1 phase of the eucaryotic cell-division cycle, between the end of cytokinesis and the start of DNA
synthesis.
G1/S-Cdk Complex formed in vertebrate cells by a G1/S-cyclin and the corresponding cyclin-dependent kinase
(Cdk).
G1-Cdk Complex formed in vertebrate cells by a G1-cyclin and the corresponding cyclin-dependent kinase (Cdk).
G2 phase Gap 2 phase of the eucaryotic cell-division cycle, between the end of DNA synthesis and the
beginning of mitosis.
GAG see glycosaminoglycan
gamete Specialized haploid cell, either a sperm or an egg, serving for sexual reproduction.
ganglion (ganglia) Cluster of nerve cells and associated glial cells located outside the central nervous system.
ganglioside Any glycolipid having one or more sialic acid residues in its structure. Found in the plasma
membrane of eucaryotic cells and especially abundant in nerve cells.
gap junction Communicating cell cell junction that allows ions and small molecules to pass from the cytoplasm
of one cell to the cytoplasm of the next.
gastrulation The stage in animal embryogenesis during which the embryo is transformed from a ball of cells to
a structure with a gut (a gastrula).
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gene activator protein A gene regulatory protein that when bound to its regulatory sequence in DNA activates
transcription.
gene control region DNA sequences required to initiate transcription of a given gene and control the rate of
initiation.
gene conversion Process by which DNA sequence information can be transferred from one DNA helix (which
remains unchanged) to another DNA helix whose sequence is altered. It occurs occasionally during general
recombination.
gene regulatory protein General name for any protein that binds to a specific DNA sequence to alter the
expression of a gene.
gene repressor protein A gene regulatory protein that prevents the initiation of transcription.
gene Region of DNA that controls a discrete hereditary characteristic, usually corresponding to a single protein
or RNA. This definition includes the entire functional unit, encompassing coding DNA sequences, noncoding
regulatory DNA sequences, and introns.
general recombination, general genetic recombination Recombination that takes place between two
homologous chromosomes (as in meiosis).
general transcription factor Any of the proteins whose assembly around the TATA box is required for the
initiation of transcription of most eucaryotic genes.
genetic code Set of rules specifying the correspondence between nucleotide triplets (codons) in DNA or RNA
and amino acids in proteins.
genetic map Map of the chromosomes in which the distance of genes relative to each other is determined by
the amount of genetic recombination that occurs between them.
genetic recombination see recombination
genetic screen A search through a large collection of mutants for a mutant with a particular phenotype.
genome The totality of genetic information belonging to a cell or an organism; in particular, the DNA that carries
this information.
genomic DNA DNA constituting the genome of a cell or an organism. Often used in contrast to cDNA (DNA
prepared by reverse transcription from messenger RNA). Genomic DNA clones represent DNA cloned directly
from chromosomal DNA, and a collection of such clones from a given genome is a genomic DNA library.
genomic imprinting Situation where a gene is either expressed or not expressed in the embryo depending on
which parent it is inherited from.
genomics The science of studying the DNA sequences and properties of entire genomes.
genotype Genetic constitution of an individual cell or organism.
germ cell Precursor cell that will give rise to gametes.
germ line The lineage of germ cells (which contribute to the formation of a new generation of organisms), as
distinct from somatic cells (which form the body and leave no descendants).
GFP see green fluorescent protein
giga- Prefix denoting 109. (From Greek gigas, giant.)
Gi see inhibitory G protein
glial cell Supporting cell of the nervous system, including oligodendrocytes and astrocytes in the vertebrate
central nervous system and Schwann cells in the peripheral nervous system.
globular protein Any protein with an approximately rounded shape. Such proteins are contrasted with highly
elongated, fibrous proteins such as collagen.
glucose Six-carbon sugar that plays a major role in the metabolism of living cells. Stored in polymeric form as
glycogen in animal cells and as starch in plant cells. (See Panel 2 4, pp. 116 117.)
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Glossary
glutaraldehyde Small reactive molecule with two aldehyde groups that is often used as a cross-linking fixative.
glycerol Small organic molecule that is the parent compound of many small molecules in the cell, including
phospholipids.
glycocalyx (cell coat) Carbohydrate-rich layer that forms the outer coat of a eucaryotic cell. Composed of the
oligosaccharides linked to intrinsic plasma membrane glycoproteins and glycolipids, as well as glycoproteins
and proteoglycans that have been secreted and reabsorbed onto the cell surface.
glycogen Polysaccharide composed exclusively of glucose units used to store energy in animal cells. Large
granules of glycogen are especially abundant in liver and muscle cells.
glycolipid Membrane lipid molecule with a sugar residue or oligosaccharide attached to the polar headgroup.
(See Panel 2 5, pp. 118 119.)
glycolysis Ubiquitous metabolic pathway in the cytosol in which sugars are incompletely degraded with
production of ATP. (Literally, "sugar splitting.")
glycoprotein Any protein with one or more oligosaccharide chains covalently linked to amino-acid side chains.
Most secreted proteins and most proteins exposed on the outer surface of the plasma membrane are
glycoproteins.
glycosaminoglycan (GAG) Long, linear, highly charged polysaccharide composed of a repeating pair of
sugars, one of which is always an amino sugar. Mainly found covalently linked to a protein core in extracellular
matrix proteoglycans. Examples include chondroitin sulfate, hyaluronic acid, and heparin.
glycosylation The process of adding one or more sugars to a protein or lipid molecule. (See also O-linked
glycosylation, N-linked glycosylation.)
glycosylphosphatidylinositol anchor (GPI anchor) Type of lipid linkage by which some membrane proteins
are bound to the membrane. It is formed as the proteins travel through the endoplasmic reticulum.
Golgi apparatus (Golgi complex) Membrane-bounded organelle in eucaryotic cells in which proteins and lipids
transferred from the endoplasmic reticulum are modified and sorted. It is the site of synthesis of many cell wall
polysaccharides in plants and extracellular matrix glycosaminoglycans in animal cells.
GPI anchor see glycosylphosphatidylinositol anchor
G-protein see GTP-binding protein
G-protein-linked receptor Cell-surface receptor that associates with an intracellular trimeric GTP-binding
protein (G protein) after receptor activation by an extracellular ligand. These receptors are seven-pass
transmembrane proteins.
Gq Class of receptor-coupled G protein that activates phospholipase Cphospholipid signaling pathway.
grana (singular granum) Stacked membrane discs (thylakoids) in chloroplasts that contain chlorophyll and are
the site of the light-trapping reactions of photosynthesis.
granulocyte Category of white blood cell distinguished by conspicuous cytoplasmic granules. Includes
neutrophils, basophils, and eosinophils.
gray crescent Band of pale pigmentation that appears in the egg of some species of amphibian opposite the
site of sperm entry following fertilization. Caused by rotation of the egg cortex and associated pigment granules.
Marks the future dorsal side.
green fluorescent protein (GFP) Fluorescent protein isolated from a jellyfish. Widely used as a marker in cell
biology.
growth cone Migrating motile tip of a growing nerve cell axon or dendrite.
growth factor Extracellular polypeptide signal molecule that can stimulate a cell to grow or proliferate.
Examples are epidermal growth factor (EGF) and platelet-derived growth factor (PDGF). Most growth factors
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Glossary
also have other actions.
growth regulator see plant growth regulator
Gs see stimulatory G protein
GTP (guanosine 5 -triphosphate) Nucleoside triphosphate produced by phosphorylating GDP (guanosine
diphosphate). Like ATP it releases a large amount of free energy on hydrolysis of its terminal phosphate group.
It has a special role in microtubule assembly, protein synthesis, and cell signaling.
GTPase Enzyme activity that converts GTP to GDP. Also the common name used for monomeric GTP-binding
proteins. (See GTP-binding protein.)
GTPase-activating protein (GAP) Protein that binds to a GTP-binding protein and inactivates it by stimulating
its GTPase activity so that it hydrolyzes its bound GTP to GDP.
GTP-binding protein, G protein Protein with GTPase activity that binds GTP, which activates the protein. The
intrinsic GTPase activity eventually converts the GTP to GDP which inactivates the protein. These GTPases act
as molecular switches in, for example, intracellular signaling pathways. One family is composed of three
different subunits (heterotrimeric GTP-binding proteins). The members of the other, very large family are
monomeric GTP-binding proteins; these are commonly referred to as monomeric GTPases.
guanine nucleotide exchange factor (GEF) Protein that binds to a GTP-binding protein and activates it by
stimulating it to release its tightly bound GDP, thereby allowing it to bind GTP in its place.
H+ see proton
haploid Having only one set of chromosomes, as in a sperm cell or a bacterium, as distinct from diploid (having
two sets of chromosomes).
heat shock protein (stress-response protein) Protein synthesized in increased amounts in response to an
elevated temperature or other stressful treatment, and which usually helps the cell to survive the stress.
Prominent examples are hsp60 and hsp70.
heavy chain (H chain) The larger of the two types of polypeptide in an immunoglobulin molecule.
HeLa cell Line of human epithelial cells that grows vigorously in culture. Derived from a human cervical
carcinoma.
helix-loop-helix (HLH) DNA-binding structural motif present in many gene regulatory proteins. Should not be
confused with the helix-turn-helix.
helper T cell Type of T cell that helps stimulate B cells to make antibodies and activates macrophages to kill
ingested microorganisms.
heme Cyclic organic molecule containing an iron atom that carries oxygen in hemoglobin and carries an
electron in cytochromes.
hemidesmosome Specialized anchoring cell junction between an epithelial cell and the underlying basal lamina.
hemoglobin The major protein in red blood cells that associates with O2 in the lungs by means of a bound heme
group.
hemopoiesis Generation of blood cells, mainly in the bone marrow.
hepatocyte Liver cell.
heterocaryon Cell with two or more genetically different nuclei; produced by the fusion of two or more different
cells.
heterochromatin Region of a chromosome that remains unusually condensed chromatin; transcriptionally
inactive during interphase.
heterodimer Protein complex composed of two different polypeptide chains.
heterozygote Diploid cell or individual having two different alleles of one or more specified genes.
high-energy bond Covalent bond whose hydrolysis releases an unusually large amount of free energy under
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Glossary
the conditions existing in a cell. A group linked to a molecule by such a bond is readily transferred from one
molecule to another. Examples include the phosphodiester bonds in ATP and the thioester linkage in acetyl CoA.
high-mannose oligosaccharide Chain of sugars attached to a glycoprotein which contains many mannose
residues. It is generated by a trimming of the original mannose-rich oligosaccharide that leaves most of the
mannose residues with no subsequent addition of further sugars. (See Figure 13 26.)
high-performance liquid chromatography (HPLC) Type of chromatography that uses columns packed with
tiny beads of matrix; the solution to be separated is pushed through under high pressure.
histidine-kinase-associated receptor Type of transmembrane receptor found in the plasma membrane of
bacteria, yeast and plant cells, and involved, for example, in sensing stimuli that cause bacterial chemotaxis.
Associated with a histidine protein kinase on its cytoplasmic side.
histone One of a group of small abundant proteins, rich in arginine and lysine, four of which form the
nucleosome on the DNA in eucaryotic chromosomes.
HIV Human immunodeficiency virus, the retrovirus that is the cause of AIDS.
HLH see helix-loop-helix
hnRNP protein (heterogeneous nuclear ribonuclear protein) Any of a group of proteins that assemble on
newly synthesized RNA, organizing it into a more compact form.
Holliday junction X-shaped structure observed in DNA undergoing recombination, in which the two DNA
molecules are held together at the site of crossing-over, also called a cross-strand exchange.
homeobox Short (180 base pairs long) conserved DNA sequence that encodes a DNA-binding protein motif
(homeodomain) famous for its presence in genes that are involved in orchestrating development in a wide range
of organisms.
homeodomain DNA-binding domain that defines a class of gene regulatory proteins important in animal
development.
homeotic mutation Mutation that causes cells in one region of the body to behave as though they were located
in another, causing a bizarre disturbance of the body plan.
homolog (1) One of two or more genes that are similar in sequence as a result of derivation from the same
ancestral gene. The term covers both orthologs and paralogs. (2) See homologous chromosome.
homologous Describes organs or molecules that are similar because of their common evolutionary origin.
