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Midterm 1 review sheet
Cell Biology (Concordia University)
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Lecture 1
Intro
-Cells can grow and reproduce in culture for extended periods. HeLa cells isolated from a
cancer patient in 1951. Contact inhibition prevents normal cells from proliferating too
much.
-Cells from different species share similar structure, composition and metabolic features.
-DNA-(transcribes)->RNA-(translates)->Protein
-Cell can reproduce
-Cells use and acquire energy. They get it from glucose. They convert glucose to ATP.
Photosynthesis provides the fuel for all organisms because it is the source of glucose.
-Cells engage in mechanical activities, respond to stimuli, are capable of self-regulation,
and they evolve.
Prokaryotes and eukaryotes
-Bacteria cell: 1x10-6 m (Prokaryotes 0.2-2.0 micro meters)
-Eukaryotes: 10-100 micrometers
-Prokaryotes: nucleoid (no true membrane bound nucleus), no membrane-bound
organelles. Red blood cells are eukaryotic but have no nucleus.
-3.5 billion years ago – prokaryotic bacteria dominate
-1.5 billion years ago – eukaryotic cells arise
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-Types of prokaryotes:
ARCHAEA: Methanogens, Halophiles, Acidophiles, Thermophiles
BACTERIA: Smallest cells known as mycoplasma, there is cyanobacteria which
partake in photosynthesis and gave rise to oxygen-rich atmosphere and green plants…
some are capable of nitrogen fixation
-Types of eukaryotes:
UNICELLULAR: an example is vorticella, which have a contractile ribbon in the stalk
and a large macronucleus that contains multiple copies of genes
MULTICELLULAR: each cell is differentiated, different numbers and arrangement of
organelles, this differentiation occurs during embryonic development
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Some organelles
-Mitochondria: Outer and inner membrane, the latter forms cristae. Own DNA. Involved
in cellular respiration and ATP production.
-Chloroplasts: In plants and algae. Outer and inner membrane + thylakoid membrane
(thylakoid contains green pigment chlorophyll). Own DNA. Involved in photosynthesis.
-Endosymbiotic theory: Mitochondria originate from aerobic (use O2 for ATP) bacteria.
Engulfed by ancestral eukaryotic cells. Chloroplasts came from photosynthetic bacteria
that were engulfed by ancestral eukaryotic bacteria already containing mitochondria.
-Chloroplast: water in, O2 out. CO2 in, glucose out. Needs sunlight. Then Mitochondria,
water out, O2 in. CO2 out, glucose in. Produces ATP.
Model organism: All cells have evolved from common ancestor. Some cells are easier to
study: look at reproduction rate, transparency. (PS: Yeast is eukaryotic)
Lecture 2
Respiration: 6O2+C6H12O6 -(enzymes)-> 6CO2+6H2O. Produces ATP.
-Aerobic vs. anaerobic. Early earth populated by anaerobes, captured and used energy
independently of oxygen. Cyanobacteria add oxygen to atmosphere without chloroplasts,
aerobes evolve and used the oxygen to extract more energy from organic molecules. In
eukaryotes this takes place in the mitochondrion.
Mitochondria
-Can be bean-shaped, round, or thread-like (in fibroblast).
-Size and number reflect energy needs of the cell.
-They can fuse or fission with one another. Major determinant of mitochondrial number,
length and degree of interconnection.
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-Outer mitochondrial membrane. 50% protein.
-Inner mitochondrial membrane – inner membrane and cristae (where ATP machinery is
located). 75% protein.
-These two enclose matrix and intermembrane space.
-Matrix contains circular DNA molecule, ribosomes where proteins are synthesized, and
enzymes.
-Outer membrane contains porins allowing even proteins through. Inner membrane not
permeable.
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Cellular Respiration
-Glycolysis first. In cytoplasm, no oxygen required.
-Glucose (C6) turns into glycogen, which turns into 2 NADH, 2 pyruvate, 2 ATP.
-Pyruvate transported across mitochondria membrane and decarboxylated to form acetyl
CoA. This process produces CO2 and NADH as well.
-Then TCA cycle/Kreb’s cycle/Critic acid cycle. Occurs in matrix and produces ATP, 3
NADH, FADH2. Multiply by two for one glucose. GTP also produced.
