Chapter 11 – The Control of Gene Expression
Gene expression – process by which genetic information
flows from genes to proteins
Gene Regulation in Prokaryotes
I. Intro
A. Therapeutic cloning vs. reproductive cloning
1. embryonic stem cells – give rise to all specialized cells
of the body (blastocyst)
B. prokaryotes and eukaryotes
1. cell specialization depends on the selective
expression of genes.
a) turn gene on when needed and off when not
2. Our earliest understanding of gene control came from
the study of the bacterium Escherichia coli.
II. Prokaryotic genes are turned on or off in response to
environmental changes
A. only turn a gene on if you need its product.
1. Don’t waste energy. Takes ATP to make polypeptides
(RNApol, tRNA synthetase, etc…)
B. E. coli
1. Bacterium found in your intestines
2. environment changes depending on what you eat
a) sweet roll - glucose and fructose
b) glass of milk –lactose
C. The LAC OPERON of E. coli:
1. Lac = short for lactose
2. 3 genes grouped together in chromosome
a) code for lactose metabolism enzymes
3. Should the genes be on or off in the presence of
lactose?
a) So how does lactose turn on these genes
b) all 3 controlled together as a unit
4. Promoter – There is a single promoter upstream from
the genes
5. Operator – DNA sequence between the promoter and
the genes – acts as a on/off switch
6. promoter + operator + genes = operon
7. Repressor – a protein that binds to the operon and
blocks binding of RNA polymerase
8. Regulatory gene – gene that codes for the repressor
9. Lactose binds to the repressor and inhibits it from
binding to the operator, allowing RNA polymerase to bind
the promoter and transcribe the genes.
D. Trp operon – controls genes involved in tryptophan
synthesis
1. So when would you want the genes to be off?
a) When tryptophan binds repressor, repressor binds
operator and turns off genes
b) Repressor alone DOES NOT bind operator
(opposite of lac operon)
E. Third type of operon use Activators – proteins that turn
operons on by binding to DNA – binds to operator and
helps RNA polymerase bind promoter.
F. Changing environments require changing gene
expression
III.Eurakaryotes
1. Producing organelles and regulating their function
requires a more complex network of gene control
B. Multicellular eukaryotes
1. added complexity in regulating what kinds of cells
should be produced where and when.
C. Zygote
1. gives rise to every type of cell in the multicellular
organism through repeated cell divisions.
2. Differentiation – cells become specialized in structure
and function
3. Each cell type has a different subset of genes turned
on and off
D. Structure of each type of cell is visibly different –
structure-function
IV.
Do differentiated cells retain their genetic potential?
A. Every cell still has all of it original DNA.
B. What about plants?
1. cloning plants evidence that the DNA in these cells is
not irreversibly changed upon differentiation.
C. What about animals? Are animals different? Do we
irreversibly alter out DNA?
1. Regeneration – growing back of lost body parts
2. Suggests that differentiated animal cells also have
their complete genetic potential
D. What about those animals that do not regenerate?
1. Nuclear transplantation – replace the nucleus of an
egg (zygote) with the nucleus of a differentiated cell
2. Will the differentiated nucleus support the
development of a normal embryo?
-1950’s - cloned tadpoles this way
-1997 – Dolly cloned using nucleus from mammary
gland cell – since cloned mice, cows, pigs
V. Reproductive cloning
1. Increase agricultural livestock with positive traits
2. make pigs with organs for human transplant
a) “knock out” (remove) immunoreactive genes and
clone
3. Remove or add a gene to the donor nuclei
a) -compare two organisms, one with gene of interest
and one without
4. Have made sheep that secrete a human blood protein
in their milk that is potentially useful in Tx cystic fibrosis.
VI.
Therapeutic cloning
A. Embryonic stem cells –
1. Cells in the early animal embryo that differentiate
during development to give rise to all the different
specialized cells in the body
2. Can divide indefinitely in culture
3. treat with certain growth factors that change gene
expression and get them to change into any cell you
desire
a) used to grow entire organs in culture or replace
lost cells in the body that can’t be naturally replaced
(neurons)
4. Ethical problems – must obtain cells initially from
human embryos
a) Adult Stem Cells – A viable solution???