Specifically it describes similarities in protein or nucleic acid sequence.
homologous chromosome (homolog) One of the two copies of a particular chromosome in a diploid cell, each
copy being derived from a different parent.
homozygote Diploid cell or organism having two identical alleles of a specified gene or set of genes.
hormone Signal molecule secreted by an endocrine cell into the bloodstream, which can then carry it to distant
target cells.
housekeeping gene Gene serving a function required in all the cell types of an organism, regardless of their
specialized role.
Hox complex Two tightly linked clusters of genes in Drosophila (the bithorax and Antennapedia complexes)
that control the differences between the different segments of the body. Homologous Hox complexes are found
in other animals, where they also determine pattern along the anteroposterior axis.
HPLC see high-performance liquid chromatography
hybridization In molecular biology, the process whereby two complementary nucleic acid strands form a double
helix. Forms the basis of a powerful technique for detecting specific nucleotide sequences.
hybridoma Cell line used in the production of monoclonal antibodies. Obtained by fusing antibody-secreting B
cells with cells of a lymphocyte tumor.
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Glossary
hydrocarbon Compound that has only carbon and hydrogen atoms. (See Panel 2 1, p 110 111.)
hydrogen bond Noncovalent bond in which an electropositive hydrogen atom is partially shared by two
electronegative atoms.
hydrolysis (adjective hydrolytic) Cleavage of a covalent bond with accompanying addition of water,
being added to one product of the cleavage and
H
OH to the other.
hydronium ion (H3O) Water molecule associated with an additional proton.
hydrophilic Describes a polar molecule or part of a molecule that forms enough energetically favorable
interactions with water molecules to dissolve readily in water. (Literally, "water loving.")
hydrophobic (lipophilic) Describes a nonpolar molecule or part of a molecule that cannot form energetically
favorable interactions with water molecules and therefore does not dissolve in water. (Literally, "water hating.")
hydrophobic force Force exerted by the hydrogen-bonded network of water molecules that brings two
nonpolar surfaces together by excluding water between them.
hydroxyl ( OH) Chemical group consisting of a hydrogen atom linked to an oxygen, as in an alcohol.
hypertonic Describes any medium with a sufficiently high concentration of solutes to cause water to move out
of a cell due to osmosis.
hypervariable region Any of three small regions within the variable region of an immunoglobulin light or heavy
chain that show the highest variability from molecule to molecule. These regions determine the specificity of the
antigen-binding site.
hypotonic Describes any medium with a sufficiently low concentration of solutes to cause water to move into a
cell due to osmosis.
IAP family Intracellular protein inhibitors of apoptosis.
Ig see immunoglobulin
Ig superfamily Large family of proteins that contain immunoglobulin domains or immunoglobulin-like domains.
Most are involved in cell-cell interactions or antigen recognition.
image processing Computer treatment of images gained from microscopy that reveal information not
immediately visible to the eye.
imaginal disc Group of cells that are set aside in the Drosophila embryo and which will develop into an adult
structure, e.g. eye, leg, wing.
immature secretory vesicle Secretory vesicle that appears to have just pinched off the Golgi stack. Its
structure resembles that of a cisterna of the trans Golgi network.
immortalization Production of a cell line capable of an unlimited number of cell divisions. Can be the result of a
chemical or viral transformation or of fusion of the original cells with cells of a tumor line.
immune response Response made by the immune system when a foreign substance or microorganism enters
its body. (See also innate immune response, adaptive immune response, primary immune response,
secondary immune response.)
immune system Population of lymphocytes and other white blood cells in the vertebrate body that defends it
against infection.
immunoglobulin (Ig) An antibody molecule. Higher vertebrates have five classes of immunoglobulin
IgE, IgG, and IgM
IgA, IgD,
each with a different role in the immune response.
immunoglobulin domain (Ig domain) Characteristic protein domain of about 100 amino acids that is found in
immunoglobulin light and heavy chains. Similar domains, known as immunoglobulin-like (Ig-like) domains, are
present in many other proteins involved in cell cell interactions and antigen recognition and define the Ig
superfamily.
immunogold electron microscopy Electron microscopy technique in which cellular structures or molecules of
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Glossary
interest are labeled with antibodies tagged with electron-dense gold particles. These show up as black spots on
the image.
immunological memory Long-lived state that follows a primary immune response to many antigens, in which
subsequent encounter with that antigen will provoke a rapid secondary immune response.
immunoprecipitation Use of a specific antibody to draw the corresponding protein antigen out of solution. The
technique can identify complexes of interacting proteins in cell extracts by using an antibody specific for one of
the proteins to precipitate the complex.
in situ hybridization Technique in which a single-stranded RNA or DNA probe is used to locate a gene or a
messenger RNA molecule in a cell or tissue by hybridization.
in vitro Term used by biochemists to describe a process taking place in an isolated cell-free extract. Also used
by cell biologists to refer to cells growing in culture (in vitro), as opposed to in an organism (in vivo). (Latin for "in
glass.")
in vivo In an intact cell or organism. (Latin for "in life.")
induction In developmental biology, a change in the developmental fate of one tissue caused by an interaction
with another tissue. Such an interaction is called an inductive interaction.
inflammatory response Local response of a tissue to injury or infection
characterized by tissue redness,
swelling, heat, and pain. Caused by invasion of white blood cells, which release various local mediators such as
histamine.
inhibitor of apoptosis family see IAP family
inhibitory G protein (Gi) G protein that can regulate ion channels and inhibit the enzyme adenylyl cyclase.
inhibitory neurotransmitter Neurotransmitter that opens transmitter-gated Cl
or K+ channels in the
postsynaptic membrane of a nerve or muscle cell and thus tends to inhibit the generation of an action potential.
initiation factor Protein that promotes the proper association of ribosomes with messenger RNA and is
required for the initiation of protein synthesis.
initiator tRNA Special tRNA that intiates translation. It always carries the amino acid methionine.
innate immune response Immune response (of both vertebrates and invertebrates) to a pathogen that involves
the pre-existing defenses of the body
the innate immune system
such as barriers formed by skin and
mucosa, antimicrobial molecules and phagocytes. Such a response is not specific for the pathogen.
inner membrane The innermost of two membranes surrounding an organelle. In the mitochondrion, it encloses
the matrix and contains the respiratory electron transport chains.
inner nuclear membrane The innermost of the two nuclear membranes. It contains binding sites for chromatin
and the nuclear lamina on its internal face.
inositol phospholipids (phosphoinositides) One of a family of lipids containing phosphorylated inositol
derivatives. Although minor components of the plasma membrane, they are important in signal transduction in
eucaryotic cells.
insulator element DNA sequence that prevents a gene regulatory protein bound to DNA in the control region of
one gene from influencing the transcription of adjacent genes.
insulin
s in the pancreas and helps regulate glucose metabolism
in animals.
integral membrane protein Protein that is held tightly in a membrane and can only be removed by treatments
that disrupt the lipid bilayer.
integrin Member of a large family of transmembrane proteins involved in the adhesion of cells to the
extracellular matrix and to each other.
intercalary regeneration Type of regeneration that fills in the missing tissues when two mismatched parts of a
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Glossary
structure are grafted together.
interferon-
-
Cytokine secreted by certain types of T cells after activation, and which enhances the
anti-viral response and macrophage activation.
interleukin Secreted peptide or protein that mainly mediates local interactions between white blood cells
(leucocytes) during inflammation and immune responses.
intermediate filament Fibrous protein filament (about 10 nm in diameter) that forms ropelike networks in animal
cells. One of the three most prominent types of cytoskeletal filaments. (See Panel 16 1, p. 909.)
intermembrane space (1) The subcompartment formed between the inner and outer mitochondrial membranes.
(2) The corresponding compartment in a chloroplast.
internal membrane Eucaryotic cell membrane other than the plasma membrane. The membranes of the
endoplasmic reticulum and the Golgi apparatus are examples.
interphase Long period of the cell cycle between one mitosis and the next. Includes G1 phase, S phase, and G2
phase.
intracellular signaling protein Protein that relays a signal as part of an intracellular signaling pathway. It may
either activate the next protein in the pathway or generate a small intracellular mediator.
intron Noncoding region of a eucaryotic gene that is transcribed into an RNA molecule but is then excised by
RNA splicing during production of the messenger RNA or other functional structural RNA.
inversion Type of mutation in which a segment of chromosome is inverted.
ion channel Transmembrane protein complex that forms a water-filled channel across the lipid bilayer through
which specific inorganic ions can diffuse down their electrochemical gradients.
ion An atom that has either gained or lost electrons to acquire a charge; for example Na+ and Cl .
ionic bond Cohesion between two atoms, one with a positive charge, the other with a negative charge. One
type of noncovalent bond.
ionophore Small hydrophobic molecule that dissolves in lipid bilayers and increases their permeability to
specific inorganic ions.
iron-sulfur center Electron-transporting group consisting of either two or four iron atoms bound to an equal
number of sulfur atoms, found in a class of electron-transport proteins.
isoelectric point The pH at which a charged molecule in solution has no net electric charge and therefore does
not move in an electric field.
isomers Molecules that are formed from the same atoms in the same chemical linkages but have different
three-dimensional conformations. (See Panel 2 4, pp. 116 117.)
isoprenoid (polyisoprenoid) Member of a large family of lipid molecules with a carbon skeleton based on
multiple five-carbon isoprene units. Examples include retinoic acid and dolichol.
isotope One of a number of forms of an atom that differ in atomic weight but have the same number of protons
and electrons, and therefore the same chemistry. May be either stable or radioactive.
Jak-STAT signaling pathway Rapid signaling pathway by which some extracellular signals (for example
interferon) activate gene expression. Involves cell-surface receptors and cytoplasmic Janus kinases (Jaks) plus
signal transducers and activators of transcription (STATs).
joule Standard unit of energy in the meter-kilogram system. One joule is the energy delivered in one second by
a one-watt power source. Approximately equal to 0.24 calories.
K see equilibrium constant
K+ leak channel A K+-transporting ion channel in the plasma membrane of animals cells that remains open
even in a "resting" cell.
karyotype Full set of chromosomes of a cell arranged with respect to size, shape, and number.
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Ka see affinity constant
keratin Member of the family of proteins that form keratin intermediate filaments, mainly in epithelial cells.
Specialized keratins are found in hair, nails, and feathers.
ketone Organic molecule containing a carbonyl group linked to two alkyl groups.
kilo-Prefix denoting 103 .
kilocalorie (kcal) Unit of heat energy equal to 1000 calories. Often used to express the energy content of food
or molecules: bond strengths, for example, are measured in kcal/mole. An alternative unit in wide use is the
kilojoule, equal to 0.24 kcal.
kilojoule Standard unit of energy equal to 1000 joules, or 0.24 kilocalories.
kinesin One type of motor protein that uses the energy of ATP hydrolysis to move along a microtubule.
kinetochore Complex structure formed from proteins on a mitotic chromosome to which microtubules attach
and which plays an active part in the movement of chromosomes to the poles. The kinetochore forms on the part
of the chromosome known as the centromere.
kinetochore microtubule In a mitotic or meiotic spindle, a microtubule with one end attached to the
kinetochore on a chromosome.
Krebs cycle see citric acid cycle
label Chemical group, radioactive atom or fluorescent dye added to a molecule in order to follow its progress
through a biochemical reaction or to locate it spatially. Also, as a verb, to add such a group or atom to a cell or
molecule.
lagging strand One of the two newly synthesized strands of DNA found at a replication fork. The lagging strand
is made in discontinuous lengths that are later joined covalently.