-Then electron transport chain or oxidative phosphorylation. Oxygen required.
Happens on inner membrane of mitochondria. NADH and FADH2 give off electrons to
proteins in inner membrane, lose energy. These high energy electrons move protons
from matrix to intermembrane space, then protons move back through ATP synthase.
Creates energy. Oxygen collects remaining electron and becomes water.
-In total, 36-38 ATP per glucose.
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Glycolysis
TCA
-Central metabolic pathway of the cell because reaction intermediates in the TCA are
common compounds generated in other catabolic reactions. These are fed into TCA.
-Two carbon acetyl group from acetyl CoA is condensed with the four carbon
oxaloacetate to form a six carbon citrate.
-Two carbons are oxidized to CO2, regenerating the four-carbon oxalocatetate needed to
continue the cycle.
-GTP produced.
Oxidative Phosphorylation
-Coupling of proton translocation to ATP synthesis is called chemiosmosis.
-Three molecules of ATP are formed from each pair of electrons donated by NADH.
-Two molecules of ATP are formed from each pair of electrons donated by FADH2.
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Overview
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ATP synthesis
-ATP hydrolysis increases 100 fold during exercise, stored creatine phosphate is used to
rapidly generate ATP.
-ATP synthase consists of F0 and F1 portions. F1 contains catalytic sites for ATP
synthesis, F0 is attached and also embedded in the inner membrane, contains channel for
conducting proteins.
-There are ring subunits called c subunits that make up the F0 ring. 10 for yeast and e
coli, 14 for eukaryotes and chloroplast ATP synthase.
-C subunits attached to gamma stalk (part of F1). Ring rotated, rotates stalk. Twisting
drives ATP synthesis.
Diseases result from abnormal mitochondria. Traced to mutations in mtDNA. These
are inherited maternally. Only part of the nucleus comes from the sperm.
Photosynthesis: 6CO2+6H2O-(light energy from sun)->6O2+C6H12O6.
-Converts energy from sunlight to chemical energy.
-Low energy electrons are removed from a donor molecule. First photoautotrophs used
H2S, then from H2O to produce O2.
Chloroplasts and leafs
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-Photosynthesis in eukaryotes takes place in the chloroplast, in the thylakoid.
-They have an inner and outer membrane, both smooth. Outer membrane contains porins.
-Thylakoids within, result of folds in inner membrane. Thylakoid membrane contains
light absorbing pigment, electron carriers, and ATP-synthesizing enzymes.
-Stack of thylakoids: granum.
-Inside chloroplast: stroma.
-Inside thylakoid: lumen.
-Chloroplasts are self-replicating and contain their own DNA.
=>Photosynthesis uses low energy electrons to form ATP and NADPH which are used to
reduce CO2 to carbohydrate. Cellular respiration removes high energy electrons from
NADH and FADH2 for form ATP.
-Light reactions use sunlight to create ATP and NADPH, dark reactions use these to form
glucose.
-Absorption spectrum: for a molecule. Action spectrum: for a process.
Light reactions
-Absorbing photons makes molecule go from ground state to excited state. Pigments
such as chlorophyll absorb the light. Chlorophyll contains porphyrin ring that absorbs
light, and phytol tail embedding it to the photosynthetic membrane.
-Photosynthetic unit contains several hundred chlorophyll molecules: most are antenna
pigments that resonate energy, and then there is at the center reaction-center
chlorophyll that absorbs energy, gets excited, and the electron is transferred to an
electron acceptor.
-The first complex is the photosystem II which absorbs 680 nm light and water replaces
the missing electrons. Water is the source of electrons.
-The second complex is the photosystem I which absorbs 700 nm light and electrons
from PSI replace missing electrons.
-Electrons pass through a chain, causes proton gradient (from stroma to lumen, then
ATP synthase), then taken up by NADP+.
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Photophosphorylation – converting energy to ATP
-ATP synthase has CF0 and CF1.
-CF1 heads project into the stroma.
Dark reactions
-Carboxylation of RuBP -(Rubisco enzyme)->PGA. Reduction of PGA -> GAP using 2
NADPH and 2 ATP form light reactions.
-RuBP is regenerated.
-GAP can be exported into cytosol in exchange for phosphate ions and used to synthesize
sucrose.
-Can also remain in chloroplast and become starch.