-Found in adults, they generate replacement cells for cells
that don’t divide themselves.
-They are partially differentiated (not true stem cell) – only
give rise to a few different types of related cells
Ex. Stem cell in bone marrow only regenerate
different kinds of blood cells.
-Difficult to grow in culture
GENE REGULATION IN EUKARYOTES
VII. DNA packing helps regulate gene expression
A. Histones – proteins involved in packaging DNA
B. Packaging the DNA
1. Nucleosome –8 histones + wrapped DNA – “bead on a
string”
2. “Beads” are wrapped into tight helical fibers
3. Helical fiber coils into a supercoil
4. Looping and folding can further compact the DNA
during mitosis
C. Tight packing tends to prevent gene expression (RNA
polymerase and other transcription proteins presumably
can’t bind)
1. Histones must loosen grip for a gene to be turned on
(transcribed).
2. Non-histone proteins will loosen histone grip
D. Long term shut down – interphase chromatin can be
highly compact similar to that found during metaphase.
1. X chromosome inactivation – in each cell of the female
mammal 1 of the 2 X-chromosomes is turned off (highly
condensed) randomly during embryonic development
resulting in a Barr body.
2. The inactivated X is inherited by the cells decendants
3. If a female is heterozygous for genes on the X
chromosome, different cells will express different Xlinked alleles.
a) Tortoiseshell pattern on a cat
b) why are they usually female?
VIII. You tell me where you think we can control gene
expression (at what level) in eukaryotes?
IX. Complex assemblies of proteins control eukaryotic
transcription
A. More complex than prokaryotes
B. Eukaryotes tend to regulate single genes in contrast to
groups on genes like operons.
C. Transcription factors – proteins involved in the
transcription of genes
1. Activators – initiate transcription
2. Other proteins – needed for initiation
3. Repressors – inhibit transcription
D. Enhancers – regions of DNA that activators bind –
usually far away from the gene in contrast to operons in
prokaryotes
E. Silencers - regions of DNA that repressors bind
X. Eukaryotic RNA may be spliced in more than one way
A. Alternative RNA splicing
1. Remember the one gene – one polypeptide rule…
2. Turns out eukaryotes have the potential to get more
than one polypeptide per gene by changing the way
introns are spliced out.
a) Introns – noncoding region of the DNA transcripts
b) Exons – coding regions of DNA transcripts
3. Introns are not always junk
a) Some contain gene regulation sequences that
function on the transcription level
b) Suggested that by making genes longer it increases
chances of crossing over and thus genetic diversity
4. Spliceosome (enzymes that splice out introns) can
hold up passage through the nuclear pore.
XI. Translation and later stages of gene expression are
also subject to regulation
A. Breakdown (Degradation) of mRNA
1. mRNA does not hang around forever
2. Specifically degraded at different times by cellular
enzymes
3. The longer they are around, the more protein the cell
can potentially make from them
a) prokaryotes – average lifetime is only a few
minutes
b) eukaryotes – can last for hours to weeks –
nonmammalian vertebrate red blood cells have no
nucleus, but have ribosomes. Hemoglobin mRNA last
the lifetime of the RBC (~ a month).
B. Initiation of Translation
1. There are proteins that control the initiation of
polypeptide synthesis.
a) Ex -There is a protein that inhibits translation of
hemoblogin in RBC’s if heme group is not present.
C. Protein Activation
1. Polypeptides may require alteration to become
functional after translation is complete
a) Often involves cleavage (cutting) of the polypeptide
to yield a smaller active product
b) Ex. Insulin
D. Protein breakdown (degradation)
1. Lifetimes of proteins are regulated (nothing is random
here)
2. Allows for cells to adjust the kinds and amounts of
proteins present according to environmental conditions
3. Proteins also get damaged and must be destroyed
(recycled, always recycled)
XII. Review: Multiple mechanisms regulate gene
expression in eukaryotes
THE GENETIC CONTROL OF EMBRYONIC
DEVELOPMENT
XIII. Cascades of gene expression and cell-to-cell
signaling direct the development of an animal – like
complicated dominoes, one event triggers another.
A. Homeotic genes – master control genes that regulate
many other genes during embryonic development that
determine the anatomy of parts of the body.
B. Improper function of these genes leads to some bizarre
changes in morphology (shape).