Virus that infects E. coli. Widely used as a DNA cloning vector
lamellipodium (lamellipodia) Flattened, sheetlike protrusion supported by a meshwork of actin filaments,
which is extended at the leading edge of a crawling animal cell.
laminin Extracellular matrix protein found in basal laminae, where it forms a sheetlike network.
lamin see nuclear lamin
lampbrush chromosome Paired chromosome in meiosis in immature amphibian eggs, in which the chromatin
forms large stiff loops extending out from the linear axis of the chromosome.
leading strand One of the two newly synthesized strands of DNA found at a replication fork. The leading strand
is made by continuous synthesis in the 5 -to-3 direction.
lectin Protein that binds tightly to a specific sugar. Abundant lectins from plant seeds are often used as affinity
reagents to purify glycoproteins or to detect them on the surface of cells.
leptotene The first phase of division I of meiosis, in which the paired duplicated homologous chromosomes
condense and become visible in the light microscope.
lethal mutation A mutation that causes the death of the cell or the organism that contains it.
leucine zipper Structural motif seen in many DNAproteins are joined together in a coiled-coil (rather like a zipper), forming a protein dimer.
leucine-rich repeat protein (LRR protein) Common type of receptor serine/threonine kinase in plants.
Characterized by a tandem array of leucine-rich repeat sequences in the extracellular portion.
leucocyte see white blood cell
leukemia Cancer of white blood cells.
ligand Any molecule that binds to a specific site on a protein or other molecule. (From Latin ligare, to bind.)
ligase Enzyme that joins together (ligates) two molecules in an energy-dependent process. DNA ligase, for
example, joins two DNA molecules together end to end through phosphodiester bonds.
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light chain One of the smaller polypeptides of a multisubunit protein such as myosin or immunoglobulin.
Abbreviated as L chain in immunoglobulins.
lineage analysis Tracing the ancestry of individual cells in a developing embryo.
linkage (1) Mutual effect of the binding of one ligand on the binding of another that is a central feature of the
behavior of all allosteric proteins. (2) Co-inheritance of two genetic loci that lie near each other on the same
chromosome. The closer together the two loci, that is, the greater the linkage, the lower the frequency of
recombination between them.
lipase Enzyme that catalyzes the cleavage of fatty acids from the glycerol moiety of a triglyceride.
lipid Organic molecule that is insoluble in water but tends to dissolve in nonpolar organic solvents. A special
class, the phospholipids, forms the structural basis of biological membranes.
lipid bilayer Thin bimolecular sheet of mainly phospholipid molecules that forms the core structure of all cell
membranes. The two layers of lipid molecules are packed with their hydrophobic tails pointing inward and their
hydrophilic heads outward, exposed to water.
lipid raft Small region of the plasma membrane enriched in sphingolipids and cholesterol.
lipophilic see hydrophobic
liposome Artificial phospholipid bilayer vesicle formed from an aqueous suspension of phospholipid molecules.
local mediator Secreted signal molecule that acts at short range on adjacent cells.
locus In genetics, the position of a gene on a chromosome. Different alleles of the same gene all occupy the
same locus.
long-term potentiation Long-lasting increase (days to weeks) in the sensitivity of certain synapses in the
hippocampus. Induced by a short burst of repetitive firing in the presynaptic neurons.
low-density lipoprotein (LDL) Large complex composed of a single protein molecule and many esterified
cholesterol molecules, together with other lipids. The form in which cholesterol is transported in the blood and
taken up into cells.
LTP see long-term potentiation
lumen Cavity enclosed by an epithelial sheet (in a tissue) or by a membrane (in a cell).
lymph Colorless fluid derived from blood by filtration through capillary walls. Carries lymphocytes in a special
system of ducts and vessels
the lymphatic vessels.
lymphocyte Type of white blood cell responsible for the specificity of adaptive immune responses. There are
two main types: B cells, which produce antibody, and T cells, which interact directly with other effector cells of
the immune system and with infected cells. T cells develop in the thymus and are responsible for cell-mediated
immunity. B cells develop in the bone marrow in mammals and are responsible for the production of circulating
antibodies.
lymphoid organ Organs involved in the production or function of lymphocytes, such as thymus, spleen, lymph
nodes, and tonsils.
lysis Rupture of a cell's plasma membrane, leading to the release of cytoplasm and the death of the cell.
lysogeny State of a bacterium in which it carries the DNA of an inactive virus integrated into its genome. The
virus can subsequently be activated to replicate and lyse the cell.
lysosome Membrane-bounded organelle in eucaryotic cells containing digestive enzymes, which are typically
most active at the acid pH found in the lumen of lysosomes.
lysozyme Enzyme that catalyzes the cutting of polysaccharide chains in the cell walls of bacteria.
M phase Period of the eucaryotic cell cycle during which the nucleus and cytoplasm divide.
M6P see mannose 6-phosphate
macromolecule Molecule such as a protein, nucleic acid, or polysaccharide with a molecular mass greater than
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Glossary
a few thousand daltons.
macrophage Phagocytic cell derived from blood monocytes, typically resident in most tissues. It has both
scavenger and antigen-presenting functions in immune responses.
major histocompatibility complex (MHC) Complex of highly polymorphic genes in vertebrates. They code for
a large family of cell-surface glycoproteins (MHC molecules) that bind peptide fragments of foreign proteins and
present them to T cells to induce an immune response.
malaria Potentially fatal human disease caused by the protozoan parasite Plasmodium, which is transmitted by
the bite of an infected mosquito.
malignant Describes tumors and tumor cells that are invasive and/or able to undergo metasis. A malignant
tumor is a cancer.
mannose 6-phosphate (M6P) Unique marker attached to the oligosaccharides on some glycoproteins destined
for lysosomes.
map unit see genetic map distance
MAP see microtubule-associated protein
MAP-kinase (mitogen-activated protein kinase) Protein kinase that performs a crucial step in relaying signals
from the plasma membrane to the nucleus. Turned on by a wide range of proliferation- or differentiation-inducing
signals.
mating-type locus (MAT locus) In budding yeast, the locus that determines the mating type (α or a) of the
haploid yeast cell.
matrix space (1) Central subcompartment of a mitochondrion, bounded by the inner mitochondrial membrane.
(2) The corresponding compartment in a chloroplast, which is more commonly known as the stroma.
M-Cdk see M-phase Cdk
Mcm proteins Proteins in the eucaryotic cell that bind to origin recognition complexes in DNA in early G 1 and
are involved in forming the pre-replicative complex.
M-cyclin Type of cyclin found in all eucaryotic cells that promotes the events of mitosis.
MDR protein see multidrug resistance protein
mega- Prefix denoting 106. (From Greek megas, huge, powerful.)
megakaryocyte Large myeloid cell with a multilobed nucleus that remains in the bone marrow when mature. It
buds off platelets from long cytoplasmic processes.
meiosis Special type of cell division by which eggs and sperm cells are produced. It comprises two successive
nuclear divisions with only one round of DNA replication, which produces four haploid daughter cells from an
initial diploid cell.
melanocyte Cell that produces the dark pigment melanin. Responsible for the pigmentation of skin and hair.
membrane The lipid bilayer plus associated proteins that encloses all cells and, in eucaryotic cells, many
organelles as well.
membrane-bound ribosome Ribosome attached to the cytosolic face of the endoplasmic reticulum. The site of
synthesis of proteins that enter the endoplasmic reticulum.
membrane channel Transmembrane protein complex that allows inorganic ions or other small molecules to
diffuse passively across the lipid bilayer.
membrane potential Voltage difference across a membrane due to a slight excess of positive ions on one side
and of negative ions on the other. A typical membrane potential for an animal cell plasma membrane is
60 mV
(inside negative relative to the surrounding fluid).
membrane protein Protein that is normally closely associated with a cell membrane. (See Figure 10 17.)
membrane transport Movement of molecules across a membrane mediated by a membrane transport protein.
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Glossary
membrane transport protein Membrane protein that mediates the passage of ions or molecules across a
membrane. Examples are ion channels and carrier proteins.
meristem An organized group of dividing cells whose derivatives give rise to the tissues and organs of a
flowering plant. Key examples are the apical meristems at the tips of shoots and roots.
mesenchyme Immature, unspecialized form of connective tissue in animals, consisting of cells embedded in a
thin extracellular matrix.
mesoderm Embryonic tissue that is the precursor to muscle, connective tissue, skeleton and many of the
internal organs.
messenger RNA (mRNA) RNA molecule that specifies the amino acid sequence of a protein. Produced by
RNA splicing (in eucaryotes) from a larger RNA molecule made by RNA polymerase as a complementary copy
of DNA. It is translated into protein in a process catalyzed by ribosomes.
metabolism The sum total of the chemical processes that take place in living cells.
metaphase Stage of mitosis at which chromosomes are firmly attached to the mitotic spindle at its equator but
have not yet segregated toward opposite poles.
metaphase plate Imaginary plane at right angles to the mitotic spindle and midway between the spindle poles;
the plane in which chromosomes are positioned at metaphase.
metaplasia A change in the pattern of cell differentiation in a tissue.
metastasis Spread of cancer cells from their site of origin to other sites in the body.
methyl ( CH3) Hydrophobic chemical group derived from methane (CH 4).
MHC molecule One of a large family of ubiquitous cell-surface glycoproteins encoded by genes of the major
histocompatibility complex (MHC). They bind peptide fragments of foreign antigens and present them to T cells
to induce an immune response. (See also class I MHC molecule, class II MHC molecule.)
MHC see major histocompatibility complex
micro- Prefix denoting 10
6.
microelectrode, micropipette Piece of fine glass tubing pulled to an even finer tip. Used to penetrate a cell to
study its physiology or to inject electric current or molecules.
microfilament see actin filament
micrograph Photograph of an image seen through a microscope. May be either a light micrograph or an
electron micrograph depending on the type of microscope employed.
microinjection Injection of molecules into a cell using a micropipette.
Unit of measurement often applied to cells and organelles. Equal to 10
10
4
6
meter or
centimeter.
micropipette see microelectrode
microsome Small vesicle that is derived from fragmented endoplasmic reticulum produced when cells are
homogenized.
microtubule Long hollow cylindrical structure composed of the protein tubulin. It is one of the three major
classes of filaments of the cytoskeleton.
microtubule-associated protein (MAP) Any protein that binds to microtubules and modifies their properties.
Many different kinds have been found, including structural proteins, such as MAP-2, and motor proteins, such as
dynein.
microtubule-organizing center (MTOC) Region in a cell, such as a centrosome or a basal body, from which
microtubules grow.
microvillus (microvilli) Thin cylindrical membrane-covered projection on the surface of an animal cell
containing a core bundle of actin filaments. Present in especially large numbers on the absorptive surface of
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Glossary
intestinal epithelial cells.
midbody Structure formed at the end of cleavage that can persist for some time as a tether between the two
daughter cells in animals.
milli- Prefix denoting 10
3.
minus end The end of a microtubule or actin filament at which the addition of monomers occurs least readily;
the "slow-growing" end of the microtubule or actin filament. The minus end of an actin filament is also known as
the pointed end. (See Panel 16 2, pp. 912 913.)
mismatch repair DNA repair process that corrects mismatched nucleotides inserted during DNA replication. A
short stretch of newly synthesized DNA including the mismatched nucleotide is removed and replaced with the
correct sequence with reference to the template strand.
mitochondrial precursor protein Mitochondrial protein encoded by a nuclear gene, synthesized in the cytosol,
and subsequently transported into mitochondria.
mitochondrion (mitochondria) Membrane-bounded organelle, about the size of a bacterium, that carries out
oxidative phosphorylation and produces most of the ATP in eucaryotic cells.
mitogen An extracellular substance, such as a growth factor, that stimulates cell proliferation.
mitogen-activated protein kinase see MAP-kinase
mitosis Division of the nucleus of a eucaryotic cell, involving condensation of the DNA into visible
chromosomes, and separation of the duplicated chromosomes to form two identical sets. (From Greek mitos, a
thread, referring to the threadlike appearance of the condensed chromosomes.)
mitotic chromosome Highly condensed duplicated chromosome with the two new chromosomes still held
together at the centromere as sister chromatids.
mitotic spindle Array of microtubules and associated molecules that forms between the opposite poles of a
eucaryotic cell during mitosis and serves to move the duplicated chromosomes apart.
model organism A species, such as Drosophila melanogaster or Escherichia coli, that has been studied
intensively over a long period and thus serves as a "model" of the biology of a particular type of organism.
module In proteins or nucleic acids, a unit of structure or function that is found in a variety of different contexts in
different molecules.
molar Describes a solution with a concentration of 1 mole of a substance dissolved in 1 liter of solution
(abbreviated as 1 M).
mole X grams of a substance, where X is its relative molecular mass (molecular weight). A mole consists of 6 ×
1023 molecules of the substance.
molecular chaperone see chaperone
molecular weight Numerically, the same as the relative molecular mass of a molecule expressed in daltons.