-These processes require 12 molecules of NADPH and 18 molecules of ATP.
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Lecture 3
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-Proteins in DNA – histones
-DNA + histones = nucleosome
-Nucleosome + coiling = chromosome
-Gene: part of DNA that codes for proteins
-2 chromatids per chromosome
-Hereditary factors consist of DNA and reside on chromosomes.
-Genome: collective body of genetic information in an organism.
-Order of gene discoveries: discovery of discrete units of inheritance, discovery of
chromosomes, discovery of homologous chromosomes, discovery of crossing over,
discovery that genes could be mapped, discovery of DNA as genetic material, discovery
of DNA structure.
Mendel
-1860
-Crossbred plants with certain characteristics and counted the number of offspring
through generations with characteristics.
-Established laws of inheritance.
-Focused on seven traits: seed color, seed shape, height, flower color, flower position,
pod color, pod shape.
Conclusions: (1) characteristics of organisms are governed by units of inheritance called
genes, one or two forms of a gene called an allele which can be identical or non identical,
and one allele dominates the other in a heterozygote. (2) Gametes contain one gene for
each trait. Somatic cells arise by the union of male and female gametes, two alleles are
inherited, one from each parent. (3) Pairs of genes are separated during gamete formation.
(4) Independent assortment – genes controlling different traits segregate independently.
Chromosomes
-Carrier of genetic information
-First observed in dividing cells using light microscope.
-23 pairs per cell. Homologous chromosomes.
-Genes on the same chromosomes do not assort independently. Part of the same linkage
group.
-BUT, if further apart, more chances of independent assortment.
Recombination/crossing over. If linkage is incomplete, maternal and paternal
chromosome can exchange pieces.
-Frequency of recombination indicates distance. Increases as distance increases.
-Points at which homologues are crossed over are chiasmata.
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Chemical Nature of the Gene
-Nucleotide is the building block of DNA. Monomer of nucleic acid.
-Consists of phosphate, sugar, and either a pyrimidine or purine nitrogenous base.
-Pyrimidine: Thymine or Cytosine
-Purine: Adenine or Guanine
-Nucleotides are linked into polymers, have polarized structure where the ends are called
5’ and 3’. Sugar and phosphate are linked by 3’ or 5’ phosphodiester bonds.
-Chargaff: base composition analysis shows # of A=T, C=G. Hydrogen bonds to each
other, A=T, G=C.
-Bases attached to the sugar.
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Watson-Crick
-DNA molecule is a double helix. Two chains of nucleotides.
-Chains are antiparallel.
-Sugar-phosphate backbone is located on the outside, bases on the inside.
-2nm wide.
-Pyrimidines are always paired with purines.
-Major groove and minor groove within double helix.
-Two chains antiparallel, complementary.
Functions of gene:
1) Storage of genetic information
2) Replication and inheritance
3) Expression
DNA Supercoiling
-More compact: supercoiled DNA.
-Underwound DNA is negatively supercoiled, overwound DNA is positively supercoiled.
The latter allows it to fit in the nucleus.
-Must be open to allow for transcription. Most organisms negative supercoiling.
-Topoisomerases change the level of DNA supercoiling. Type 1: transient break in one
nucleotide strand. Type 2: transient break in both nucleotide strands.
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-Genome: cell’s unique content of genetic information.
-Important property of DNA: denaturation – ability to separate into two strands. Protein
loses its function.
-Absorption of light higher when strands separated.
-Renaturation or reanneling – single strands capable of associating. Has led to
development of nucleic acid hybridization…nucleic acids from different sources can
form hybrid molecules.
Lecture 4
DNA fingerprinting
-DNA is digested by restriction endonucleases and DNA fragments are size separated
by gel electrophoresis
Modification of DNA Sequences
1) Whole Genome Duplication (Polyploidization)
-Offspring receive more than two sets of chromosomes from their parents. Could be the
result of duplicate chromosomes not separating properly in embryonic cells.
2) Gene Duplication
-Occurs within a small portion of a single chromosome
=>Occurs due to unequal crossing over between misaligned homologous chomosomes.
-Shorter segment for one chromosome compared to another.
-Important role in evolution of multigene families.
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Transposition
-Genes or elements of genes can move within a chromosome.
-Transposable elements.
