Transcript Regulation of Gene Expression - mvhs

Regulation of Gene Expression
Prokaryotes and Eukaryotes
Unit 2
AP Biology
Regulation of Gene Expression
• A cell contains the entire genome of an
organism– ALL the DNA.
• Gene expression = transcribing and
translating the gene
• Regulation allows an organism to
selectively transcribe (and then translate)
only the genes it needs to.
• Genes expressed depend on
– the type of cell
– the particular needs of the cell at that time.
Regulation of Gene Expression
• Up-regulation = increased
expression of gene
• Down-regulation = decreased
expression of gene
• Each of these can be due to a
variety of factors (to be discussed
later)
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Gene Regulation in Prokaryotes
• Prokaryotes organize their genome into operons
• Operon = a group of related genes
– One promoter sequence at the very beginning
– All of the genes will be transcribed together (in one
long strand of RNA).
Question…
• What is the benefit of organizing the
genome into operons?
– It’s more efficient – transcribe everything you
need for a process at once.
Repressible Operon: Trp Operon
• Repressible Operon = Operon that is usually
“ON” but can be inhibited
• The Trp Operon
– example of a repressible operon
– Genes that code for enzymes needed to make the
amino acid tryptophan
TrpR Gene
• TrpR gene is the regulatory gene
for the Trp operon
– Found somewhere else on the
genome
– NOT part of the Trp operon
– TrpR gene codes for a protein =
TrpR repressor
– TrpR gene is transcribed and
translated separately from the Trp
operon genes.
TrpR Repressor
• Repressor protein is translated in an inactive
form
• Tryptophan is called a corepressor
– When tryptophan binds to the TrpR repressor, it
changes it into the active form
Operator Region
• There is also an operator region of DNA in the
Trp Operon
– Just after the promoter region
– The TrpR Repressor can bind to the operator if it’s
in the active form
Trp Operon
• Transcription is “ON”
– Occurs when there is no tryptophan available to the
cell.
– Repressor is in inactive form (due to the absence of
tryptophan)
– RNA Polymerase is able to bind to promoter and
transcribe the genes.
Trp Operon
• Transcription is “OFF”
– Occurs when
tryptophan is available
– Tryptophan binds to the
TrpR repressor 
converts it to active
form
– TrpR protein binds to
operator  blocks RNA
Polymerase  no
transcription
Question…
• Under what conditions would you expect
the trp operon to go from “OFF” to “ON”
again?
– When there is no longer tryptophan available–
all of it has been used up
Inducible Operon: Lac Operon
• Inducible operon = operon is usually “OFF”
but can be stimulated/activated
• Lac Operon
– Example of an inducible operon
– Genes code for enzymes that break down
lactose
LacI gene
• LacI gene is the regulatory
gene for the lac operon
– Found somewhere else on the
genome
– NOT part of the lac operon
– LacI gene codes for a protein
= lacI repressor
– LacI gene is transcribed and
translated separately from the
lac operon genes.
LacI Repressor
• The lacI repressor protein
is translated into an active
form
• When the lacI repressor is
bound by lactose (also
called allolactose) it
becomes inactive
– Lactose is the inducer
Lac Operon
• Transcription is “OFF”
– When there is no lactose that needs to be
digested
– lacI repressor is in active form  binds to
operator  blocks RNA Polymerase  no
transcription
Lac Operon
• Transcription is “ON”
– When there is lactose that needs to be digested
– Lactose binds to lacI repressor  inactivates it
– RNA Polymerase is able to bind to promoter 
transcribe genes
Do all operons have operator regions?
• NO
• There are some genes that always need to
be transcribed  they do not need to have
operators to regulate them in this manner.
• Ex. genes that participate in cellular
respiration
Positive Gene Regulation
• In the lac operon there are other molecules
to further stimulate transcription.
• Lactose will only be digested for energy
when there isn’t much glucose around
• When glucose levels are low, level of
cAMP molecule builds up
cAMP and CAP
• CAP = regulatory protein
that binds to cAMP
• CAP is inactive unless
cAMP binds to it
Positive gene regulation
• If there isn’t much
glucose high levels
of cAMP
• CAP and cAMP bind
 CAP can bind to
the promoter 
stimulates RNA
Polymerase to bind
Positive gene regulation
• When glucose levels rise again, cAMP
levels will drop  no longer bound to CAP
• CAP can’t bind to promoter 
transcription slows down
Positive gene regulation
• The lac operon is controlled on 2 levels:
– Presence of lactose determines if transcription
can occur
– CAP in the active form determines how fast
transcription occurs
Gene Regulation in Eukaryotes
• Eukaryotes have large genomes
• Other molecules have to help RNA
Polymerase find the promoter and start
transcription
– Transcription factors
– Enhancer sequences
Transcription Factors
• Series of proteins that bind to the
DNA to control transcription
– Activators: increase expression (ex.
bind to promoter to help RNA
Polymerase bind)
– Repressors: decrease expression of
gene
• RNA Polymerase also has to bind
certain transcription factors in
order to be able to start
transcription.
Question…
• How might binding transcription factors
help RNA Polymerase bind?
– Creates an area that chemically attracts RNA
Polymerase more
Enhancer sequences
• Sequences of DNA that are
far away from the gene they
help transcribe
• Process:
– Activator molecules bind to
the Enhancer sequence
– Enhancer loops around so
that the activators can also
bind to the transcription
factors
– Together with RNA
polymerase they all cause
transcription to start
Cell-specific Regulation
• Each cell has the DNA to
transcribe any gene
• Different activators and
transcription factors in
specific cells will determine
which genes are transcribed
 which proteins are
translated