For example, a protein of relative molecular mass 20,000 has a molecular weight of 20,000.
molecule Group of atoms joined together by covalent bonds.
monoclonal antibody Antibody secreted by a hybridoma clone. Because each such clone is derived from a
single B cell, all of the antibody molecules produced are identical.
monocyte Type of white blood cell that leaves the bloodstream and matures into a macrophage in tissues.
monomer Small molecular building block that can serve as a subunit, being linked to others of the same type to
form a larger molecule (a polymer).
monosaccharide Simple sugar with the general formula (CH2O)n, where n = 3 to 8.
morphogen Signal molecule that can impose a pattern on a field of cells by causing cells in different places to
adopt different fates.
mosaic In developmental biology, an organism made of a mixture of cells with different genotypes.
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Glossary
motif Element of structure or pattern that recurs in many contexts. Specifically, a small structural domain that
can be recognized in a variety of proteins.
motor protein Protein that uses energy derived from nucleoside triphosphate hydrolysis to propel itself along a
protein filament or another polymeric molecule.
M-phase Cdk (M-Cdk) Complex formed in vertebrate cells by an M-cyclin and the corresponding
cyclin-dependent kinase (Cdk).
mRNA see messenger RNA
MTOC see microtubule-organizing center
multidrug resistance protein (MDR protein) Type of ABC transporter protein that can pump hydrophobic
drugs (such as some anti-cancer drugs) out of the cytoplasm of eucaryotic cells.
multipass transmembrane protein Membrane protein in which the polypeptide chain crosses the lipid bilayer
more than once.
mutant Organism in which a mutation has occurred that makes it different from wild-type or from the 'normal'
extent of variation in the population.
mutation rate The rate at which observable changes occur in a DNA sequence.
mutation Heritable change in the nucleotide sequence of a chromosome.
myelin sheath Insulating layer of specialized cell membrane wrapped around vertebrate axons. Produced by
oligodendrocytes in the central nervous system and by Schwann cells in the peripheral nervous system.
myeloid cell Any white blood cell other than lymphocytes.
myoblast Mononucleated, undifferentiated muscle precursor cell. A skeletal muscle cell is formed by the fusion
of multiple myoblasts.
myoepithelial cell Type of unstriated muscle cell found in epithelia, e.g. in the iris of the eye and in glandular
tissue.
myofibril Long, highly organized bundle of actin, myosin, and other proteins in the cytoplasm of muscle cells
that contracts by a sliding filament mechanism.
N terminus see amino terminus
Na+-K+ pump (Na+-K+ ATPase) Transmembrane carrier protein found in the plasma membrane of most animal
cells that pumps Na+ out of and K+ into the cell, using energy derived from ATP hydrolysis.
NAD+ (nicotine adenine dinucleotide) Activated carrier that participates in an oxidation reaction by accepting
a hydride ion (H ) from a donor molecule. The NADH formed is an important carrier of electrons for oxidative
phosphorylation.
NADH dehydrogenase complex First of the three electron-driven proton pumps in the mitochondrial
respiratory chain. It accepts electrons from NADH.
NADP+ (nicotine adenine dinucleotide phosphate) Activated carrier closely related to NAD+ that is used
extensively in biosynthetic, rather than catabolic, pathways. The reduced form is NADPH.
9.
nano- Prefix denoting 10
nanometer (nm) Unit of length commonly used to measure molecules and cell organelles. 1 nm = 10
9
3
meter.
natural killer cell (NK cell) Cytotoxic cell of the innate immune system that can kill virus-infected cells.
N-CAM see neural cell adhesion molecule
negative control Type of control of gene expression in which the active DNA-binding form of the regulatory
protein turns the gene off.
negative staining Staining technique for use in the electron electron microscope in which a reverse, or negative,
image of the object is created.
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Glossary
Nernst equation Quantitative expression that relates the equilibrium ratio of concentrations of an ion on either
side of a permeable membrane to the voltage difference across the membrane. (See Panel 11 2, p. 634.)
nerve cell see neuron
neural cell adhesion molecule (N-CAM) Cell adhesion molecule of the immunoglobulin superfamily,
expressed by many cell types including most nerve cells. It mediates Ca2+-independent cell-cell attachment in
vertebrates.
neural tube Tube of ectoderm that will form the brain and spinal cord in a vertebrate embryo.
neurite Long process growing from a nerve cell in culture. A generic term that does not specify whether the
process is an axon or a dendrite.
neurofilament Type of intermediate filament found in nerve cells.
neuromuscular junction Specialized chemical synapse between an axon terminal of a motor neuron and a
skeletal muscle cell.
neuron (nerve cell) Cell with long processes specialized to receive, conduct, and transmit signals in the
nervous system.
neuropeptide Peptide secreted by neurons as a signaling molecule either at synapses or elsewhere.
neurotransmitter Small signal molecule secreted by the presynaptic nerve cell at a chemical synapse to relay
the signal to the postsynaptic cell. Examples include acetylcholine, glutamate, GABA, glycine, and many
neuropeptides.
neutron Uncharged subatomic particle that forms part of an atomic nucleus.
neutrophil White blood cell that is specialized for the uptake of particulate material by phagocytosis and which
enters tissues that become infected or inflamed.
nicotine adenine dinucleotide phosphate see NADP+
nicotine adenine dinucleotide see NAD+
nitric oxide (NO) Gaseous signal molecule in both animals and plants. In animals it regulates smooth muscle
contraction, for example; in plants it is involved in responses to injury or infection.
nitrogen cycle The natural circulation of nitrogen between molecular nitrogen in the atmosphere, inorganic
molecules in the soil, and organic molecules in living organisms.
nitrogen fixation Biochemical process carried out by certain bacteria that reduces atmospheric nitrogen (N 2) to
ammonia, leading eventually to various nitrogen-containing metabolites.
nitrogenase complex Complex of enzymes in nitrogen-fixing bacteria that catalyzes the reduction of
atmospheric N2 to ammonia.
NK cell see natural killer cell
N-linked oligosaccharide Chain of sugars attached to a protein through the NH2 group of the side chain of an
asparagine residue.
NMR (nuclear magnetic resonance) Resonant absorption of electromagnetic radiation at a specific frequency
by atomic nuclei in a magnetic field, due to flipping of the orientation of their magnetic dipole moments. The
NMR spectrum provides information about the chemical environment of the nuclei. Two-dimensional NMR is
used widely to determine the three-dimensional structure of small proteins.
nm see nanometer
noncovalent attraction Chemical bond in which, in contrast to a covalent bond, no electrons are shared.
Noncovalent bonds are relatively weak, but they can sum together to produce strong, highly specific interactions
between molecules.
noncyclic photophosphorylation Photosynthetic process that produces both ATP and NADPH in plants and
cyanobacteria.
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Glossary
nonenveloped virus Virus consisting of a nucleic acid core and protein capsid only.
nonpolar (apolar) Lacking any asymmetric accumulation of positive and negative charge. Nonpolar molecules
are generally insoluble in water.
nonsense-mediated mRNA decay Mechanism for removing aberrant mRNAs containing in-frame internal stop
codons before they can be translated.
Northern blotting Technique in which RNA fragments separated by electrophoresis are immobilized on a paper
sheet. A specific RNA is then detected by hybridization with a labeled nucleic acid probe.
NO see nitric oxide
Notch Receptor protein involved in many instances of choice of cell fate in animal development, for example in
the specification of nerve cells from ectodermal epithelium. Its ligands are cell-surface proteins such as Delta
and Serrate.
notochord Stiff rod of mesoderm that runs along the back of all chordate embryos. In vertebrates it does not
persist and becomes incorporated into the vertebral column.
NSF Protein with ATPase activity that disassembles a complex of a v-SNARE and a t-SNARE.
nuclear envelope Double membrane surrounding the nucleus. Consists of an outer and inner membrane and is
perforated by nuclear pores.
nuclear export signal Sorting signal contained in the structure of molecules and complexes, such as RNA and
new ribosomal subunits, that are transported from the nucleus to the cytosol through nuclear pore complexes.
nuclear lamin Protein subunit of the intermediate filaments of the nuclear lamina.
nuclear lamina Fibrous meshwork of proteins on the inner surface of the inner nuclear membrane. It is made up
of a network of intermediate filaments formed from nuclear lamins.
nuclear localization signal (NLS) Signal sequences or signal patches found in proteins destined for the
nucleus and which enable their selective transport into the nucleus from the cytosol through the nuclear pore
complexes.
nuclear magnetic resonance see NMR
nuclear pore complex Large multiprotein structure forming a channel (the nuclear pore) through the nuclear
envelope that allows selected molecules to move between nucleus and cytoplasm.
nuclear receptor superfamily Intracellular receptors for hydrophobic signal molecules such as steroids and
retinoic acid. The receptor-ligand complex acts as a transcription factor in the nucleus.
nuclear transport Movement of macromolecules into or out of the nucleus mediated by nuclear transport
receptors.
nucleation Critical stage in the assembly of a polymeric structure, such as a microtubule, at which a small
cluster of monomers aggregates in the correct arrangement to initiate rapid polymerization. (See Panel 16 2,
pp. 912 913.) More generally, the rate-limiting step in an assembly process.
nucleic acid RNA or DNA, a macromolecule consisting of a chain of nucleotides joined together by
phosphodiester bonds.
nucleolar organizer Region of a chromosome containing a cluster of ribosomal RNA genes that gives rise to a
nucleolus.
nucleolus Structure in the nucleus where ribosomal RNA is transcribed and ribosomal subunits are assembled.
nucleoporin Any of a number of different proteins that make up nuclear pore complexes.
nucleoside Molecule composed of a purine or pyrimidine base covalently linked to a ribose or deoxyribose
sugar
nucleosome Beadlike structure in eucaryotic chromatin. It is composed of a short length of DNA wrapped
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Glossary
around a core of histone proteins, and is the fundamental structural unit of chromatin.
nucleotide Nucleoside with one or more phosphate groups joined in ester linkages to the sugar moiety. DNA
and RNA are polymers of nucleotides. (See Panel 2 6, pp. 120 121.)
nucleus Prominent membrane-bounded organelle in a eucaryotic cell, containing DNA organized into
chromosomes.
nurse cell Cell connected by cytoplasmic bridges to a developing oocyte and which thereby supplies it with
ribosomes, mRNAs, and proteins needed for the development of the early embryo.
occluding junction Type of cell junction that seals cells together in an epithelium, forming a barrier through
which even small molecules cannot pass.
Okazaki fragments Short lengths of DNA produced on the lagging strand during DNA replication. They are
rapidly joined by DNA ligase to form a continuous DNA strand.
oligodendrocyte Type of glial cell in the vertebrate central nervous system that forms a myelin sheath around
axons.
oligomer Short polymer, usually consisting (in a cell) of amino acids (oligopeptides), sugars (oligosaccharides),
or nucleotides (oligonucleotides). (From Greek oligos, few, little.)
oligosaccharide Short linear or branched chain of covalently linked sugars (see Panel 2 4, pp. 116 117.)
