Transcript Virus - Perry Local Schools

Chapter 18
Regulation of Gene
Expression
• Classify these things as occurring in
prokaryotes, eukaryotes, or both.
•
Single loop of DNA
•
Chromosomes wound around histones
•
Telomeres
•
Multiple sites of origin in DNA replication
•
Single origin site for DNA Replication
•
Uses DNA polymerase
•
RNA is processed after transcription
•
Transcription/translation can be coupled
•
Translation occurs at the ribosome
•
Uses codons and anticodons
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.
Gene Structures
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.
trp Operon
• Makes Tryptophan.
• Repressible 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.
Wednesday, February 18
• Predict what would happen in the lac
operon for each of these scenarios.
•
•
•
•
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Lactose is present, glucose is scarce
A mutation in the operator so the repressor cannot bind
Lactose is absent
Glucose is present
The repressor has a mutation so that it always binds to
the operator
• CAP and cAMP levels are high
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
(-CH3) 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
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!!
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
•
•
•
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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 translation.
• 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 multicellular 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.
Monday, February 23
• During mitotic cell division, each
daughter cell receives an exact copy of
the DNA from the parent cell. Explain
two mechanisms how eukaryotes have
exactly the same copy of DNA in each
cell, yet different proteins can be
expressed.
Gene Expression and Cancer
• Cancer - loss of the genetic
control of cell division.
• Balance between growthstimulating 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 (common example)
• 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, proto-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.