Chapter 18
Regulation of
Gene Expression
Regulation of
Gene Expression
• Important for cellular control and
differentiation.
• Understanding “expression” is a “hot” area in
Biology.
General Mechanisms
1. Regulate Gene Expression
2. Regulate Protein Activity
Operon Model
• Jacob and Monod (1961) - Prokaryotic model
of gene control.
• Always on the National AP Biology exam!
Operon Structure
1. Regulatory Gene
2. Operon Area
a. Promoter
b. Operator
c. Structural Genes
Gene Structures
Regulatory Gene
• Makes Repressor Protein which may bind to
the operator.
• Repressor protein blocks transcription.
Promoter
• Attachment sequence on the DNA for RNA
polymerase to start transcription.
Operator
• The "Switch”, binding site for Repressor
Protein.
• If blocked, will not permit RNA polymerase to
pass, preventing transcription.
Structural Genes
• Make the enzymes for the metabolic pathway.
Lac Operon
• For digesting Lactose.
• Inducible Operon - only works (on) when the
substrate (lactose) is present.
If no Lactose
• Repressor binds to operator.
• Operon is "off”, no transcription, no enzymes
made
If Lactose is absent
If Lactose is present
• Repressor binds to Lactose instead of
operator.
• Operon is "on”, transcription occurs, enzymes
are made.
If Lactose is present
Enzymes
• Digest Lactose.
• When enough Lactose is digested, the
Repressor can bind to the operator and switch
the Operon "off”.
Net Result
• The cell only makes the Lactose digestive
enzymes when the substrate is present, saving
time and energy.
Animation
• http://www.biostudio.com/d_%20Lac%20Ope
ron.htm
trp Operon
• Makes/synthesizes Tryptophan.
• Repressible Operon.
– Predict how it is different from the inducible
operon…
If no Tryptophan
• Repressor protein is inactive, Operon "on”
Tryptophan made.
• “Normal” state for the cell.
Tryptophan absent
If Tryptophan present
• Repressor protein is active, Operon "off”,
no transcription, no enzymes
• Result - no Tryptophan made
If Tryptophan present
Repressible Operons
• Are examples of Feedback Inhibition.
• Result - keeps the substrate at a constant
level.
Positive Gene Regulation
• Positive increase of the level of
transcription.
• Uses CAP - Catabolite Activator Protein
• Uses cAMP as a secondary cell signal.
CAP - Mechanism
• Binds to cAMP.
• Complex binds to the Promoter, helping RNA
polymerase with transcription.
Result
• If the amount of glucose is low (as shown by
cAMP) and lactose is present, the lac operon
can kick into high gear.
Eukaryotic Gene Regulation
• Can occur at any stage between DNA and
Protein.
• Be prepared to talk about several mechanisms
in some detail.
Chromatin Structure
• Histone Modifications
• DNA Methylation
• Epigenetic Inheritance
Histone Acetylation
• Attachment of acetyl groups (-COCH3) to AAs in
histones.
• Result - DNA held less tightly to the nucleosomes,
more accessible for transcription.
DNA Methylation
• Addition of methyl groups(-C H3) to DNA bases.
• Result - long-term shut-down of DNA
transcription.
• Ex: Barr bodies, genomic imprinting
Epigenetics
• Another example of DNA methylation effecting
the control of gene expression.
• Long term control from generation to generation.
• Tends to turn genes “off”.
Do Identical Twins have Identical DNA?
• Yes – at the early stages of their lives.
• Later – methylation patterns change their DNA
and they become less alike with age.
Transcriptional Control
• Enhancers and Repressors
• Specific Transcription Factors
• Result – affect the transcription of DNA into
mRNA
Enhancers
• Areas of DNA that increase transcription.
• May be widely separated from the gene
(usually upstream).
Posttranscriptional Control
• Alternative RNA Processing/Splicing
– Ex. - introns and exons
• Can have choices on which exons to keep
and which to discard.
• Result – different mRNA and different
proteins.
Another Example
Results
– inhibits apoptosis
Bcl-XS – induces apoptosis
• Bcl-XL
•
• Two different and opposite
effects!!
DSCAM Gene
• Found in fruit flies
• Has 100 potential splicing sites.
• Could produce 38,000 different
polypeptides
• Many of these polypeptides have been
found
Commentary
• Alternative Splicing is going to be a BIG
topic in Biology.