-Some moderately repeated sequences (Alu and L1) are transposable.
(Ends with –ase….enzyme)
-Retrotransposons use an RNA intermediate which produces a complementary DNA via
reverse transcriptase.
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Genetic Variation within Human Population
1) Variation due to genetic polymorphisms. Most common is the single nucleotide
polymorphism (SNP). Example: C may be replaced by T.
2) Chromosomal aberrations can be from structural or numerical chromosome
differences.
-Structural variation. Large segments of the DNA (structural variants) can change.
-Loss or exchange of a segment between different chromosomes, potentially due to DNAdamaging agents.
-Different consequences depending on if in somatic or germ cells.
1) Deletions result when there is loss of a portion of a chromosome.
2) Duplications occur when a portion of a chromosome is repeated.
3) Inversions involve the breakage of a chromosome and resealing of the segment in a
reverse order.
4) Translocations are the result of the attachment of all or one piece of one chromosome
to another chromosome.
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Lecture 5
Operons
-Same genes in every cell, but cells express their genetic information selectively. This
expressions is controlled by regulatory machinery in the nucleus. Only find messenger
RNA for active genes.
-Bacteria selectively express genes to use resources effectively.
1) Lactose induces synthesis of enzyme Beta-galactosidase.
2) Tryptophan represses the genes that encode for tryptophan synthesis.
-Operons activate/inactivate genes. They are functional complexes (set in continuation)
of genes containing the information for enzymes of a metabolic pathway. All proteins
associated with operons operate for one metabolic pathway.
-Operons include:
(1) structural genes – code for enzymes and are translated from a single mRNA that is
usually polycistronic (encodes for more than one protein)
(2) promoter – where RNA polymerase binds
(2) operator – site next to promoter where the regulatory protein can bind. Decides
whether gene is expressed or not.
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Types of operons: repressible operons (tryptophan) and inducible operons (lactose).
-Lac operon: inducible operon which is turned on in the presence of the inducer lactose.
-Lactose binds to the repressor, changing its conformation and making it unable to bind to
the operator. RNA polymerase can transcribe and operon is expressed. Normally without
lactose, can’t transcribe.
-Contains three structural genes: Beta-galactosidase, Permease, Transacetylase
-Beta-galactosidase: enzyme cleaves lactose into glucose and galactase
-Permease: proteins insert in the plasma membrane and allow more lactose molecules to
move in.
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Nucleus and nuclear membrane
-Contents of nucleus enclosed by nuclear envelope
-Non-dividing nucleus includes: chromosomes as extended fibers of chromatin, nucleoli
for rRNA synthesis, nucleoplasm
-Nuclear envelope: two membranes separated by intermembrane space. These
membranes fuse at sites forming nuclear pores. The inner surface of the envelope is
lined by proteins known as nuclear lamina. Nuclear envelope contains 60 distinct
transmembrane protein.
-The nuclear lamina supports the envelope and is composed of lamins. Integrity of
lamina regulated by phosphorylation/dephosphorylation.
-Human conditions caused by mutations in lamina: lamin A/C mutation causes
Hutchinson-Gilford Progeria syndrome, lamin B mutation causes leukodystrophy
(loss of myelin…lipid presence on neuron axon).
-Nuclear pore contains nuclear pore complex. Composed of ~30 proteins called
nucleoporins.
-Proteins and RNA are transported in and out of the nucleus.
-FG (phenylalanine-glycine) domains form a hydrophobic sieve that blocks the diffusion
of larger macromolecules (greater than about 40,000 Daltons).
-Proteins synthesized in the cytoplasm are targeted for the nucleus by the nuclear
localization agent (a protein).
-Proteins with an NLS strength bind to an NLS receptor (importin)
-Conformation of the nuclear pore complex changes as the protein passes through.
-RNAs move through the nuclear pore complex as ribonucleoprotein and carry nuclear
export signals to pass through.
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Lecture 6
Control of Gene Expression in Eukaryotes
-Each nucleosome includes a core particle of supercoiled DNA and histone H1 serving as
a linker.
-The histone core complex consists of two molecules each of H2A, H2B, H3, and H4
forming an octamer. These 8 subunits form the core.
-30 nm filament is another level of chromatin packaging maintained by H1.
-Can be in solenoid form or zig-zag form.
