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Gene Expression
Cells use information in genes to
build hundreds of different
proteins, each with a specific
function.
But, not all proteins are required
by the cell at any one time. By
regulating gene expression, cells
are able to control which proteins
are made.
Role of Gene Expression
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Gene expression is
the activation of a
gene that results in
the formation of a
protein.
A gene is said to be
expressed when
transcription occurs
Role of Gene Expression
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Some proteins may
play a structural
role, while other
are enzymes that
catalyze chemical
reactions
Cells do not always
need all of the
proteins present in
the genome
Role of Gene Expression
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Gene expression occurs
in two steps:
transcription and
translation
Gene expression begins
when RNA polymerase
transcribes DNA into
mRNA. This sets off a
chain of events that
results in the production
of a specific protein.
Gene Expression in Prokaryotes
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Scientists first studied
what controlled gene
expression by looking at
prokaryotes.
The DNA of a prokaryote
exists as a single
circular chromosome of
DNA
Two scientists (Francios
Jacob and Jaques Monod
studied the bacteria
Escherichia coli and it’s
ability to metabolize the
sugar lactose
Gene Expression in Prokaryotes
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E. coli is found in the
intestines of humans
Normally E. coli uses
glucose as a food source
However, when you drink
cow’s milk, the sugar
present is a two part
sugar, or disaccharide,
composed of glucose
and galactose
E. coli must alter it’s
proteins in order to break
down this new sugar.
Gene Expression in Prokaryotes
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The metabolism of
lactose is controlled by
three particular enzymes.
The genes for these three
enzymes are adjacent to
each other on the E. coli
chromosome.
Gene Expression in Prokaryotes
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The production of these
enzymes is controlled by
three “regulatory
elements” found on the
DNA of E. coli. These
elements are:
1.
2.
3.
Structural genes--this is the actual code
for each of the three
enzymes
Promoter----In front of
the genes that code
for our enzymes is a
region of DNA that
does not code for a
protein, but instead
acts as a binding site
for RNA polymerase.
(recall: what does
RNA polymerase do?)
Operator--just next to
the promoter is a
second region of DNA
that does not code for
a protein but is also a
binding site. At this
site a protein that
blocks
transcription can
bind.
Gene Expression in Prokaryotes
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Together, all of
these elements
make up a unit
called an operon.
This clustered
pattern of related
genes and their
regulators is a
common pattern in
bacterial DNA.
This particular
operon is called the
lac operon
because the
structural genes
involved regulate
lactose metabolism
Gene Expression in Prokaryotes
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In their work with E. coli, Jacob
and Monod found that the
genes for lactose metabolic
proteins were only expressed
when lactose was present.
How was that possible?
Because the presence of
lactose induced the genes to
be transcribed. Here is how it
works.
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Gene Expression in Prokaryotes
Repression: in the absence
of lactose a protein called a
repressor protein attaches to
one of the binding sites in front
of the structural genes…the
operator.
The repressor protein
physically blocks RNA
polymerase from being able
to transcribe the genes.
The action of a repressor
protein blocking transcription is
called repression.
Gene Expression in Prokaryotes
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Activation: when lactose
is present in the E. coli
cell it binds to the
repressor protein. When
this binding occurs, it
changes the physical
shape of the repressor
protein and renders it
unable to bind to the DNA.
Now RNA polymerase
can bind at the promoter
region of the gene, and
transcription can occur.
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Gene Expression in Prokaryotes
An inducer is a molecule
that initiates gene
expression. In this
example lactose acts as
an inducer
The initiation of
transcription by removing
a repressor protein is
called activation
The advantage to this
type of system is that the
cell doesn’t waste
energy producing
proteins it doesn’t need.
Gene Expression in Eukaryotes
Question: Kwashiorkor is a
disease in children caused by
a diet high in carbohydrates
but lacking in complete
protein. When children with
kwashiorkor are suddenly put
on a diet rich in protein they
may become very ill with
ammonia poisoning, and some
even die. The high level of
ammonia in their blood is due
to the inadequate metabolism
of protein. What does this tell
you about the enzymes that
metabolize protein?
Gene Expression in Eukaryotes
Eukaryotes are vastly different
than prokaryotes
Genomes are much larger
Several linear chromosomes
instead of single circular one.
Multiple, specialized cells, that
only produce a subset of
proteins instead of single cells
Therefore gene expression is
much more complicated in
eukaryotes than prokaryotes!
Gene Expression in Eukaryotes
Much of the control of gene
expression in eukaryotes
occurs at the level of the
chromosome.
Recall that during cell
division the DNA coils
into chromosomes, but
after cell division it
relaxes into chromatin.
In some cells, gene
regulation is
accomplished when
certain regions of the
DNA remain tightly
coiled to prevent
transcription of genes.
Gene Expression in Eukaryotes
Another form of gene
regulation has to do with
the way that eukaryotic
genes are organized.
Remember that genes are
preceeded by a short
segment of DNA called
the promoter region,
where RNA polymerase
can bind.
After the promoter, the
structural genes of
eukaryotes are made up
of introns and exons.
Gene Expression in Eukaryotes
Introns are segments of the
gene that do not code for
amino acids.
Exons do code for amino
acids.
We are not sure why
eukaryotic genes are
organized in this fashion,
but think it might have
something to do with
crossing over
Gene Expression in Eukaryotes
When a gene is transcribed,
both the introns and
exons are transcribed
into mRNA. This mRNA
that contains both introns
and exons is called premRNA.
True RNA is formed when
the introns are
removed by and the
exons are spliced beck
together by enzymes.
The nucleotides from the
introns are then broken
down and recycled into
new RNA molecules
Gene Expression in Eukaryotes
Gene Expression in Eukaryotes
So, what does this
have to do with
which genes are
expressed?
Well, the splicing of
these genes
determines
whether a protein
is translated.
Sometimes, a single
gene can be
spliced in
alternative ways
and result in
totally different
proteins
Gene Expression in Eukaryotes
Enzymes drive this
splicing process
How these enzymes
are triggered and
controlled is still
being figured out.
Gene Expression in Eukaryotes
Eukaryotic genes also often have
regions that are non-coding that
enhance the transcription of
genes.
These regions are called enhancer
regions.
Much like the promoter region of
prokaryotic genes, these regions
determine whether RNA
polymerase will transcribe a gene.
Again, how these enhancer regions
work is still being understood by
scientists.