• About 60% of genes are estimated to
have alternative splicing sites. (way to
increase the number of our genes)
• One “gene” does not equal one
polypeptide (or RNA).
Other post transcriptional control points
• RNA Transport - moving the mRNA into the
cytoplasm.
• RNA Degradation - breaking down old mRNA.
Translation Control
• Regulated by the availability of initiation
factors.
• Availability of tRNAs, AAs and other protein
synthesis factors. (review Chapter 17).
Protein Processing and Degradation
• Changes to the protein structure after
translation.
• Ex: Cleavage
– Modifications
– Activation
– Transport
– Degradation
Protein Degradation
• By Proteosomes using Ubiquitin to mark the
protein.
Noncoding RNA
• Small RNA molecules that are not translated
into protein.
• Whole new area in gene regulation.
• Ex - RNAi
Types of RNA
• MicroRNAs or miRNAs.
• RNA Interference or RNAi using small
interfering RNAs or siRNAs.
• Both made from RNA molecule that is
“diced” into double stranded (ds)
segments.
RNAi
• siRNAs or miRNAs can interact with mRNA and
destroy the mRNA or block transcription.
• A high percentage of our DNA produces
regulatory RNA.
Morphogenesis
• The generation of body form is a prime
example of gene expression control.
• How do cells differentiate from a single
celled zygote into a multi-cellular organism?
Clues?
• Some of the clues are already in the egg.
• Cytoplasmic determinants – chemicals in
the egg that signal embryo development.
• Made by Maternal genes, not the embryo’s.
Induction
• Cell to cell signaling of neighboring cells gives
position and clues to development of the
embryo.
Fruit Fly Studies
• Have contributed a great deal of information
on how an egg develops into an embryo and
the embryo into the adult.
Homeotic (Hox) Genes
• Any of the “master” regulatory genes
that control placement of the body parts.
• Usually contain “homeobox” sequences
of DNA (180 bases) that are highly
conserved between organisms.
Comment
• Evolution is strongly tied to gene regulation.
Why?
• What happens if you mutate the homeotic
genes?
• Stay tuned for more “evo-devo” links in the
future.
When things go wrong
Example case
• Bicoid (two tailed) – gene that controls the
development of a head area in fruit flies.
• Gene produces a protein gradient across the
embryo.
Result
• Head area develops where Bicoid protein
levels are highest.
• If no bicoid gradient – get two tails.
Other Genes
• Control the development of segments and
the other axis of the body.
Gene Expression and Cancer
• Cancer - loss of the genetic control of cell
division.
• Balance between growth-stimulating pathway
(accelerator) and growth-inhibiting pathway
(brakes).
Proto-oncogenes
• Normal genes for cell growth and cell
division factors.
• Genetic changes may turn them into
oncogenes (cancer genes).
• Ex: Gene Amplification, Translocations,
Transpositions, Point Mutations
Proto-oncogenes
Tumor-Suppressor Genes
• Genes that inhibit cell division.
• Ex - p53, p21
Cancer Examples
• RAS - a G protein.
• When mutated, causes an increase in cell
division by over-stimulating protein kinases.
• Several mutations known.
Cancer Examples
• p53 - involved with several DNA repair genes
and “checking” genes.
• When damaged (e.g. cigarette smoke), can’t
inhibit cell division or cause damaged cells to
apoptose.
Carcinogens
• Agents that cause cancer.
• Ex: radiation, chemicals
• Most work by altering the DNA, or interfering
with control or repair mechanisms.
Multistep Hypothesis
• Cancer is the result of several control
mechanisms breaking down (usually).
• Ex: Colorectal Cancer requires 4 to 5
mutations before cancer starts.
Colorectal Cancer
News Flash
• Severe damage to a chromosome that causes
it to “shatter” can lead to immediate cancer.
• Doesn’t always take a long time and multiple
steps.
Can Cancer be Inherited?
• Cancer is caused by genetic changes but is not
inherited.
• However, oncogenes can be inherited.
• Multistep model suggests that this puts a
person “closer” to developing cancer.
Example – BRAC1
• BRAC1 is a tumor suppressor gene linked
with breast cancer.
• Normal BRAC1 – 2% risk.
• Abnormal BRAC1 – 60% risk.
• Runs in families. Some will have breasts
removed to avoid cancer risk.
Summary
• Know Operons
• Be able to discuss several control
mechanisms of gene expression.
• Be familiar with gene expression and
development of organisms.
• How control of DNA can lead to cancer.