-Uncoil for transcription.
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Euchromatin and Heterochromatin
-Euchromatin (active) returns to a dispersed state after mitosis.
-Heterochromatin (inactive) is condensed during interphase.
-Latter subdivided into two groups: (1) constitutive heterochromatin which remains
condensed all the time, is found mostly around centromeres and telomeres, and
consists of highly repeated sequences. (2) facultative heterochromatin is inactivated
during certain phases of the organism’s life.
Histone Modification
-Removal of the acetyl groups from H3 and H4 histones is among the initial steps in
conversion of euchromatin to heterochromatin.
-Histone deacetylation is accompanied by methylation of H3K9 by histone
methyltransferase in humans.
-Methylated H3K9 binds to proteins with a chromodomain, for example
heterochromatic protein 1 (HP1)
-Once HP1 is bound to the histone tails, HP1-HP1 interactions facilitate chromatin
packaging into a heterochromatin state.
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Structure of chromosome during mitosis
-Most highly condensed state of chromatin
-Karyotype: preparation of homologous pairs according to size. Used to screen
abnormalities.
-Centromere at marked location. It contains heterochromatin.
-Centromeric DNA is the site of microtubule attachment during mitosis.
Telomere
-End of each chromosome (heterochromatin).
-Set of repeated sequences.
-New repeats are added by telomerase, a reverse transcriptase that synthesizes DNA
from RNA template. Prevents telomere from diminishing.
-Telomeres are required because they protect the end from being degraded. But they
diminish over time, and not that prominent at first. Cell dies.
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-In somatic cells (not germ cells), telomere lengths are reduced after every cell division to
limit cell doublings.
-A critical point occurs from telomere shortening when cells stop their growth and
division.
-In contrast, cells that are able to resume telomerase expression continue to proliferate.
-Apoptosis. Abnormal cell death. If this regulation is removed, if telomerase is active,
cancer occurs.
Gene regulation in eukaryotes
-Cells of a complex eukaryote exist in many differentiated states.
-Differentiated cells retain a full set of genes.
-Nuclei from cells of adult animals are capable of supporting the development of a new
individual, as demonstrated in experiments.
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Lecture 7
-Genes are turned on and off as a result of interaction with regulatory proteins.
-Each cell type contains a unique set of proteins.
-Regulation of gene expression occurs on FOUR levels:
Transcriptional control
RNA Process control
Translational control
Post-translational control (ubiquitination)
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Transcriptional control
-Differential transcription is the most important mechanism by which eukaryotic cells
determine which proteins are synthesized.
-Differential gene expression is found in various conditions:
a. Cells at different stages of embryonic development
b. Cells in different tissues
c. Cells that are exposed to different types of stimuli
DNA Microarrays
- DNA microarrays can monitor the expression of thousands of genes simultaneously.
1. Immobilized fragments of DNA are hybridized with fluorescent cDNAs.
2. Genes that are expressed show up as fluorescent spots on immobilized genes.
-Stimulus. Want to know what genes are transcribed to mRNA. Then reverse transcription
for cDNA.
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Yellow-both
Red-expressed in ethanol
Green-expressed in glucose
Dark-nothing
Transcription profiling. DNA microarrays can facilitate the diagnosis and treatment of
human diseases.
-Human breast tumors can be Estrogen receptor-positive (ER+) or Estrogen receptornegative (ER-)
-These tumor subtypes show differences in a subset of genes.
Transcription factors
-Transcription factors are the proteins that either act as transcription activators or
transcription inhibitors
-A single gene can be controlled by different regulatory proteins.
-A single DNA-binding protein may control the expression of many
different genes.
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-Gene expression gives rise to phenotypes within cells and tissues of an organism.
-Ectopic or forced gene expression can alter the phenotype.
EX1: Expression of MyoD causes fibroblasts to become muscle cells.
EX2: Expression of eyeless gene can trigger eye formation in the leg of flies.
-Embryonic stem (ES) cells are: Capable of indefinite self-renewal and are pluripotent:
capable of differentiating into all of the different types of cells.
-Introducing a combination of genes encoding only four specific transcription factors
(Oct4, Sox2, Myc, and Klf4) was sufficient to reprogram the fibroblasts and convert
them into undifferentiated cells that behaved like ES cells.