C. The gene product of one gene turns on other genes.
These gene products turn on other genes and so on… Fig.
11.12B.
XIV. Signal-transduction pathways (cell signaling)
convert messages received at the cell surface into
response within the cell
A. The gene expression of one cell can affect the gene
expression of other cells – how is that possible?
B. Signal transduction pathway – a series of molecular
events that converts a signal on the target cell’s surface to
a specific response within the cell.
C. Elements of signal transduction
1. The signaling cell secretes the signal molecule
2. This molecule binds to a receptor protein embedded in
the target cells plasma membrane
3. The binding activates the first in a series of relay
proteins within the target cell. Each relay molecule
activates another
4. The last relay molecule in the series activates a
transcription factor that
5. Triggers transcription of a specific gene (or inhibits
transcription)
6. Translation of the mRNA produces a protein
XV. Key developmental genes are very ancient
A. Almost every homeotic gene found from yeast to fruit
flies to humans contains a common 180 nucleotide
sequence (how many amino acids is the product?) – called
homeoboxes
B. arose early in animal history and have remained
remarkably unchanged for eons of animal evolution
1. Each homeobox is translated into a small 60 amino
acid long segment of the total protein that it is found in.
2. It is a DNA binding domain that allows the protein to
interact with specific sequences of DNA to turn genes on
or off during development
C. Unity in Diversity – just the 1 millionth another example
D. Protein signals cell from outside – cell sends signal to
nucleus and turns on a homeotic gene – homeobox protein
is made, which will turn on 100’s of genes in the cell
related to the type of cell it should become.
THE GENETIC BASIS OF CANCER
XVI. Cancer results from mutations in genes that control
cell division
A. Oncogene – a gene that can cause cancer when present
as a single copy (onkos is greek for tumor)
B. Virus can cause cancer if it inserts an oncogene into the
chromosome or…
C. Proto-oncogene – a normal gene with the potential to
become and oncogene (your own gene)
1. they usually code for proteins involed in the cell cycle
like growth factors
2. Can become an oncogene by
a) Mutations resulting in hyperactivity
b) Gene duplication – too much protein produced
c) Chromosomal translocation – gene moved to new
locus under new genetic controls resulting in too
much protein produced
Ex) Signal transduction – ras protein
D. Tumor-suppressor gene – genes whose products inhibit
cell division (like p53 – “guardian angel”)
E. A mutation in the DNA of proto-oncogenes or tumorsuppressor may increase the chances of getting cancer.
XVII. Oncogene proteins and faulty tumor-supressor
proteins can interfere with normal signal-transduction
pathways
A.
XVIII.
Multiple genetic changes underlie the
development of cancer
A. One single mutation, even in a proto-oncogene, is not
enough to get cancer
B. Cancer takes a while to develop usually because
multiple mutations in different genes are necessary –
usually need 3 or 4 mutations in the right places in a cell
C. Colon cancer illustrates this nicely (150,000 people a
year are stricken with colon or rectal cancer)
XIX. Mary-Claire King discusses mutations that cause
breast cancer
A. Jewish women have a higher percentage of mutations to
lead to “familial” breast cancer
B. We said you need ~4 mutations, what if you were born
with 1 or 2 already?
C. BRCA1 – gene on chromosome 17 – mutation in this
gene gives you a >80% chance of getting breast cancer
D. BRCA2 – found shortly after BRCA1
E. Among breast cancer patients of Jewish ancestry, 10%
had mutations in one of these two genes.
F. Should all women be tested for BRCA1 and BRCA2
mutations?
XX. Avoiding carcinogens can reduce the risk of cancer
A. Cancer is 2nd leading cause of death next to heart
disease
B. Carcinogens – cancer-causing agents
1. Most mutagens are carcinogens
a) Two very potent carcinogens
(1) X-rays – leukemia, brain cancer
(2) UV light – skin cancer
b) Largest group of carcinogens are chemical
compounds
(1) Tobacco – known to cause more cases and types
of cancer than any other single agent
(a) 69 carcinogens in tobacco smoke
(i) benzopyrene
2. Some carcinogens work by increasing rate of cell
division
3. Avoid carcinogens
4. Growing evidence that diet can reduce cancer risk