Saladin 5e Extended Outline
Chapter 4
Genetics and Cellular Function
I. Genes and Nucleic Acids (pp. 124–129)
A. Johann Friedrich Miescher found in 1869 that cell nuclei contained a substance he called
nuclein, which we now know to be deoxyribonucleic acid (DNA). (p. 124)
B. The DNA molecule and other nucleic acids are composed of nucleotides, each of which is made
up of a sugar, a phosphate group, and a nitrogenous base. (p. 124–127)
1. Three bases, cytosine (C), thymine (T), and uracil, are pyrimidines; two bases, adenine
(A) and guanine (G) are purines. (Fig. 4.1a, b)
2. The structure of DNA resembles a spiral staircase; each sidepiece is a backbone
composed of phosphate groups alternative with the sugar deoxyribose. (Fig. 4.2)
3. The nitrogenous bases form the stepwise connections between the two backbones.
a. Adenine and thymine form two hydrogen bonds with each other; guanine and
cytosine form three hydrogen bonds. (Fig. 4.2b)
b. The A–T and C–G combinations are called base pairs
c. The law of complementary base pairing allows prediction of the sequence of
one strand if the other strand’s sequence is known.
3. The essential function of DNA is to code for the proteins a cell synthesizes.
Insight 4.1 Discovery of the Double Helix (Fig. 4.3)
C. DNA is complexed with proteins to form a fine filamentous material called chromatin. (p. 127–
128)
1. Human chromatin consists of 46 long filaments called chromosomes.
2. The DNA is about 2 m in length if fully unwound but is packed into the nucleus.
3. View with electron microscopy, the chromatin has an appearance like beads on a string
and is divided into segments called nucleosomes. (Fig. 4.4a)
a. Each nucleosome consists of a core particle and a short segment of linker
DNA.
b. The core particle is a disk-shaped cluster of eight proteins called histones with
the DNA wound around it like ribbon around a spool. (Fig. 4.4b)
4. Nucleosomes are arranged in a zigzag pattern, folding the chromatin like an accordion
5. The folded strand is then packed into irregular loops and coils that make up the
chromosome.
6. Each chromosome is packed into its own region of the nucleus called a chromosome
territory, permeated with channels.
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7. When a chromosome is preparing to divide, it copies all its nuclear DNA, such that
each chromosome is made up of two sister chromatids that then coil further; these are
visible in the light microscope.
a. In early cell division, a chromosome is compacted to its greatest extent.
b. Sister chromatids are joined at a centromere, which has a kinetochore on each
side. (Fig. 4.5)
D. Ribonucleic acid (RNA) is produced in three types: messenger RNA (mRNA), ribosomal RNA
(rRNA), and transfer RNA (tRNA). (p. 128)
1. RNA and DNA have significant differences although both are nucleic acids. (Table
4.1)
a. RNA is much smaller than DNA, ranging from 70 to 90 bases in tRNA to
over 10,000 bases in the largest mRNA; DNA averages more than 100 million
base pairs.
b. RNA is a single nucleotide chain, not a double helix.
c. The sugar in RNA is ribose, not deoxyribose; and thymine is replaced by
uracil. (Fig. 4.2)
2. The essential function of RNA is to interpret the code in DNA and direct the synthesis
of proteins.
II. Genes and Their Action (pp. 129–138)
A. The classical concept of a gene was as an abstract “unit of heredity” by which a trait passes to
offspring. (p. 129)
B. We now define a gene as an information-containing segment of DNA that codes for the
production of a molecule of RNA, which in most cases goes on to play a role in protein synthesis.
(p. 129)
C. The 46 human chromosomes come in two sets of 23 each, one set from each parent; all the
DNA in one 23-chromosome set is called the genome. (pp. 129–130
1. The human genome consists of about 3.1 billion nucleotide pairs.
2. The Human Genome Project (HGP) revealed the base sequence of more than 99% of
the genome.
3. Genomics is a new field of biology that studies a genome and how its genes and
noncoding DNA interact.
4. Revelations from the HGP include six findings major findings.
a. Humans have 25,000 to 35,000 genes, not 100,000 as was thought.
b. These genes generate million of different proteins; a single gene can code for
many proteins through alternative splicing of mRNA.
c. Genes average about 3,000 bases long but range up to 2.4 million bases.