(See also complex oligosaccharide, high-mannose oligosaccharide, N-linked oligosacharide, O-linked
glycosylation.)
O-linked glycosylation Addition of an oligosaccharide chain to a protein through the OH group of a serine or
threonine side chain.
oncogene An altered gene whose product can act in a dominant fashion to help make a cell cancerous.
Typically, an oncogene is a mutant form of a normal gene (proto-oncogene) involved in the control of cell growth
or division.
oocyte The developing egg. It is usually a large and immobile cell.
oogenesis Formation and maturation of oocytes in the ovary.
operator Short region of DNA in a bacterial chromosome that controls the transcription of an adjacent gene.
operon In a bacterial chromosome, a group of contiguous genes that are transcribed into a single mRNA
molecule.
ORC see origin recognition complex
organelle Membrane-enclosed compartment in a eucaryotic cell that has a distinct structure, macromolecular
composition, and function. Examples are nucleus, mitochondrion, chloroplast, Golgi apparatus.
Organizer see Spemann's Organizer
origin recognition complex (ORC) Large protein complex that is bound to the DNA at origins of replication in
eucaryotic chromosomes throughout the cell cycle.
osmolarity A term used to describe the concentration of a solute in terms of the osmotic pressure it can exert.
osmosis Net movement of water molecules across a semipermeable membrane driven by a difference in
concentration of solute on either side. The membrane must be permeable to water but not to the solute
molecules.
osteoblast Cell that secretes matrix of bone.
osteoclast Macrophage-like cell that erodes bone, enabling it to be remodeled during growth and in response to
stresses throughout life.
outer membrane Outermost of the two membranes surrounding an organelle; the membrane adjacent to the
cytosol.
outer nuclear membrane The outermost of the two nuclear membranes. It is continuous with the endoplasmic
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Cell
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Glossary
reticulum and is studded with ribosomes on its cytosolic face.
overlap microtubule In the mitotic or meiotic spindle, a microtubule interdigitating at the equator with the
microtubules emanating from the other pole.
ovulation Release of an egg from the ovary.
ovum see egg
oxidation (verb oxidize) Loss of electrons from an atom, as occurs during the addition of oxygen to a molecule
or when a hydrogen is removed. Opposite of reduction. (See Figure 2 43.)
oxidative phosphorylation Process in bacteria and mitochondria in which ATP formation is driven by the
transfer of electrons from food molecules to molecular oxygen. Involves the intermediate generation of a proton
gradient (pH gradient) across a membrane and chemiosmotic coupling.
p53 Tumor suppressor gene found mutated in about half of human cancers. It encodes a gene regulatory
protein that is activated by damage to DNA and is involved in blocking further progression through the cell cycle.
pachytene Third stage of division I of meiosis, in which synapsis is complete.
palindromic sequence Nucleotide sequence that is identical to its complementary strand when each is read in
the same chemical direction
for example, GATC.
paracrine signaling Short-range cell-cell communication via secreted signal molecules that act on adjacent
cells.
parthenogenesis Production of a new individual from an egg cell in the absence of fertilization by sperm.
passive transport Transport of a solute across a membrane down its concentration gradient or its
electrochemical gradient, using only the energy stored in the gradient.
patch-clamp recording Electrophysiological technique in which a tiny electrode tip is sealed onto a patch of
cell membrane, thereby making it possible to record the flow of current through individual ion channels in the
patch.
pathogen (adjective pathogenic) An organism or other agent that causes disease.
PCR (polymerase chain reaction) Technique for amplifying specific regions of DNA by the use of
sequence-specific primers and multiple cycles of DNA synthesis, each cycle being followed by a brief heat
treatment to separate complementary strands.
peptide bond Chemical bond between the carbonyl group of one amino acid and the amino group of a second
amino acid
a special form of amide linkage. Peptide bonds link amino acids together in proteins.
peptide map Characteristic two-dimensional pattern (on paper or gel) formed by the separation of the mixture of
peptides produced by the partial digestion of a protein.
peripheral lymphoid organ (secondary lymphoid organ) Lymphoid organ in which T cells and B cells interact
with foreign antigens. Examples are spleen, lymph nodes, and mucosal-associated lymphoid tissue.
peripheral membrane protein Protein that is attached to one face of a membrane by noncovalent interactions
with other membrane proteins, and which can be removed by relatively gentle treatments that leave the lipid
bilayer intact.
peroxisome Small membrane-bounded organelle that uses molecular oxygen to oxidize organic molecules.
Contains some enzymes that produce and others that degrade hydrogen peroxide (H 2O2).
pH Common measure of the acidity of a solution: "p" refers to power of 10, "H" to hydrogen. Defined as the
negative logarithm of the hydrogen ion concentration in moles per liter (M). Thus on the pH scale, pH 3 (10
H+) is acidic and pH 9 (10
9
3
M H+) is alkaline.
PH domain see pleckstrin homology domain
phage display Technique for detecting proteins that interact with each other by screening a protein against a
library of genetically modified phage, each displaying a potential binding protein on their surface.
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Glossary
phage see bacteriophage
phagocyte General term for a professional phagocytic cell
that is, a cell such as a macrophage or neutrophil
that is specialized to take up particles and microorganisms by phagocytosis.
phagocytosis Process by which particulate material is endocytosed ("eaten") by a cell. Prominent in
carnivorous cells, such as Amoeba proteus, and in vertebrate macrophages and neutrophils. (From Greek
phagein, to eat.)
phagosome Large intracellular membrane-bounded vesicle that is formed as a result of phagocytosis. Contains
ingested extracellular material.
phase-contrast microscope Type of light microscope that exploits the interference effects that occur when
light passes through material of different refractive indexes. Used to view living cells.
phenotype The observable character of a cell or an organism.
phosphatase Enzyme that removes phosphate groups from a molecule.
phosphatidylinositol 3-kinase (PI 3-kinase) A kinase involved in intracellular signaling pathways activated by
a variety of cell-surface receptors. It phosphorylates inositol phospholipids at the 3 position of the inositol ring.
phosphatidylinositol An inositol phospholipid. (See Figure 15 34.)
phosphodiester linkage Set of covalent chemical bonds formed when two hydroxyl groups are linked in ester
linkage to the same phosphate group. This linkage joins adjacent nucleotides in RNA or DNA.
phosphoinositide see inositol phospholipid
phospholipase C-
-
Enzyme bound to the cytoplasmic face of the plasma membrane that converts
membrane phosphatidylinositol 4,5-bisphosphate to diacylglycerol (which remains in the plasma membrane)
and inositol 1,4,5-trisphosphate (IP3). It is activated by certain G proteins to trigger the inositol phospholipid
signaling pathway.
phospholipase C-
-
Like phospholipase C-
diacylglycerol and IP3 to trigger the inositol phospholipid signaling pathway. Activated by certain receptor
tyrosine kinases.
phospholipid exchange protein Water-soluble carrier protein that transfers a phospholipid molecule from one
membrane to another.
phospholipid The main category of lipid molecules used to construct biological membranes. Generally
composed of two fatty acids linked through glycerol phosphate to one of a variety of polar groups.
phosphoprotein phosphatase Enzyme that removes a phosphate group from a protein by hydrolysis.
phosphorylation Reaction in which a phosphate group becomes covalently coupled to another molecule.
photochemical reaction center The part of a photosystem that converts light energy into chemical energy.
photon Elementary particle of light and other electromagnetic radiation.
photoreceptor Cell or molecule that is sensitive to light.
photosynthesis Process by which plants, algae and some bacteria use the energy of sunlight to drive the
synthesis of organic molecules from carbon dioxide and water.
photosynthetic electron-transfer Light-driven reactions in photosynthesis in which electrons move along the
electron-transport chain in the thylakoid membrane, generating ATP and NADPH.
photosystem Multiprotein complex involved in photosynthesis that captures the energy of sunlight and converts
it to useful forms of energy.
phragmoplast Structure made of microtubules and actin filaments that forms in the prospective plane of
division of a plant cell and guides formation of the cell plate.
phylogeny Evolutionary history of an organism or group of organisms, often presented in chart form as a
phylogenetic tree.
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pinocytosis Type of endocytosis in which soluble materials are taken up from the environment and
incorporated into vesicles for digestion. Literally, "cell drinking." (See also fluid-phase endocytosis.)
PKA see cyclic AMP-dependent protein kinase
PKC see protein kinase C
plant growth regulator Signal molecule (also known as a plant hormone) that helps coordinate growth and
development. Examples are ethylene, auxins, gibberellins, cytokines, abscisic acid, and the brassinosteroids.
plasma membrane Membrane that surrounds a living cell.
plasmid Small circular DNA molecule that replicates independently of the genome. Modified plasmids are used
extensively as plasmid vectors for DNA cloning.
plasmodesma (plasmodesmata) Communicating cell cell junction in plants in which a channel of cytoplasm
lined by plasma membrane connects two adjacent cells through a small pore in their cell walls.
plastid Cytoplasmic organelle in plants, bounded by a double membrane, that carries its own DNA and is often
pigmented. Chloroplasts are plastids.
platelet Cell fragment, lacking a nucleus, that breaks off from a megakaryocyte in the bone marrow and is found
in large numbers in the bloodstream. It helps initiate blood clotting when blood vessels are injured.
PLC-
see phospholipase C-
PLC-
see phospholipase C-
pleckstrin homology domain (PH domain) Protein domain found in intracellular signaling proteins by which
they bind to inositol phospholipids phosphorylated by PI 3-kinase.
ploidy The number of complete sets of chromosomes in a genome. Diploid organisms have two sets in their
somatic cells, polyploid organisms more than two. Natural polyploidy is the result of previous duplications of the
whole genome or the introduction of complete genomes from another species during evolution.
plus end The end of a microtubule or actin filament at which addition of monomers occurs most readily; the
"fast-growing" end of a microtubule or actin filament. The plus end of an actin filament is also known as the
barbed end. (See Panel 16 2, pp. 912 913.)
point mutation Change of a single nucleotide in DNA, especially in a region of DNA coding for protein.
polar In the electrical sense, describes a structure (for example, a chemical bond, chemical group, or molecule)
with positive charge concentrated toward one end and negative charge toward the other as a result of an
uneven distribution of electrons. Polar molecules are likely to be soluble in water.
polyisoprenoid
see isoprenoid
polymer Large molecule made by covalently linking multiple identical or similar units (monomers) together.
polymerase chain reaction see PCR
polymorphic Describes a gene with many different alleles, none of which is predominant in the population.
polypeptide Linear polymer composed of multiple amino acids. Proteins are large polypeptides, and the two
terms can be used interchangeably.
polypeptide backbone The chain of repeating carbon and nitrogen atoms, linked by peptide bonds, in a
polypeptide or protein. The side chains of the amino acids project from this backbone.
polyploid Describes a cell or an organism that contains more than two sets of homologous chromosomes.
polyribosome (polysome) Messenger RNA molecule to which are attached a number of ribosomes engaged in
protein synthesis.
polysaccharide Linear or branched polymer of monosaccharides. They include glycogen, starch, hyaluronic
acid, and cellulose.
polytene chromosome Giant chromosome in which the DNA has undergone repeated replication without
separation into new chromosomes.
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position effect Differences in gene expression that depend on the position of the gene on the chromosome and
probably reflect differences in the state of the chromatin along the chromosome.
positional information Information supplied to or possessed by cells according to their position in a
multicellular organism. A cell's internal record of its positional information is called its positional value.
positive control Type of control of gene expression in which the active DNA-binding form of the regulatory
protein turns the gene on.
posterior Situated toward the tail end of the body.
posttranscriptional control Any control on gene expression that is exerted at a stage after transcription has
begun.
posttranslational Describes any process involving a protein that occurs after protein synthesis is completed.
posttranslational modification The enzyme-catalyzed change to a protein made after it is synthesized.
