Download Regulation of Gene Expression

Regulation of Gene
Expression
Step 1 of Transcription
What is “Gene Expression”?
Some proteins are always necessary
for an organism’s survival and
continuously produced:
Expression of the
– RNA/DNA Polymerase
gene is always
– Glycoproteins
occurring
Other proteins need to be
produced at certain times and
in certain amounts.
Expression of the
gene is turned “on”
or “off”
Why would a
cell want to turn
off a gene?
All cells carry the same genetic
information, but each type of cell
expresses only a fraction of its genes
(cellular differentiation).
Insects change
through a variety
of body shapes
during their life
cycle which can be
accounted for by
changing gene
expression.
Expression of
different genes at
different times also
accounts for the
different stages
vertebrate
embryos go
through as they
develop.
***Control of gene expression is complex, interrelated and much is still being discovered***
Mechanisms of
regulating gene expression:
Epigenics
Proteins
Environment
Nucleosomes
Regulation of Gene Expression by Proteins
Recall with your partner:
what a promoter is and
what it does.
Regulation of Gene Expression by Proteins
• A promoter is a sequence of base pairs located
near a gene that onto which RNA polymerase
binds.
• It does not code for a protein but plays a role in
protein synthesis by initiating transcription.
Regulation of Gene Expression by Proteins
Proteins can attach to the DNA near the
promoter to make the DNA more or less
accessible for RNA polymerase
PROTEINS
DNA
Transcription factors
AKA Activators
Enhancer
(increase gene expression)
(increase gene expression)
Repressor proteins
Silencer
(decrease gene expression)
(decrease gene expression)
Regulation of Gene Expression by Proteins
For example:
E.coli has genes to produce the enzyme necessary to digest
the sugar lactose- but it only activates them in the presence
of lactose. (WHY?)
An operator is a
sequence next to the
promoter. Transcription
factors can bind to it
and turn on gene
expression.
Regulation of Gene Expression
by Proteins
Repressor proteins are normally produced and will bind to
the operator preventing transcription for the lactase gene.
If lactose if present, it will bind to the repressor protein which
will change its shape and prevent it from binding on the
operator.
This is known as
the lac operon in
prokaryotes.
Operon refers to
a cluster of genes
under the control
of a single
promoter.
Enhancers
Sequence on
the DNA that
increase the
rate of
transcription
when
proteins bind
to them.
Silencers
Sequence on
the DNA that
decreases
the rate of
transcription
when
proteins bind
to them.
Mechanisms of
regulating gene expression:
Epigenics
Proteins
Environment
Nucleosomes
Nucleosomes Regulate Transcription
Eukaryotic DNA is associated
with proteins called
histones.
The DNA is wrapped around
a core of 8 histone molecules
which is known as a
nucleosome.
Nucleosomes are then
compacted in chromatin
fibers.
BIG IDEA: chemical modification of histone tails
through acetyl/methyl groups can either activate or
deactivate genes by decreasing or increasing the
accessibility of genes to transcription factors.
acetyl group
Nucleosomes Regulate Transcription
“Chemical
tag”
Acetylation is the adding of an acetyl group
Methylation is the adding of a methyl group
Nucleosomes Regulate Transcription
Histone acetylation
allows a less condensed
DNA with higher levels
of transcription.
Non-acetylated histones form a
tightly bound structure that
inhibits transcription of the genes
there.
Nucleosomes Regulate Transcription
DNA methylation typically turns off genes
(through a mechanism that isn’t well understood yet).
Mechanisms of
regulating gene expression:
Epigenics
Proteins
Environment
Nucleosomes
Reading at:
https://www.genome.gov/27532724
1. What is the
epigenome?
2. What does it do?
3. What makes it up?
4. Can it change?
There is substantial evidence that
smoking increases the rate of DNA
methylation.
Mothers who smoke while pregnant
have been shown to influence the
methylation patterns of their
unborn children too!
• What does DNA
methylation do?
• What if a tumor
suppressor gene is
methylated?
Epigenetics
Extra Credit Opportunity:
There is even mounting
evidence that the chemical
modifications that occur to
the DNA in gametes could in
certain circumstances be
passed on to the next
generation both at the
cellular as well as whole
organism level.
Mechanisms of
regulating gene expression:
Epigenic
tags
Proteins
Environment
Nucleosomes
Regulation of Gene Expression by the Environment
Example #6 of regulating gene expression:
UV exposure increases expression of the
gene that codes for the melanin protein.
Regulation of Gene Expression by the Environment
Regulation of Gene Expression by the Environment
For some traits, it can be hard
to tell how much influence the
environment has over gene
expression.
Comparing identical to
fraternal twins can be helpful
in determining this.
Why do you think this is?
Think:
Determine the
percentage of
identical twins
where both have
diabetes.
Explain why a higher
% of identical twins
sharing a trait
suggests that a
genetic component
contributes to the
onset of the trait.