The Structure of Transcription Factors
-Transcription factors contain a DNA-binding domain and an activation domain.
-Many transcription factors can bind a protein of identical or similar structure to form a
dimer.
-The DNA-binding domains of most transcription factors have related structures (motifs)
that interact with DNA sequences.
Most motifs contain a segment that binds to the major groove of the DNA.
-Transcription factor motifs:
-The zinc finger motif – zinc ion of each finger is held in place by two cysteines and two
histidines.
-The helix-loop-helix (HLH) motif – has two α-helical segments separated by a loop.
-The leucine zipper motif – has a leucine at every seventh amino acid of an α-helix.
DNA Binding Sites
-Researchers use several techniques to find DNA sequences involved in regulation:
-Genome-wide location analysis
-Allows simultaneous monitoring of all the sites within the genome that are bound by a
given transcription factor under a given set of physiologic conditions
-Chromatin immuno-precipitation (ChIP): experimental technique used to investigate the
interaction between proteins and DNA in the cell.
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-If interested in new transcription factor, choose a new antibody. Antibodies pull the
transcription factor bound to DNA.
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RNA Processing Control
-Introns (exons are useful) must be removed by splicing and a poly-A tail is added
before the mRNA goes out of the nucleus.
-Protein diversity can be generated by alternative splicing. Single gene codes for
multiple proteins.
-Alternative splicing can become complex, allowing different combinations of exons in
the final mRNA product.
-There are factors that can influence splice site selection.
RNA Editing
-Specific nucleotides can be converted to other nucleotides through mRNA editing.
-RNA editing can create new splice sites, generate stop codons, or lead to amino acid
substitutions.
Example 1: It is important in the nervous system, where messages need to have A
(adenine) converted to (not all the time, but not mutation…nucleotide modification) I
(inosine) to generate a glutamate receptor. This is the receptor protein in generation
of action-potential in neuron. Resultant glutamate receptor’s internal channel is
impermeable to calcium ions.
Example 2: Conversion to cholesterol carrying protein (hydrophobic fat
apolipoprotein B) mRNA to a truncated version in the intestines (apolipoprotein B48) produces a smaller protein that works to help absorb fats. Full length of mRNA is
approximately 14,000 nucleotides long. Cytidine at 6666th position is converted to
uridine: generates a STOP codon (UAA). 3 base pairs (nucleotide) per codon. Stop
codons are UAA, UGA, UAG.
Translational control
-Affects the translation of mRNAs previously transported from the nucleus to the
cytoplasm.
- Types of control:
1. Localization of mRNA to certain sites within a cell
2. Whether or not a mRNA is translated?
3. How often a mRNA is translated?
4. Half life of mRNA: how long the message is translated for?
-mRNA contains non-coding regions called untranslated regions (UTRs) at both their 5’
and 3’ ends
-5’ UTR contains certain nucleotides to mediate translational control
-Many mRNA are stored in unfertilized eggs that remain inactive until fertilization and
subsequent development
-Initiation of translation of mRNA during early development involves at least 2 distinct
events:
1 Increase in the length of polyA tail
2 Removal of bound inhibitory proteins
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The Control of mRNA Translation
-When iron concentrations are low, iron regulatory protein (IRP), an inhibitor, binds to
the iron-response element (IRE), which is part of the mRNA, to prevent translation of
ferritin (intracellular iron storage protein)
-When iron becomes available, it binds to the IRP, changing its conformation and causing
it to dissociate from the IRE, allowing the translation of the mRNA to form ferritin.
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Lecture 8
Plasma Membrane
-The outer boundary of the cell that separates it form the world is a thin, fragile structure
about 5-10 nm thick. Not detectable with light microscope.
-Inner an outer polar surfaces, non-polar interior. Inner tails are water fearing, outer heads
water loving.
-All membranes (of all types) have same ultrastructure.
-Proteins + phospholipid bilayer.
Overview of membrane functions
1) Compartmentalization Membranes form continuous sheets that enclose intracellular
compartments.
2) Scaffold for biochemical activities Membranes provide a framework that organizes
enzymes for effective interaction.
3) Movement of solvent Membranes allow movement of water between compartments.
4. Transporting solutes Membrane proteins facilitate the movement of substances
between compartments.