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d. All humans are at least 99.99% genetically identical, but even this small
variation means differences in more than 3 million base pairs.
i. Combinations of these differences, called single-nucleotide
polymorphisms, account for all human genetic variation.
Insight 4.2 Genomic Medicine
D. The genetic code is a system that enables four nucleotides to code for the amino acid sequences
of all proteins. (p. 130–131)
1. The minimum code to symbolize 20 amino acids is 3 nucleotides per amino acid.
a. Each group of three nucleotides coding for an amino acid is called a base
triplet.
b. Messenger RNA contains a “mirror image” of each base triplet, and this
group of three bases is called a codon.
2. The possible combinations of three bases would yield a maximum of 64 codons;
therefore, some amino acids are specified by more than one codon.
3. The code also contains stop codons that end the translation to amino acids and a start
codon that also codes for methionine. (Table 4.2)
E. DNA and RNA collaborate to make proteins. (p. 131–134) (Fig. 4.10)
1. The genetic code specifies which proteins a cell can make.
2. Different cells have different active genes; a given cell uses only 1/3 to 2/3 of its
genes.
3. When a gene is activated, an mRNA copy of a gene is made; this migrates to the
cytoplasm where its code is “read” by a ribosome.
a. Ribosomes are composed of rRNA and enzymes.
b. tRNA carries amino acids to the ribosome, where they are linked to form a
protein chain.
4. Transcription is the process by which DNA is copied into RNA.
a. RNA polymerase is the enzyme that binds to the DNA and assembles the
RNA strand.
i. Certain DNA sequences such as TATATA tell the RNA polymerase
where to begin.
ii. The enzyme opens up the DNA helix, reads the bases in one strand,
and matches them with complementary bases.
iii. At the end of the gene is a terminator sequence that tells the enzyme
to stop.
b. The RNA produced is pre-mRNA that contains sense portions called exons
and nonsense portions called introns.
i. Enzymes excise the introns and splice together the exons.
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ii. Through alternative splicing, a gene can code for more than one
protein. (Fig. 4.6)
5. Translation is the process by which the language of nucleotides is converted into the
language of amino acids. (Fig. 4.7)
a. This process is carried out by ribosomes, which consist of two subunits, a
large subunit and a small subunit.
b. The mRNA begins with a leader sequence to which a ribosome may attach;
the small subunit binds first, the large subunit joins it, and the mRNA is pulled
upon like a ribbon.
c. tRNA is a molecules that coils on itself to form an angular L shape that has a
3-nucleotide anticodon on one end, and a binding site for one amino acid on the
other. (Fig. 4.8)
i. An ATP is used to bind an amino acid to its site on a tRNA and later
this energy is used to form the bond between amino acids.
d. The small ribosomal subunit reads codons on the mRNA transcript and
matches them with anticodons on charged tRNA molecules.
i. Some mismatches are tolerated in the third base of the codon; this is
called “wobble,” and it allows 48different tRNAs to pair up with the
61 codons the represent amino acids.
e. When the correct charged tRNA is in position, the large ribosomal subunit
connects the amino acid the tRNA carries to the growing chain of amino acids
that is becoming the protein.
f. One mRNA can hold 10 or 20 ribosomes in a cluster called a polyribosome,
and the mRNA is translated by all of these ribosomes at once. (Fig. 4.9)
6. Transcription and translation are sequential processes that begin with DNA and end
with a protein product. (Fig. 4.10)
F. Protein synthesis requires more than the primary sequence of amino acids; it also involves
processing that folds and coils the protein chain into secondary and tertiary structures, and in some
cases association with other protein chains (quaternary structure), or binding with other molecules.
(pp. 134–136)
1. Chaperone proteins guide a new protein strand into proper configuration; some of these
are called stress proteins or heat-shock proteins because they are produced under these
conditions.
2. The endoplasmic reticulum (ER) and the Golgi complex have important roles in
protein processing and secretion. (Fig. 4.11)
a. A protein assembled on the ER surface moves into the ER cisterna where it is
modified by enzymes (posttranslational modification).