Examples are acetylation, cleavage, glycosylation, methylation, phosphorylation, and prenylation.
pre-B cell Iimmediate precursor of a B cell.
prenylation Covalent attachment of an isoprenoid lipid group to a protein.
preprophase band Circumferential band of microtubules and actin filaments that forms around a plant cell
under the plasma membrane prior to mitosis and cell division.
primary immune response Adaptive immune response to an antigen that is made on first encounter with that
antigen.
primary structure Sequence of monomer units in a linear polymer, such as the amino acid sequence of a
protein.
primordial germ cell Cell set aside early in embryonic development that is a precursor to germ cells that give
rise to gametes.
primosome A complex of DNA primase and DNA helicase that is formed on the lagging strand during DNA
replication, improving the efficiency of replication.
prion An infectious abnormal form of a normal protein that is replicated in the host by forcing the normal proteins
of the same type to adopt the aberrant structure.
prion disease Transmissible spongiform encephalopathies such as Kreutzfeld Jacob disease in humans,
scrapie in sheep and bovine spongiform encephalopathy (BSE) in cattle, that are apparently caused and
transmitted by abnormal forms of a protein (prions).
probe Defined fragment of RNA or DNA, radioactively or chemically labeled, used to locate specific nucleic acid
sequences by hybridization.
procaryote (prokaryote) Single-celled microorganism whose cells lack a well-defined, membrane-enclosed
nucleus. The procaryotes comprise two of the major domains of living organisms
the Bacteria and the
Archaea.
programmed cell death see apoptosis
prometaphase Phase of mitosis preceding metaphase in which the nuclear envelope breaks down and
chromosomes first attach to the spindle.
promoter Nucleotide sequence in DNA to which RNA polymerase binds to begin transcription.
prophase First stage of mitosis, during which the chromosomes are condensed but not yet attached to a mitotic
spindle.
protease (proteinase, proteolytic enzyme) Enzyme such as trypsin that degrades proteins by hydrolyzing
some of their peptide bonds.
proteasome Large protein complex in the cytosol with proteolytic activity that is responsible for degrading
proteins that have been marked for destruction by ubiquitylation or by some other means.
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protein The major macromolecular constituent of cells. A linear polymer of amino acids linked together by
peptide bonds in a specific sequence.
protein domain Portion of a protein that has a tertiary structure of its own. Larger proteins are generally
composed of several domains, each connected to the next by short flexible regions of polypeptide chain.
protein glycosylation Posttranslational addition of oligosaccharide side chains to a protein.
protein kinase Enzyme that transfers the terminal phosphate group of ATP to a specific amino acid of a target
protein.
protein kinase C (PKC) Ca2+-dependent protein kinase that, when activated by diacylglycerol and an increase
in the concentration of Ca2+, phosphorylates target proteins on specific serine and threonine residues.
protein module see module
protein phosphatase see phosphoprotein phosphatase
protein phosphorylation The covalent addition of a phosphate group to a side chain of a protein catalyzed by a
protein kinase.
protein translocator Membrane-bound protein that mediates the transport of another protein across an
organelle membrane.
proteoglycan Molecule consisting of one or more glycosaminoglycan (GAG) chains attached to a core protein.
proteolysis Degradation of a protein by hydrolysis at one or more of its peptide bonds.
proteolytic enzyme see protease
protofilament A linear chain of protein subunits joined end to end, which associates laterally with other
protofilaments to form cytoskeletal components such as microtubules and intermediate filaments.
proton Positively charged subatomic particle that forms part of an atomic nucleus. Hydrogen has a nucleus
composed of a single proton (H+).
proton-motive force Driving force that moves protons across a membrane as a result of an electrochemical
proton gradient.
proto-oncogene Normal gene, usually concerned with the regulation of cell proliferation, that can be converted
into a cancer-promoting oncogene by mutation.
protozoa Free-living or parasitic, nonphotosynthetic, single-celled, motile eucaryotic organisms, such as
Paramecium and Amoeba. Free-living protozoa feed on bacteria or other microorganisms.
pseudogene Gene that has accumulated multiple mutations that has rendered it inactive and nonfunctional.
pseudopodium (pseudopodia) Large cell-surface protrusion formed by amoeboid cells as they crawl. More
generally, any dynamic actin-rich extension of the surface of an animal cell.
pulse-chase Technique for following the movement of a substance through a biochemical or cellular pathway,
by briefly adding the radioactively labeled substance (the pulse) followed by the unlabeled substance (the
chase).
pump Transmembrane protein that drives the active transport of ions or small molecules across the lipid bilayer.
purine One of the two categories of nitrogen-containing ring compounds found in DNA and RNA. Examples are
adenine and guanine. (See Panel 2 6, pp. 120 121.)
pyrimidine One of the two categories of nitrogen-containing ring compounds found in DNA and RNA. Cytosine,
thymine and uracil are pyrimidines. (See Panel 2 6, pp. 120 121.)
quaternary structure Three-dimensional relationship of the different polypeptide chains in a multisubunit
protein or protein complex.
quinone (Q) Small, lipid soluble, mobile electron carrier molecule found in the respiratory and photosynthetic
electron-transport chains. (See Figure 14 24.)
Rab protein Any of a large family of monomeric GTPases present in the plasma membrane and organelle
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Glossary
membranes that are involved in conferring specificity on vesicle docking.
radioactive isotope Form of an atom with an unstable nucleus that emits radiation as it decays.
Ran Monomeric GTPase present in both cytosol and nucleus that is required for the active transport of
macromolecules into and out of the nucleus through nculear pore complexes. Hydrolysis of GTP to GDP is
thought to provide the energy required for this transport.
Ras protein The most famous member of a large family of GTP-binding proteins (called monomeric GTPases)
that help relay signals from cell-surface receptors to the nucleus. Named for the ras gene, first identified in
viruses that cause rat sarcomas.
reaction In chemistry, any process in which one molecule is converted into another by the removal or addition
of atoms, or in which the arrangement of atoms in a molecule or molecules is altered by a change in chemical
bonds.
reading frame The phase in which nucleotides are read in sets of three to encode a protein. A messenger RNA
molecule can be read in any one of three reading frames, only one of which will give the required protein.
RecA protein The prototype for a class of DNA-binding proteins that catalyze synapsis of DNA strands during
genetic recombination.
receptor Protein that binds a specific extracellular signal molecule (ligand) and initiates a response in the cell.
Cell-surface receptors, such as the acetylcholine receptor and the insulin receptor, are located in the plasma
membrane, with their ligand-binding site exposed to the external medium. Intracellular receptors, such as
steroid hormone receptors, bind ligands that diffuse into the cell across the plasma membrane.
receptor-mediated endocytosis Internalization of receptor-ligand complexes from the plasma membrane by
endocytosis, It is used to take up some macromolecules, such as cholesterol-containing lipoproteins, from the
extracellular fluid, and is also a means of recycling receptor proteins once they have bound their ligands.
recessive In genetics, refers to the member of a pair of alleles that fails to be expressed in the phenotype of the
organism when the dominant allele is present. Also refers to the phenotype of an individual that has only the
recessive allele.
recombinant DNA Any DNA molecule formed by joining DNA segments from different sources. Recombinant
DNAs are widely used in the cloning of genes, in the genetic modification of organisms, and in molecular biology
generally.
recombination Process in which DNA molecules are broken and the fragments are rejoined in new
combinations. Can occur in the living cell
for example, through crossing-over during meiosis
or in vitro
using purified DNA and enzymes that break and ligate DNA strands.
recycling endosomes Large intracellular membrane-bounded vesicle formed from a fragment of an endosome
that is an intermediate stage on the passage of recycled receptors back to the cell membrane.
red blood cell see erythrocyte
redox pair Pair of molecules in which one acts as an electron donor and one as an electron acceptor in an
oxidation-reduction reaction; for example, NADH (electron donor) and NAD+ (electron acceptor).
redox potential The affinity of a redox pair for electrons, generally measured as the voltage difference between
an equimolar mixture of the pair and a standard reference. NADH/NAD+ has a low redox potential and O2/H2 has
a high redox potential (high affinity for electrons).
redox reaction A reaction in which one component becomes oxidized and the other reduced; an
oxidation-reduction reaction.
reduction (verb reduce) Addition of electrons to an atom, as occurs during the addition of hydrogen to a
molecule or the removal of oxygen from it. Opposite of oxidation. (See Figure 2 43.)
regulatory sequence DNA sequence to which a gene regulatory protein binds to control the rate of assembly of
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Glossary
the transcirptional complex at the promoter.
regulatory site Site on an enzyme, other than the active site, that binds a molecule that affects enzyme activity.
replication fork Y-shaped region of a replicating DNA molecule at which the two daughter strands are formed
and separate.
replication origin Location on a DNA molecule at which duplication of the DNA begins.
replicative cell senescence Phenomenon observed in primary cell cultures as they age, in which cell
proliferation slows down and finally halts.
repressor Protein that binds to a specific region of DNA to prevent transcription of an adjacent gene.
residue General term for the unit of a polymer. That portion of a sugar, amino acid, or nucleotide that is retained
as part of the polymer chain during the process of polymerization.
respiration General term for a process in a cell involving the oxidative breakdown of sugars or other organic
molecules, and requiring the uptake of O2 while producing CO2 and H2O as waste products.
respiratory chain Electron-transport chain in the inner mitochondrial membrane that receives high-energy
electrons derived from the citric acid cycle and generates the proton gradient across the membrane that is used
to power ATP synthesis.
respiratory control Regulatory mechanism that controls the rate of electron transport in the respiratory chain
according to need via a direct influence of the electrochemical proton gradient.
respiratory enzyme complex Any of the major protein complexes of the mitochondrial respiratory chain that
act as electron-driven proton pumps to generate the proton gradient across the inner membrane.
resting membrane potential The membrane potential in equilibrium conditions in which there is no net flow of
ions across the plasma membrane.
restriction map Diagrammatic representation of a DNA molecule indicating the sites of cleavage by various
restriction enzymes.
restriction nuclease (restriction enzyme) One of a large number of nucleases that can cleave a DNA
molecule at any site where a specific short sequence of nucleotides occurs. Extensively used in recombinant
DNA technology.
restriction point Important checkpoint in the mammalian cell cycle. Passage through the restriction point
commits the cell to enter S phase. It corresponds to Start in the yeast cell cycle.
retrotransposon Type of transposable element that moves by being first transcribed into an RNA copy that is
then reconverted to DNA by reverse transcriptase and inserted elsewhere in the chromosomes.
retrovirus RNA-containing virus that replicates in a cell by first making a double-stranded DNA intermediate.
reverse genetics Approach to discovering gene function that starts from the DNA (gene) and protein and then
creates mutants to analyze the gene's function.
reverse transcriptase Enzyme first discovered in retroviruses that makes a double-stranded DNA copy from a
single-stranded RNA template molecule.
rhodopsin G-protein-linked light-sensitive receptor protein in the rod photoreceptor cells of the retina.
ribonuclease Enzyme that cuts an RNA molecule by hydrolyzing one or more of its phosphodiester bonds.
ribonucleic acid see RNA
ribosomal RNA (rRNA) Any one of a number of specific RNA molecules that form part of the structure of a
ribosome and participate in the synthesis of proteins. Often distinguished by their sedimentation coefficient,
such as 28S rRNA or 5S rRNA.
ribosome Particle composed of ribosomal RNAs and ribosomal proteins that associates with messenger RNA
and catalyzes the synthesis of protein.
ribozyme RNA with catalytic activity.
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Glossary
RNA (ribonucleic acid) Polymer formed from covalently linked ribonucleotide monomers. (See also
messenger RNA, ribosomal RNA, transfer RNA.)
RNA editing Production of a functional mRNA by insertion or alteration of individual nucleotides in an RNA
molecule after it is synthesized.