5. Responding to external signals Membrane receptors transduce signals from outside
the cell in response to specific ligands.
6. Intercellular interaction Membranes mediate recognition and interaction between
adjacent cells.
7. Energy transduction Membranes transduce photosynthetic energy, convert chemical
energy to ATP, and store energy.
Plasma Membrane Structure
-Membranes were found to be mostly composed of lipids
-The most energetically favored (lowest energy) orientation for polar head groups is
facing the aqueous compartments outside of the bilayer
-Nature and importance of the lipid bilayer:
1) Lipid composition can influence the activity of membrane proteins and determine the
physical state of the membrane. Saturated chain = single bonds between carbons.
Unsaturated = double bonds. Double bonds twist and turn the structure.
2) The cohesion of bilayers to form a continuous sheet makes cells deformable and
facilitates splitting and fusion of membranes.
-Protein-lines pores in the membrane account for the movement of polar solutes and ions
across cell boundaries. Normally, smaller hydrophobic can pass, bigger hydrophobic pass
less, hydrophilic not.
-Fluid-mosaic model: core lipid bilayer exists in a fluid state, capable of movement.
Membrane particles form a mosaic penetrating the lipids.
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Chemical Composition of Membranes
-The lipid and protein components are bound together by non-covalent bonds.
-Membranes also contain carbohydrates. Present on outer side.
-Protein/lipid ratios vary among membrane types. Myelin on neuron has more fat.
-Membrane lipids are amphipathic (polar and non polar) with three main types.
1) Phosphoglycerides are diacylglycerides with small function head groups linked to the
glycerol backbone by phosphate ester bonds.
2) Sphingolipids are ceramides formed by the attachment of sphingosine to fatty acids.
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3) Cholesterol is a smaller and less amphipathic lipid that is only found in animals.
Mostly hydrophobic.
Cholesterol
-A sterol that makes up to 50% of animal membrane lipids.
-The -OH group is oriented toward membrane surface
-Carbon rings are flat and rigid; interfere with the movement of phospholipid fatty acid
tails.
The Nature and Importance of the Lipid Bilayer
-Membrane lipid composition is characteristic of specific membranes.
-Lipid bilayers assemble spontaneously in aqueous solutions as liposomes.
The Asymmetry of Membrane Lipids
-Inner and outer membrane leaflets have different lipid compositions.
-Provides different physico-chemical properties appropriate for different interactions
-PE helps with membrane curvature
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-Membranes contain carbohydrates covalently linked to lipids and proteins on the
extracellular surface of the bilayer.
-Glycoproteins have short, branched carbohydrates for interactions with other cells and
structures outside the cell.
-Glycolipids have larger carbohydrate chains that may be cell-to-cell recognition sites.
Structure and Functions of Membrane Proteins
-Membrane proteins attach to the bilayer asymmetrically, giving the membrane a distinct
“sidedness”
-Membrane proteins can be grouped into three distinct classes:
1) Integral proteins
-Penetrate and pass through lipid bilayer
-Make up 20 -30% of all encoded proteins
-Are amphipathic, with hydrophobic domains anchoring them in the bilayer and
hydrophilic regions forming functional domains outside of the bilayer.
-Channel proteins have hydrophilic cores that form aqueous channels in the membranespanning region.
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2) Peripheral proteins
-Attached to the polar head groups of the lipid bilayer and/or to an integral membrane
protein by weak bonds
-Easily solubilized.
-Located entirely outside of bilayer on either the extracellular or cytoplasmic side.
3. Lipid-anchored membrane proteins
-Covalently bonded (strong bond) to a lipid group that resides within the membrane
-Glycophosphatidylinositol (GPI)-linked proteins found on the outer leaflet can be
released by inositol-specific phospholipases.
-Some inner-leaflet proteins are anchored to membrane lipids by long hydrocarbon
chains.
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Membrane Lipids and Membrane Fluidity
-Membrane lipids exist in gel or liquid-crystal phases depending on temperature, lipid
composition, saturation and presence of cholesterol.
-Liquid-crystal membranes predominate
-When lowering the temperature, a point is reached where liquid-crystal phaseàgel phase
occurs which is known as the transition temperature
-The greater the degree of unsaturation of fatty acids of the bilayer is, the lower the
transition temperature is for the membrane.
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