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b. The ER pinches off a bubblelike transport vesicle coated with clathrin that
holds completed proteins; these transport vesicles may fuse, and they move to
the Golgi complex.
c. The cluster of vesicles fuses with the Golgi complex, releasing its contents
into the Golgi cisterna.
d. The Golgi complex modifies proteins further, often by adding carbohydrate
chains.
e. The Golgi cisterna farthest from the ER buds off or breaks up into Golgi
vesicles.
f. Some of the Golgi vesicles become lysosomes, while others become secretory
vesicles that merge with the plasma membrane, releasing cell products by
exocytosis.
g. Proteins synthesized in this way have a variety of destinations and functions.
(Table 4.3)
G. Genes are regulated by a number of factors that turn them on or off as their products are
needed. (p. 136–137)
1. The production of casein by mammary gland cells provides an example of gene
regulation. (Fig. 4.12)
a. The hormone prolactin binds to receptors in the plasma membrane of a
mammary gland cell.
b. The binding to receptors triggers activation of a regulatory protein in the
cytoplasm.
c. The regulatory protein moves into the nucleus and binds to DNA near the
gene for casein, a milk protein.
d. This binding enables RNA polymerase to transcribe the gene, producing
mRNA.
e. Casein mRNA moves to the cytoplasm and is translated by ribosomes on the
rough ER.
f. The Golgi complex packages casein into secretory vesicles.
g. The vesicles release casein outside the cells, and it becomes part of breast
milk.
3. Regulatory proteins can turn gene transcription on or off in a number of ways,
including attracting and positioning RNA polymerase, and coiling or uncoiling DNA in a
nucleosome to make it accessible.
H. Compounds other than proteins are synthesized in cells, and they are made through the activity
of enzymes, which are proteins encoded by genes. (p. 137)
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1. There is no gene for the steroid testosterone, but a cell of the testis enzymatically
converts cholesterol to testosterone if genes for the enzymes are activated. (Fig. 4.13)
2. DNA codes for RNA and protein synthesis, but in this way indirectly controls
synthesis of a wide range of substances.
III. DNA Replication and the Cell Cycle (pp. 138–142)
A. DNA replication is necessary prior to cell division, and it is accomplished through
complementary base pairing. (p. 138) (Fig. 4.14)
1. The first step is the unwinding of the double helix from the histones.
2. An enzyme called DNA helicase opens up a short segment of the helix, exposing the
bases; the point where the DNA is opened is called the replication fork.
3. Molecules of DNA polymerase match the exposed bases with complementary free
nucleotides.
a. The two strands are copied by separate DNA polymerase molecules moving
in opposite directions.
i. One polymerase makes a long, continuous strand; the other makes
short strands moving away from the replication fork, and these are
joined together by DNA ligase.
b. Two new daughter DNA molecules are thus synthesized from the original
parental molecule, and each daughter DNA consists of one old (conserved) helix
and one newly synthesized helix.
i. This process is therefore called semiconservative replication.
4. While DNA is synthesized in the nucleus, new histones are being synthesized in the
cytoplasm and are transported into the nucleus for combination with each new DNA helix
to make new nucleosomes.
5. Thousands of polymerase molecules work simultaneously on each DNA molecule.
B. Changes in DNA structure are called mutations and result from replication errors or
environmental factors. (pp. 138–139)
1. DNA polymerase may make errors in copying, but there are many modes of error
correction.
2. Some mutations have no effect because they do not change the amino acid for which
they code, for example, TGG and TGC both code for threonine. (Table 4.2)
3. Since 98% of the DNA does not code for proteins, the majority of mutations do not
affect protein structure at all.
C. The cell cycle is the life cycle period from one cell division to the next; it has four main phases:
G1, S, G2, and M. (pp. 139–140) (Fig. 4.15)
1. G1, the first gap phase, is an interval between cell division and DNA replication.
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a. During this time the cell synthesizes proteins, grows, and carries out its tasks
in support of the body.
b. Cells in G1 also accumulate the materials needed to replicate DNA in the next
phase.
2. S, the synthesis phase, is the period during which the cell makes a duplicate copy of its
centrioles and all of its nuclear DNA.
3. G2, the second gap phase, is a relatively brief interval during which the cell finishes
replicating its centrioles and synthesizes enzymes that control cell division; it also check
replication for errors.
4. M, the mitotic phase, is the period during which a cell replicates its nucleus and then
pinches in two to form two new daughter cells.
5. Phases G1, S, and G2 are collectively called interphase—the time between M phases.
6. The length of the cell cycle varies depending on cell type; skin cells divide rapidly;
bone cells slowly, and skeletal muscle and nerve cells not at all.
