Transcript Biology 10.2 Gene Regulation and Structure

Biology 10.2 Gene Regulation and Structure
Gene
Regulation
and
Structure
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Protein Synthesis in Prokaryotes
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Although prokaryotes, such as
bacteria seem simple because of
their small size, prokaryotes
cells typically have about 2000
genes.
The human genome; the largest
sequenced to date, has about
30,000 genes.
Not all genes are transcribed
and translated all the time. Cells
are able to regulate which genes
are expressed and which are not,
depending on the needs of the
cell.
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Protein Synthesis in Prokaryotes
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An example of gene regulation
can be found in the bacterium
Escherichia Coli. (E. Coli)
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When you eat or drink a dairy
product, the lactose (milk sugar)
reaches the intestinal track and
becomes available to the E. coli
living there. The bacteria can
absorb the lactose and break it
down for energy or for making
other compounds.
In E. Coli; recognizing,
consuming, and breaking down
lactose into it’s parts requires
three different types of
enzymes, each of which is coded
for by a different gene.
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Protein Synthesis in Prokaryotes
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The three lactose-
metabolizing genes are
located next to each other
and are controlled by the
same promoter site.
There is an on-off switch
that “turns on”
(transcribes and than
translates) the three
genes when lactose is
available and “turns-off”
the genes when lactose is
not available.
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Protein Synthesis in Prokaryotes
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The piece of DNA that overlaps
the promoter site and serves as
the on-off switch is called an
operator.
In bacteria, a group of genes
that code for enzymes involved
in the same function, their
promoter site, and the operator
that controls them all function
together as an operon.
In prokaryotes; gene expression
is controlled by these operons.
The operon that controls the
metabolism of lactose in our
example is called the lac operon.
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Protein Synthesis in Prokaryotes
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What determines if the lac
operon is in the on or off mode?
When there is no lactose in the
bacterial cell, a repressor turns
the operon off.
A repressor is a protein that
binds to an operator and
physically blocks RNA
polymerase from binding to a
promoter site. This blocking of
the RNA polymerase STOPS the
transcription of the genes in the
operon.
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Protein Synthesis in Prokaryotes
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When lactose is present, the
lactose binds to the repressor
and changes the shape of the
repressor.
The change in shape causes the
repressor to fall off the
operator.
Now the bacterial cell can begin
transcribing the genes that code
for the lactose-metabolizing
enzymes.
By producing the enzymes only
when the nutrient is available,
the bacterium saves energy.
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Protein Synthesis in Prokaryotes
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In Summary:
In prokaryotes, gene expression
is regulated by operons.
Gene expression is switched OFF
when repressor proteins block
RNA polymerase from
transcribing a gene.
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Protein Synthesis in Eukaryotes:
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Eukaryote cells contain much
more DNA than prokaryote
cells do. Like prokaryotes cells,
eukaryote cells must
continually turn certain genes
on/off in response to signals
from their environment.
Operons have NOT been found
often in eukaryote cells.
Instead, genes with related
functions are often scattered
on different chromosomes.
Because a nuclear envelope
physically separates
transcription from translation
in a eukaryote cell, more
opportunities exist for
regulating
gene expression.
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Protein Synthesis in Eukaryotes:
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Controlling the Onset of
Transcription:
Predominantly, gene regulation
in eukaryotes controls the
onset of transcription .
Like prokaryotes, eukaryotes
cells use regulatory proteins
(proteins to start, stop and
regulate the process)
These regulatory proteins in
eukaryotes are called
transcription factors
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Protein Synthesis in Eukaryotes:
 Transcription factors help
arrange RNA polymerases in
the correct position on the
promoter. A gene can be
influenced by many different
transcription factors.
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An enhancer is a sequence of
DNA that can be bound by a
transcription factor.
Enhancers are typically located
thousands of nucleotides bases
away from the promoter.
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Protein Synthesis in Eukaryotes:
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A loop in the DNA may bring
the enhancer and it’s attached
transcription factor (called an
activator) into contact with the
transcription factors and RNA
polymerase at the promoter.
Transcription factors bound to
enhancers can activate
transcription factors bound to
promoters.
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Intervening DNA in Eukaryote cells:
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While it is tempting to
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In eukaryotes, many
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Exons are the portions
think of a gene as an
unbroken stretch of
nucleotides that code
for a protein, this simple
arrangement is usually
found only in
prokaryotes.
genes are interrupted by
introns, long series of
nucleotides that have
NO coding information.
(blank)
of the genes that are
translated (copied or
expressed) into proteins.
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Intervening DNA in Eukaryote cells:
 After a eukaryotic
gene is transcribed,
the introns in the
resulting mRNA are cut
out by complex
assemblies of RNA and
protein called
spliceosomes.
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The exons that remain
are “stitched” back
together by the
spliceosome to form a
smaller RNA molecule
that is than translated.
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Intervening DNA in Eukaryote cells:
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Many biologists think
this organization of
genes adds evolutionary
flexibility. Each exon
encodes a different
part of a protein.
By having introns and
exons, cells can
occasionally shuffle
exons between genes
and make new genes.
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Intervening DNA in Eukaryote cells:
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The thousands of
proteins that occur in
human cells appear to
have arisen as
combinations of only a
few thousand exons.
Some genes in your cells
exist in multiple copies,
in clusters of as few as
three or as many as
several hundred.
For example, your cells
may each contain 12
different hemoglobin
genes, all of which came
from one original
hemoglobin gene.
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Mutations:
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Although changes in an
organisms hereditary
information are rare, they can
occur.
A change in the DNA of a gene
is called a mutation.
Mutations in gametes can be
passed on to offspring of he
affected individual, but
mutations in body cells affect
only the individual in which
they occur.
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Mutations:
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Mutations that move an entire
gene to a new location are
called gene rearrangements.
Changes in a gene’s position can
often disrupt the gene’s
function because the gene is
exposed to new regulatory
controls in it’s new place (like
moving to a place where no one
spoke your language and you
couldn’t communicate with
anyone near you).
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Mutations:
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Mutations that change a gene
are called gene alterations.
Gene alterations usually result
in the placement of the wrong
amino acid during protein
assembly. This error will
usually disrupt a proteins
function.
I a point mutation, a single
nucleotide changes.
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Mutations:
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In an insertion mutation, a
sizable length of DNA is
inserted into the gene.
Insertions often result when
mobile segments of DNA,
called transposons, move
randomly from one position to
another on chromosomes.
Transposons make up 45
percent of the human genome.
In a deletion mutation,
segments of a gene are lost,
often during meiosis.
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