Control of gene expression

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• Control of gene expression
• Prokaryotes
• Eukaryotes

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Prokaryotes Feedback inhibition enzymes are
already available end product of pathway binds
to enzyme Gene regulation operons genes
involved in a pathway are synthesized together T
ranscription and translation are coupled
in prokaryotes (protein synthesis begins
before transcription is finished)
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Repressor is synthesized continuously but at
low levels
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The trp operon is repressible it is turned off
by the binding of the repressor and co-repressor
(tryptophan) to the operator Other operons are
inducible they are turned on (induced) when a
molecule binds to a regulatory protein
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These operons are negatively controlled represso
rs turn genes off (Allolactose is a derepressor
because it works by binding to the
repressor Positive regulation is also seen
(molecule interacts with DNA to activate
genes) lac operon E. coli detects that lactose
is present and also that glucose is not
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Catabolite repression glucose is the
preferred energy source. It inhibits synthesis
of enzymes that catabolize other
molecules. How? When glucose levels are low,
cyclic AMP accumulates (glucose inhibits
adenylyl cyclase Levels are sufficient to bind
to a regulatory protein that activates
transcription of the lac operon (this works on
other inducible operons, too)
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Notice that active CRP is enhancing
transcription, but also that the inducer
(allolactose) is present Two levels of
controls lactose-metabolizing enzymes are
synthesized only when needed
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Other regulatory factors in prokaryotes Sigma
factors (promote transcription of specific sets
of genes) Attentuator sites reduce
synthesis structures form in RNA that block
trans- cription if end product is plentiful
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It was initially thought that regulatory
processes were very similar in prokaryotes
and eukaryotes Both are regulated at the
transcriptional level promoters and regulatory
proteins More elaborate in eukaryotes More
posttranscriptional control in eukaryotes
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Eukaryote genome is much larger and more complex
(most of the DNA is not transcribed at
all!) Much RNA processing occurs in the nucleus
of prokaryotes Genome structure in eukaryotes
is more elaborate and complex no operons-
response elements can regulate one or more genes
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RNA is more stable in eukaryotes Prokaryotes
need rapid response to environ- mental
change Cells within a multicellular organism are
less sensitive and may require a less
diverse response (only a particular subset
of genes will be transcribed)
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Eukaryotic proteins undergo extensive posttransl
ational modification (after polypeptide
sequence is made it may be edited, other active
groups added, etc.) Protein turnover
(degradation) is more important in eukaryotes.
Prokaryotes can dilute out unwanted proteins
through rapid cell division some
eukaryotic cells dont divide at all!
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Cell differentiation (from stem cells) Multiple
levels of regulation in eukaryotic
cells accessibility of genes transcription
factors RNA processing (e.g., alternative
splicing) mRNA stability protein folding,
assembly, modification, etc. protein turnover