RNA interference (RNAi) Selective intracellular degradation of RNA that is intended to remove foreign RNAs,
such as those of viruses. Fragments cleaved from free double-stranded RNA direct the degradative mechanism
to other similar RNA sequences. Widely exploited in a technique used to silence the expression of selected
genes.
RNA polymerase II holoenzyme Large pre-assumbled complex of RNA polymerase II, most of the general
transcription factors required for its function, and the mediator protein complex.
RNA polymerase Enzyme that catalyzes the synthesis of an RNA molecule on a DNA template from nucleoside
triphosphate precursors. (See Figure 6 8.)
RNA primer Short stretch of RNA synthesized on a DNA template. It is required by DNA polymerases to start
their DNA synthesis.
RNA processing control Control of gene expression by controlling how the RNA transcript is spliced or
otherwise processed.
RNA splicing Process in which intron sequences are excised from RNA transcripts in the nucleus during
formation of messenger and other RNAs.
RNAi see RNA interference
rod photoreceptor (rod) Photoreceptor cell type in the retina that is responsible for noncolor vision in dim light.
rough endoplasmic reticulum (rough ER) Endoplasmic reticulum with ribosomes on its cytosolic surface.
Involved in the synthesis of secreted and membrane-bound proteins.
rRNA see ribosomal RNA
rRNA gene Gene that specifies a ribosomal RNA (rRNA).
S phase Period of a eucaryotic cell cycle in which DNA is synthesized.
Saccharomyces Genus of yeasts that reproduce asexually by budding or sexually by conjugation.
Economically important in brewing and baking, they are also widely used in genetic engineering and as simple
model organisms in the study of eucaryotic cell biology.
sarcoma Cancer of connective tissue.
sarcomere Repeating unit of a myofibril in a muscle cell, composed of an array of overlapping thick (myosin)
and thin (actin) filaments between two adjacent Z discs.
sarcoplasmic reticulum Network of internal membranes in the cytoplasm of a muscle cell that contains high
concentrations of sequestered Ca2+ which is released into the cytosol during muscle excitation.
satellite DNA Regions of highly repetitive DNA from a eucaryotic chromosome, usually identifiable by its
unusual nucleotide composition. Satellite DNA is not transcribed and has no known function.
saturated Describes a molecule containing carbon carbon bonds that has only single covalent bonds.
scaffold protein Protein that organizes groups of interacting intracellular signaling proteins into signaling
complexes.
scanning electron microscope Type of electron microscope that produces an image of the surface of an
object.
S-Cdk Complex formed in vertebrate cells by an S-cyclin and the corresponding cyclin-dependent kinase (Cdk).
Schwann cell Glial cell responsible for forming myelin sheaths in the peripheral nervous system.
SDS polyacrylamide gel electrophoresis (SDS-PAGE) Type of electrophoresis in which the protein mixture to
be separated is run through a gel containing the detergent sodium dodecyl sulfate (SDS) which unfolds the
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Glossary
proteins and frees them from association with other molecules.
second messenger Small molecule that is formed in or released into the cytosol in response to an extracellular
signal and helps to relay the signal to the interior of the cell. Examples include cAMP, IP 3, and Ca2+.
secondary immune response Adaptive immune response to an antigen that is made on a second or
subsequent encounter with a given antigen. It is more rapid in onset, stronger, and more specific than the
primary immune response.
secondary structure
secretory vesicle Membrane-bounded organelle in which molecules destined for secretion are stored prior to
release. Sometimes called secretory granule because darkly staining contents make the organelle visible as a
small solid object.
section A very thin slice of tissue, suitable for viewing under the microscope.
selectin Member of a family of cell-surface carbohydrate-binding proteins that mediate transient,
Ca2+-dependent cell-cell adhesion in the bloodstream, for example between white blood cells and the
endothelium of the blood vessel wall.
selectivity filter That part of an ion channel structure that determines which ions it can transport.
septate junction Main type of occluding cell junction in invertebrates; their structure is distinct from that of
vertebrate tight junctions.
serine protease Type of protease that has a reactive serine in the active site.
sex chromosome Chromosome that may be present or absent, or present in a variable number of copies,
according to the sex of the individual. In mammals, the X and Y chromosomes.
sexual reproduction Type of reproduction in which the genomes of two individuals are mixed in the formation
of a new organism. Individuals produced by sexual reproduction differ from either of their parents and from each
other.
SH2 domain Src homology region 2, a protein domain present in many signaling proteins; it binds a short amino
acid sequence containing a phosphotyrosine.
side chain The part of an amino acid that differs between different amino acids, giving the amino acid its unique
physical and chemical properties.
signal molecule Extracellular or intracellular molecule that cues the response of a cell to the behavior of other
cells or objects in the environment.
signal patch Protein sorting signal that consists of a specific three-dimensional arrangement of atoms on the
folded protein's surface.
signal peptidase Enzyme that removes a terminal signal sequence from a protein once the sorting process is
complete.
signal-recognition particle (SRP) Ribonucleoprotein particle that binds an ER signal sequence on a partially
synthesized polypeptide chain and directs the polypeptide and its attached ribosome to the endoplasmic
reticulum.
signal sequence Short continuous sequence of amino acids that determines the eventual location of a protein
in the cell. An example is the N-terminal sequence of 20 or so amino acids that directs nascent secretory and
transmembrane proteins to the endoplasmic reticulum.
signal transduction Relaying of a signal by conversion from one physical or chemical form to another. In cell
biology, the process by which a cell converts an extracellular signal into a response.
single-nucleotide polymorphism (SNP) Variation between individuals at certain nucleotide positions in the
genome.
single-pass transmembrane protein Membrane protein in which the polypeptide chain crosses the lipid
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Glossary
bilayer only once.
single-strand DNA-binding protein Protein that binds to the single strands of the opened-up DNA double helix,
preventing helical structures from reforming while the DNA is being replicated.
sister chromatid see chromatid
site-directed mutagenesis Technique by which a mutation can be made at a particular site in DNA.
site-specific recombination Type of recombination that does not require extensive similarity in the two DNA
sequences undergoing recombination. Can occur between two different DNA molecules or within a single DNA
molecule.
small intracellular mediator see second messenger
small nuclear RNA (snRNA) Small RNA molecules that are complexed with proteins to form the
ribonucleoprotein particles involved in RNA splicing.
smooth endoplasmic reticulum (smooth ER) Region of the endoplasmic reticulum not associated with
ribosomes. It is involved in lipid synthesis.
smooth muscle cell Type of long, spindle-shaped mononucleate muscle cell making up the muscular tissue
found in the walls of arteries and of the intestine and other viscera, and in some other locations of the vertebrate
body. Called "smooth" because it lacks the striated myofibrils of skeletal and cardiac muscle cells.
SNAREs Large family of transmembrane proteins present in organelle membranes and the vesicles derived
from them. They are involved in guiding vesicles to their correct destinations. They exist in pairs
a v-SNARE
in the vesicle membrane that binds specifically to a complementary t-SNARE in the target membrane.
SNP see single-nucleotide polymorphism
snRNA see small nuclear RNA
solute Any molecule that is dissolved in a liquid. The liquid is called a solvent.
somatic cell Any cell of a plant or animal other than a germ cell or germ-cell precursor. (From Greek soma,
body.)
somite One of a series of paired blocks of mesoderm that form during early development and lie on either side
of the notochord in a vertebrate embryo. They give rise to the vertebral column, muscles and associated
connective tissue. Each somite produces the musculature of one vertebral segment, plus associated connective
tissue.
sorting signal Amino acid sequence that directs the delivery of a protein to a specific location outside the
cytosol.
Southern blotting Technique in which DNA fragments separated by electrophoresis are immobilized on a
paper sheet. Specific fragments are then detected with a labeled nucleic acid probe. (Named after E.M.
Southern, inventor of the technique.)
spectrin Abundant protein associated with the cytosolic side of the plasma membrane in red blood cells,
forming a rigid network that supports the membrane.
Spemann's Organizer Specialized tissue at the dorsal lip of the blastopore in an amphibian embryo; a source
of signals that help to orchestrate formation of the embryonic body axis. (After H. Spemann and H. Mangold,
co-discoverers.)
sperm (spermatozoon,spermatozoa) The mature male gamete in animals. It is motile and usually small
compared with the egg.
spermatogenesis Development of sperm.
spindle-attachment checkpoint Checkpoint that operates during mitosis to ensure that all chromosomes are
properly attached to the spindle before sister-chromatid separation starts.
spliceosome Large assembly of RNA and protein molecules that performs pre-mRNA splicing in eucaryotic
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Glossary
cells.
Src family Family of cytoplasmic tyrosine kinases (pronounced "sark") that associate with the cytoplasmic
domains of some enzyme-linked receptors (for example, the T cell antigen receptor) that lack intrinsic tyrosine
kinase activity. They transmit a signal onwards by phosphorylating the receptor itself and other signaling
proteins.
SRP see signal-recognition particle
standard free-energy change ( G°) Free-energy change of two reacting molecules at standard temperature
and pressure when all components are present at a concentration of 1 mole per liter.
starch Polysaccharide composed exclusively of glucose units, used as an energy storage material in plant cells.
start-transfer signal Short amino-acid sequence that enables a polypeptide chain to start being translocated
across the endoplasmic reticulum membrane through a protein translocator. Multipass membrane proteins have
both N-terminal (signal sequence) and internal start-transfer signals.
stem cell Relatively undifferentiated cell that can continue dividing indefinitely, throwing off daughter cells that
can undergo terminal differentiation into particular cell types.
stereocilium A large, rigid microvillus found in "organ pipe" arrays on the apical surface of hair cells in the ear.
A stereocilium contains a bundle of actin filaments, rather than microtubules, and is thus not a true cilium.
steroid Hydrophobic lipid molecule with a characteristic four-ringed structure. Many important hormones such
as estrogen and testosterone are steroids.
stimulatory G protein (Gs) G protein that, when activated, activates the enzyme adenylyl cyclase and thus
stimulates the production of cyclic AMP.
stop-transfer signal Hydrophobic amino acid sequence that halts translocation of a polypeptide chain through
the endoplasmic reticulum membrane, thus anchoring the protein chain in the membrane
strand-directed mismatch repair see mismatch repair
striated muscle Muscle composed of transversely striped (striated) myofibrils. Skeletal and heart muscle of
vertebrates are the best-known examples.
stroma (1) The connective tissue in which a glandular or other epithelium is embedded. (2) The large interior
space of a chloroplast, containing enzymes that incorporate CO2 into sugars.
structural gene Region of DNA that codes for a protein or for an RNA molecule that forms part of a structure or
has an enzymatic function. Distinguished from regions of DNA that regulate gene expression.
substrate Molecule on which an enzyme acts.
substratum Solid surface to which a cell adheres.
subunit Component of a multicomponent complex
for example, one protein component of a protein complex
or one polypeptide chain of a multichain protein.
sucrose Disaccharide composed of one glucose unit and one fructose unit. The major form in which glucose is
transported between plant cells.
sugar Small carbohydrates with a monomer unit of general formula (CH 2O)n. Examples are the
monosaccharides glucose, fructose and mannose, and the disacharide sucrose (composed of a molecule of
glucose and one of fructose linked together).
sulfhydryl (thiol,
SH) Chemical group containing sulfur and hydrogen found in the amino acid cysteine and
other molecules. Two sulfhydryls can join to produce a disulfide bond.
supercoiled DNA Region of DNA in which the double helix is further twisted on itself.
survival factor Extracellular signal required for a cell to survive; in its absence the cell will undergo apoptosis
and die.
symbiosis Intimate association between two organisms of different species from which both derive a long-term
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Glossary
selective advantage.
symporter Carrier protein that transports two types of solute across the membrane in the same direction.
synapse Communicating cell cell junction that allows signals to pass from a nerve cell to another cell. In a
chemical synapse the signal is carried by a diffusible neurotransmitter; in an electrical synapse a direct
connection is made between the cytoplasms of the two cells via gap junctions.
synapsis (1) In genetic recombination, the initial formation of base pairs between complementary DNA strands
in different DNA molecules that occurs at sites of crossing-over between chromosomes. (2) In meiosis, the
pairing of maternal and paternal copies of a chromosome as they become attached to each other along their
length.
synaptic signaling Type of cell cell communication that occurs across chemical synapses in the nervous
system.
synaptic vesicle Small neurotransmitter-filled secretory vesicle formed at the axon terminals of nerve cells and
whose contents are released into the synaptic cleft by exocytosis when an action potential reaches the axon
terminal.
synaptonemal complex Structure that holds paired chromosomes together during prophase I of meiosis and
promotes genetic recombination.
syncytium Mass of cytoplasm containing many nuclei enclosed by a single plasma membrane. Typically the
result either of cell fusion or of a series of incomplete division cycles in which the nuclei divide but the cell does
not.
synteny The presence in different species of regions of chromosomes with the same genes in the same order.