7. Some cells cease to divide for days, years, or the rest of one’s life; these are said to be
in G0 (G-zero) phase.
a. An inability to stop cycling and enter G0 is characteristic of cancer cells.
D. Cells divide by two mechanisms: mitosis and meiosis; meiosis is restricted to production of
eggs and sperm, and mitosis serves all the other functions of cell division. (pp. 140–142)
1. Mitosis is responsible for development of an individual from a 1-celled fertilized egg.
2. Mitosis brings about growth of all tissues and organs after birth.
3. Cells that die are replaced by mitosis.
4. Repair of damaged tissues is accomplished by mitosis.
5. Mitosis has four phases: Prophase, metaphase, anaphase, and telophase. (Fig. 4.16)
a. Prophase is the period during which the chromosomes shorten and thicken,
becoming compact rods, and then align in the middle of the cell.
i. There are two chromatids per chromosome.
ii. The nuclear envelope disintegrates and the chromosomes are
released into the cytosol.
iii. The centrioles sprout elongate microtubules called spindle fibers,
which push the centrioles apart.
iv. Some spindle fibers grow toward the chromosomes and attach to the
kinetochore on each side of the centromere. (Fig. 4.5)
v. The chromosomes are tugged back and forth until they line up at the
cell’s midline.
b. Metaphase is the period during which the aligned chromosomes, anchored by
the mitotic spindle, await the signal to split apart.
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c. Anaphase is the cleaving of the centromere of each chromatid pair. After
splitting apart, each chromatid is a daughter chromosome.
i. One daughter chromosome migrates to one pole, and the other
daughter chromosome to the opposite pole.
ii. Migration is achieved by motor proteins in the kinetochores crawling
along the spindle fiber.
iii. Sister chromatids/daughter chromosomes are genetically identical to
each other, and so the daughter cells that result are genetically identical.
d. Telophase is the last phase of mitosis, during which the daughter
chromosomes are clustered on each side of the cell; a new nuclear envelope
forms and the chromosomes begin to uncoil, while the mitotic spindle vanishes.
4. Cytokinesis overlaps telophase; it is the division of the cytoplasm into two cells.
a. The motor protein myosin pulls on microfilaments of actin in the membrane
skeleton, creating a crease called the cleavage furrow.
b. The parent cell eventually pinches in two as the furrow is deepened, and the
daughter cells enter interphase.
E. The timing of cell division and its inhibition are areas of intense research. (p. 142)
1. Cells divide when a number of conditions are met:
a. When they grow large enough to have sufficient cytoplasm for two daughter
cells.
b. When they have replicated their DNA.
c. When they receive an adequate supply of nutrients.
d. When the are stimulated by growth factors.
e. When neighboring cells die, creating space for new cells.
2. Cells cease dividing under other conditions:
a. When nutrients are limited.
b. When growth factors are removed.
c. When they are snugly in contact with neighboring cells, a phenomenon called
contact inhibition.
IV. Chromosomes and Heredity (pp. 142–147)
A. Heredity is the transmission of genetic characteristics from parent to offspring. (p. 142)
B. A karyotype is a chart of the chromosomes, arranged in pairs and by size and other physical
features. (pp. 142–143) (Fig. 4.17)
1. The two members of each chromosome pair are called homologous chromosomes; one
is inherited from the mother and one from the father.
2. Homologous chromosomes generally look alike with the exception of the sex
chromosomes (X and Y); the non-sex chromosomes are called autosomes.
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3. A female has two homologous X chromosomes; a male has one X chromosome and a
smaller Y chromosome.
4. Any cell with 23 pairs of chromosomes is said to diploid; sperm and egg cells,
however, are haploid, having half as many chromosomes.
a. Sperm and eggs, and cells that will become these cells, are called germ cells.
b. All other cells of the body are called somatic cells.
