Control of Eukaryotic gene expression

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Control of Eukaryotic gene expression
Each cell in a multicellular eukaryote is
transcribing 1/3 - 2/3 of its genes!
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Genes are turned on off in response to signals
from their internal external environments
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Important during cellular differentiation the
divergence btwn cells form and function as cells
specialize
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Eukaryotes involves some of the principles
described for prokaryotes more complexity so,
additional stages where control can occur
each stage provides a potential control point
where expression can be turned on or off, sped
up or slowed down
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A eukaryote with operons!
C. elegans
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These levels of control include 1. DNA
packing 2. transcription 3. RNA processing
4. transport to cytoplasm 5. translation
6. various alterations to the protein
product 7. degradation of protein
Fig. 19.3
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1. Packing DNA inside the nucleus
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Chromatin DNA protein complex
DNA from a developing salamander egg
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1. DNA packaging affect the availability of
genes for transcription
genes densely condensed (i.e., heterochromatin)
are usually not expressed, because transcription
proteins cannot reach the DNA
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Histone acetylation (COCH3) deacetylation
Acetylated histones grip DNA less tightly
allowing transcription
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Acetylation of histone tails promotes loose
chromatin structure that permits transcription
Unacetylated histones
Acetylated histones
19.4 b
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Chemical modifications of DNA affects its
structure and transcription regulation Methylation
attachment of methyl groups (CH3) to DNA
Inactive DNA is highly methylated! Ex. Women, one
of your X chromosomes is heavily methylated
Demethylating inactivating genes turns them on
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2. Transcription most important and universally
used control point! controlled by proteins that
interact with DNA each other
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A eukaryotic gene and the DNA segments that
control transcription include introns and exons,
a promoter sequence, and a large number of other
control elements
Control elements noncoding DNA segments that
regulate transcription by binding transcription
factors
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Enhancers distant control elements that may be
thousands of nucleotides away from the promoter
or downstream of the gene or within an intron!
Eukaryotic RNA polymerase is dependent on
transcription factors before transcription can
begin
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Activator proteins stimulate transcription
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Bending proteins bend the DNA enabling distal
transcription factors, i.e., activators, bound to
enhancers to contact the protein initiation
complex at the promoter
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3. RNA processing in the nucleus and 4. export
of mRNA to the cytoplasm provide opportunities
for gene regulation alternative RNA splicing
different mRNA molecules are produced from the
sameprimary transcript,depending onwhich
RNAsegments aretreated as exons and which as
introns.
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The life span of a mRNA molecule is an important
factor determining the pattern of protein
synthesis prokaryotes ? minutes eukaryotes ?
minutes or days or weeks! Ex. the mRNAs for the
hemoglobin polypeptides are stable and translated
repeatedly mRNA degradation ? shortening the poly
a tail, removal of the phosphate cap, followed by
rapid degradation of the mRNA by nucleases
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5. Translation of mRNAs can also be blocked by
regulatory proteins that bind to specific
sequences or structures preventing ribosome
attachment
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6. Eukaryotic polypeptides must often be
processed to yield functional proteins Ex.
cleavage, chemical modifications, transport to
the appropriate destination 7. The cell limits
the lifetimes of normal proteins by selective
degradation Ex. cyclins in the cell cycle, must
be short-lived to function appropriately
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Proteins intended for degradation are marked by
the attachment of ubiquitin proteins Giant
proteosomes recognize the ubiquitin and degrade
the tagged protein