T cell (T lymphocyte) Type of lymphocyte responsible for cell-mediated immunity; includes both cytotoxic T
cells and helper T cells.
TATA box Consensus sequence in the promoter region of many eucaryotic genes that binds a general
transcription factor and hence specifies the position at which transcription is initiated.
TCA cycle see citric acid cycle
telomerase Enzyme that elongates telomere sequences in DNA.
telomere End of a chromosome, associated with a characteristic DNA sequence that is replicated in a special
way. Counteracts the tendency of the chromosome otherwise to shorten with each round of replication. (From
Greek telos, end.)
telophase Final stage of mitosis in which the two sets of separated chromosomes decondense and become
enclosed by nuclear envelopes.
temperature-sensitive (ts) mutant Organism or cell carrying a genetically altered protein (or RNA molecule)
that performs normally at one temperature but is abnormal at another (usually higher) temperature.
template A single strand of DNA or RNA whose nucleotide sequence acts as a guide for the synthesis of a
complementary strand.
terminator Signal in bacterial DNA that halts transcription.
tertiary structure Complex three-dimensional form of a folded polymer chain, especially a protein or RNA
molecule.
TGF-
see transforming growth factor-
TGN see trans Golgi network (TGN)
thioester bond High-energy bond formed by a condensation reaction between an acid (acyl) group and a thiol
group ( SH); seen, for example, in acetyl CoA and in many enzyme-substrate complexes.
thiol see sulfhydryl
thylakoid Flattened sac of membrane in a chloroplast that contains chlorophyll and other pigments and carries
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Glossary
out the light-trapping reactions of photosynthesis. Stacks of thylakoids form the grana of chloroplasts.
tight junction Cell cell junction that seals adjacent epithelial cells together, preventing the passage of most
dissolved molecules from one side of the epithelial sheet to the other.
TIM complexes Protein translocators in the mitochondrial inner membrane. The TIM23 complex mediates the
transport of proteins into the matrix and the insertion of some proteins into the inner membrane; the TIM22
complex mediates the insertion of a subgroup of proteins into the inner membrane.
Toll-like receptor family (TLR) Important family of mammalian pattern recognition receptors abundant on
macrophages, neutrophils and the epithelial cells of the gut. They recognize pathogen-associated
immunostimulants such as lipopolysacharide and peptidoglycan.
TOM complex Multisubunit protein complex that transports proteins across the mitochondrial outer membrane.
topoisomerase (DNA topoisomerase) Enzyme that makes reversible cuts in a double-helical DNA molecule
for the purpose of removing knots or unwinding excessive twists.
tracer Molecule or atom that has been labeled either chemically or radioactively so that it can be followed in a
biochemical process or readily located in a cell or tissue.
trans face Face of a Golgi stack at which material leaves the organelle for the cell surface or another cell
compartment. It is adjacent to the trans Golgi network.
trans Golgi network (TGN) Network of interconnected cisternae and tubules at the trans face of the Golgi
apparatus, through which material is transferred out of the Golgi.
transcellular transport Transport of solutes, such as nutrients, across an epithelium, by means of membrane
transport proteins in the apical and basal faces of the epithelial cells.
transcript RNA product of DNA transcription.
transcription (DNA transcription) Copying of one strand of DNA into a complementary RNA sequence by the
enzyme RNA polymerase.
transcription attenuation Inhibition of gene expression in bacteria by the premature termination of transcription.
transcription factor Term loosely applied to any protein required to initiate or regulate transcription in
eucaryotes. Includes both gene regulatory proteins as well as the general transcription factors.
transcriptional control Control of of gene expression by controlling when and how often the gene is
transcribed.
transcytosis The uptake of material at one face of a cell by endocytosis, its transfer across a cell in vesicles,
and its discharge from another face by exocytosis.
transfection Introduction of a foreign DNA molecule into a eucaryotic cell. It is usually followed by expression of
one or more genes in the newly introduced DNA.
transfer RNA (tRNA) Set of small RNA molecules used in protein synthesis as an interface (adaptor) between
messenger RNA and amino acids. Each type of tRNA molecule is covalently linked to a particular amino acid.
transforming growth factor-
-
Large family of structurally related, secreted
proteins that act as hormones and local mediators to control a wide range of functions in animals, including
during development. It includes TGF-
enetic proteins (BMPs).
transgenic organism Plant or animal that has stably incorporated one or more genes from another cell or
organism and can pass them on to successive generations.
transition state Structure that forms transiently in the course of a chemical reaction and has the highest free
energy of any reaction intermediate. Its formation is a rate-limiting step in the reaction.
translation (RNA translation) Process by which the sequence of nucleotides in a messenger RNA molecule
directs the incorporation of amino acids into protein. It occurs on a ribosome.
translational control Control of gene expression by selection of which mRNAs in the cytoplasm are translated
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Glossary
by ribosomes.
translocation Type of mutation in which a portion of one chromosome is broken off and attached to another.
transmembrane protein Membrane protein that extends through the lipid bilayer, with part of its mass on either
side of the membrane.
transmitter-gated ion channel Ion channel in the postsynaptic plasma membranes of nerve and muscle cells
that opens only in response to the binding of a specific extracellular neurotransmitter. The resulting inflow of ions
leads to the generation of a local electrical signal in the postsynaptic cell.
transposable element Segment of DNA that can move from one position in a genome to another. Also called a
transposon.
transposition The movement of a DNA sequence from one site to another within the genome. See also
cut-and-paste transposition.
trans-splicing Type of RNA splicing present in a few eucaryotic organisms in which exons from two separate
RNA molecules are joined together to form an mRNA.
treadmilling The process by which a polymeric protein filament is maintained at constant length by addition of
protein subunits at one end and loss of subunits at the other. (See Panel 16 2, pp. 912 913.)
triacylglycerol Molecule composed of three fatty acids esterified to glycerol. The main constituent of fat
droplets in animal tissues (where the fatty acids are saturated) and of vegetable oils (where the fatty acids are
mainly unsaturated). Also known as triglyceride. (See Panel 2 5, pp. 118 119.)
tricarboxylic acid (TCA) cycle see citric acid cycle
trimeric GTP-binding protein see GTP-binding protein
tRNA see transfer RNA
t-SNARE see SNAREs
tubulin The protein subunit of microtubules.
tumor progression The process by which an initial mildly disordered cell behavior gradually evolves into a
full-blown cancer.
tumor suppressor gene Gene that appears to prevent formation of a cancer. Loss-of-function mutations in
such genes enhance susceptibility to cancer.
two-dimensional gel electrophoresis Type of electrophoresis in which the protein mixture is run first in one
direction and then in a direction at right angles to the first. It enables better separation of individual proteins.
two-hybrid system Technique for identifying interacting proteins using genetically engineered yeast cells.
type III secretion system A bacterial system for delivering toxic proteins into the cells of their host.
ubiquitin Small, highly conserved protein present in all eucaryotic cells that becomes covalently attached to
lysines of other proteins. Attachment of a short chain of ubiquitins to such a lysine tags a protein for intracellular
proteolytic destruction by a proteasome.
ubiquitin ligase Any one of a large number of enzymes that attach ubiquitin to a protein, thus marking it for
destruction in a proteasome. The process catalyzed by a ubiquitin ligase is called ubiquitylation.
unfolded protein response Cellular response triggered by an accumulation of misfolded proteins in the
endoplasmic reticulum. It involves increased transcription of ER chaperones and degradative enzymes.
uniporter Carrier protein that transports a single solute from one side of the membrane to the other.
unsaturated Describes a molecule that contains one or more double or triple carbon-carbon bonds, such as
isoprene or benzene.
V gene segment Gene segment encoding most of the variable region of the polypeptide chains of
immunoglobulins and T cell receptors.
V (D) J joining Recombination process by which gene segments are brought together to form a functional gene
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Glossary
for a polypeptide chain of an immunoglobulin or T cell receptor.
vacuole Very large fluid-filled vesicle found in most plant and fungal cells, typically occupying more than a third
of the cell volume.
van der Waals attraction Type of (individually weak) noncovalent bond that is formed at close range between
nonpolar atoms.
variable region Region of an immunoglobulin light or heavy chain that differs from molecule to molecule; it
comprises the antigen-binding site.
vector In cell biology, the DNA of an agent (virus or plasmid) used to transmit genetic material to a cell or
organism. (See also cloning vector, expression vector.)
vegetal pole The end at which most of the yolk is located in an animal egg. The end opposite the animal pole.
ventral Situated toward the belly surface of an animal, or towards the underside of a wing or leaf.
vesicle Small, membrane-bounded, spherical organelle in the cytoplasm of a eucaryotic cell.
vesicular transport Transport of proteins from one cellular compartment to another by means of
membrane-bounded intermediaries such as vesicles or organelle fragments.
virulence gene Gene that contributes to an organism's ability to cause disease.
virus Particle consisting of nucleic acid (RNA or DNA) enclosed in a protein coat and capable of replicating
within a host cell and spreading from cell to cell. Many viruses cause disease.
voltage-gated cation channel Type of ion channel found in the membranes of excitable cells (such as nerve
cells and muscle) which opens in response to a shift in membrane potential past a threshold value.
v-SNARE see SNAREs
Western blotting Technique by which proteins are separated by electrophoresis and immobilized on a paper
sheet and then analyzed, usually by means of a labeled antibody.
white blood cell (leucocyte) General name for all the nucleated blood cells lacking hemoglobin. Includes
lymphocytes, neutrophils, eosinophils, basophils, and monocytes.
wild-type Normal, nonmutant form of an organism; the form found in nature (in the wild).
Xenopus laevis (South African clawed toad) Species of frog (not toad) frequently used in studies of early
vertebrate development.
XIC see X-inactivation center
X-inactivation Inactivation of one copy of the X chromosome in the somatic cells of female mammals.
X-inactivation center (XIC) Site in an X chromosome at which inactivation is initiated and spreads outwards.
X-ray crystallography Technique for determining the three-dimensional arrangement of atoms in a molecule
based on the diffraction pattern of X-rays passing through a crystal of the molecule.
yeast Common term for several families of unicellular fungi. Includes species used for brewing beer and making
bread, as well as pathogenic species (that is, species that cause disease).
yolk Nutritional reserves rich in lipids, proteins and polysaccharides, present in the eggs of many animals.
Z disc (Z line) Platelike region of a muscle sarcomere to which the plus ends of actin filaments are attached.
Seen as a dark transverse line in micrographs.
zinc finger DNA-binding structural motif present in many gene regulatory proteins. Composed of a loop of
polypeptide chain held in a hairpin bend bound to a zinc atom.
zona pellucida Glycoprotein layer on the surface of the unfertilized egg. It is often a barrier to fertilization across
species.
zygote Diploid cell produced by fusion of a male and female gamete. A fertilized egg.
zygotene Second stage of division I of meiosis, in which the synaptonemal complex begins to form between the
two sets of sister chromatids in each bivalent chromosome.
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