5. At fertilization, a sperm cell and egg cell fuse, restoring the diploid number.
C. Each chromosome carries many genes; the location of a gene is called its locus, and
homologous chromosomes may carry different forms of a single gene, called alleles, at the same
locus. (pp. 143–144)
1. Alleles produce alternative forms of a trait.
2. Frequently, one allele is dominant, and the other allele is recessive.
a. A dominant allele masks the effect of a recessive allele; recessive alleles are
expressed only when present on both homologous chromosomes.
b. Typically, dominant alleles code for a normal, functional protein and
recessive alleles for a nonfunctional variant.
c. An example of a dominant trait is the trait for cleft chin. (Fig. 4.18)
3. Individuals with two identical alleles at a locus are said to be homozygous for that
trait; individuals with different alleles at a locus are said to be heterozygous.
4. The alleles that an individual possesses constitute the genotype; an observable trait is
called the phenotype.
5. An allele is said to be expressed if it appears in the phenotype.
a. The only way most recessive alleles can be expressed is if an individual
inherits them from both parents.
6. Recessive traits can skip one or more generations.
a. A diagram called a Punnett square shows how heterozygous parents can
produce a child with a homozygous recessive trait.
b. Each parent produces two types of germ cells; the phenotype of 75% of
children would be the dominant phenotype, but 25% would have the recessive
phenotype.
c. Heterozygous individuals with normal phenotypes may be carriers of
hereditary diseases in the form of recessive alleles, such in the case of cystic
fibrosis.
d. For some hereditary diseases, tests are available, and a genetic counselor can
have tests performed and advise potential parents of risk.
D. Some genes have multiple alleles within the gene pool of the population. (p. 144–145)
1. The ABO blood types have two dominant alleles, IA and IB, and one recessive allele, i.
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a. To have type A blood, a person must have either IA IA or IA i alleles.
b. To have type B blood, a person must have either IB IB or IB i alleles.
c. To have type AB blood, a person must have IA IB alleles.
d. To have type O blood, a person must have i i alleles.
2. The two dominant alleles for the ABO blood types are an example of alleles that are
equally dominant, or codominant.
a. The A and B alleles code for enzymes that produce different surface
glycolipids on red blood cells.
b. In type AB, both types of glycolipids are present; in type O, neither type is
present.
3. Some alleles exhibit incomplete dominance, in which the phenotype is intermediate if
two alleles are present.
E. Polygenic or multiple gene inheritance occurs when genes at two or more loci, even on different
chromosomes, contribute to a single phenotypic trait. (p. 145) (Fig. 4.19)
1. Human eye and skin color are examples of polygenic traits.
F. Pleiotropy occurs when a single gene produces multiple phenotypic effects. (p. 145)
1. Alkaptonuria is a disorder caused by a mutation on chromosome 3 that blocks the
breakdown of tyrosine, resulting in accumulation of homogenistic acid. (Fig. 4.20)
a. Oxidation of homogenistic acid causes connective tissues to turn gray to
bluish black and degeneration of these tissues.
b. The urine also turns dark when exposed to air.
c. In addition to darkening, damage also occurs to joints, heart valves, and other
internal organs.
2. Sickle cell anemia is another example of pleiotropy.
G. Sex-linked traits are carried on the X or Y chromosome and therefore tend to be inherited b y
one sex more than the other. (pp. 145–146)
1. Men are more likely to exhibit red-green colorblindness or hemophilia than are
women. (Fig. 4.21)
2. These phenotypes are caused by alleles on the X chromosome; a woman has two X
chromosomes, so that she may be heterozygous, whereas a man has only one X
chromosome and normally expresses any allele found there.
3. The X chromosome carries about 260 genes, most of which are not related to sex; the
Y chromosome has very few genes, so that most sex-linked traits are associated with the
X chromosome.
G. In some cases, a genotype does not produce the expected phenotype. (p. 146)
1. Penetrance refers to the percentage of a population with a given genotype that actually
exhibits the predicted phenotype
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a. The penetrance of the polydactyly allele may be 80%, indicating that 80% of
those having the polydactyly genotype exhibit extra digits.
2. Environmental factors play an important role in gene expression; at the very least, all
gene expression depends on nutrition. (Fig. 4.22
H. Dominance and recessiveness are unrelated to how common an allele is in the population; type
O is the most common ABO blood type in North America, and polydactyly is rare. (p. 146)
Insight 4.3 Cancer (Fig. 4.23)
Cross References
Additional information on topics mentioned in Chapter 4 can be found in the chapters listed below.
Chapter 18: Sickle Cell Anemia
Chapter 27: Meiosis.
Chapter 29: Hereditary defects and nonhereditary birth defects.