gene regulation
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GENE REGULATIONGENE REGULATION
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aka. when to turn genes on!
aka. when to turn genes on!
Consider the difference:Consider the difference:
Prokaryotic cells exist AS ONE CELL
That’s right…
ONE CELL DOES IT ALL!
Prokaryotic cells exist AS ONE CELL
That’s right…
ONE CELL DOES IT ALL!
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Consider the difference:Consider the difference:
Prokaryote genes are regulated via. Transcriptional control
Prokaryote genes are regulated via. Transcriptional control
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This means that genes This means that genes are are turnedturned onon and and offoff in response to the in response to the
need of a particular need of a particular gene product(s) at a gene product(s) at a
particular timeparticular time
This means that genes This means that genes are are turnedturned onon and and offoff in response to the in response to the
need of a particular need of a particular gene product(s) at a gene product(s) at a
particular timeparticular timeQuickTime™ and a
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This is different than EUKARYOTES
This is different than EUKARYOTES
Eukaryotic cells have a long life span, during which they may need to respond repeatedly to many different stimuli.
New enzymes are not synthesized each time the cells respond to a stimulus
Eukaryotic cells have a long life span, during which they may need to respond repeatedly to many different stimuli.
New enzymes are not synthesized each time the cells respond to a stimulus
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EUKARYOTESEUKARYOTES Most enzymes &
proteins are transformed from an inactive state to an active state, there is a RESERVE.
Some eukaryotic cells have a large store of inactive mRNA
Most enzymes & proteins are transformed from an inactive state to an active state, there is a RESERVE.
Some eukaryotic cells have a large store of inactive mRNA
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THINK ABOUT IT…THINK ABOUT IT…» RBC’s produce
hemoglobin (O2
transporting protein)
» Muscle cells produce myoglobin
(O2 storing protein)
» RBC’s produce hemoglobin
(O2 transporting protein)
» Muscle cells produce myoglobin
(O2 storing protein)
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THINK ABOUT IT…THINK ABOUT IT…»BOTH CELLS HAVE GENES THAT WILL NEVER BE USED.. IT WOULD BE WASTEFUL TO MAKE HEMOGLOBIN IF YOU WERE A MUSCLE CELL
»BOTH CELLS HAVE GENES THAT WILL NEVER BE USED.. IT WOULD BE WASTEFUL TO MAKE HEMOGLOBIN IF YOU WERE A MUSCLE CELL
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It has 4288 genes that code for proteins… some are always needed (ex. glycolysis enzymes)
Some are needed only when there are certain
environmental conditions
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An E. coli living in an adult cow intestine is not normally
exposed to LACTOSE (disaccharide)
HOWEVER, if you were in the colon of a calf… lactose would be a primary energy source.
Should the E. coli invest energy and
materials to produce lactose-produce lactose-
metabolizing metabolizing enzymesenzymes just IN
CASE it ends up in the digestive
system of a calf?
HOW can an HOW can an individual individual
bacterium, locked bacterium, locked into the genome into the genome
that it has that it has inherited…inherited…
cope with the cope with the ever “changing” ever “changing” environment????environment????
????
Turns out, the three genes to produce enzymes for lactose
metabolism are found together in a complex
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These three genes, turns out, are linked by a common control
mechanism
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OperonA gene complex, A gene complex, consisting of a group of consisting of a group of genes w/ related genes w/ related functionsfunctions& DNA sequences that & DNA sequences that control them. control them.
OperonThis is the mechanism This is the mechanism by which bacteria by which bacteria control gene expressioncontrol gene expression
Operon ModelJacob and Monod Jacob and Monod (1965 Nobel Prize for physiology (1965 Nobel Prize for physiology or medicine) - for their discovery or medicine) - for their discovery of the of the Prokaryotic model of gene control.AlwaysAlways on the national AP Biology on the national AP Biology exam !exam !
So, if you remember… to use lactose as an
energy source…
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The lactose is cleaved by enzyme galactosidase…
Another enzyme then converts galactose to
glucose
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And still a third enzyme (function not clear) is needed to complete the process
for glucose to move across the cell membrane
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E. Coli growing on glucose produces very little galactosidase…
NO NEED!!
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However, when grown on lactose, there are SEVERAL
THOUSAND galactosidase molecules
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Bacteria can, in
one sweep, turn off or on
genes that code for
the production of these enzymes
Operon Model
1. Operon Area a. Operatorb. Promoter (where RNA
polymerase binds to DNA to begin transcription)
c. Structural Genes2. Regulatory Gene
Operon Structure
1. Operon Area a. Operatorb. Promoterc. Structural Genes
OPERON AREA
OPERATOR ->
PROMOTER ->
GENES THEY CONTROL
OperatorOperator - segment of DNA that “turns on” the RNA polymerase that is binding to the gene coding area
OPERATOROPERATORSegment of DNA that “turns on” the ability for RNA polymerase to bind these genes
Pix of switch here
OPERATOROPERATORIf blocked, will NOT permit RNA polymerase to pass - preventing transcription of genes
Pix of switch here
PromotorPromotor - area where RNA polymerase binds to the DNA
OPERATOR OFFOPERATOR OFFIf blocked, will NOT permit RNA polymerase to pass - preventing transcription RNA RNA
polymepolymeraseblraseblockedocked
OperatorOperator ONON
RNA RNA polymerase polymerase not not blockedblocked
OperatorOperator ONON
RNARNA polymerasepolymerase can bind to can bind to promotorpromotor
OperatorOperator ONON
via.via. RNA RNA polymerasepolymerase
GenesGenes transcritranscribedbed
OPERATOR ->
PROMOTER ->
GENES THEY CONTROL ->
2. Regulatory Gene - codes for repressor
molecules (this is upstream)
Repressor Protein - switches OFF the Operator and the OPERON cannot be transcribed
In this case, continuously made by the
regulatory gene
Repressor Protein - switches OFF the operonRNARNA
polymerasepolymerase cannot move cannot move down the down the operonoperon
no mRNA made no mRNA made - no enzymes - no enzymes mademade
Repressor Protein - switches OFF the operonRNARNA
polymerasepolymerase
ex. lac Operon - codes for three genes that function in the production of an enzyme that breakdowns the disaccharide lactose
(no breakdown of lactose)
lac Operon - usually off, only works when
substrate (in this case: lactose) is present
(no breakdown of lactose)
Promoter - (remember) area where RNA polymerase binds to the DNA to promote transcription
GenesGenes - plural… code for enzymes that break down lactose
withwith nono lactoselactose inin thethe systemsystem -> -> regulatory gene regulatory gene codes for codes for repressorrepressor.. repressor binds to repressor binds to operatoroperator - - no RNA no RNA polymerase attachmentpolymerase attachment - - no digestive enzyme made- no digestive enzyme made- NO NEED`NO NEED`
repressor
withwith lactoselactose presentpresent - - inducer (an isomer of inducer (an isomer of lactose) binds lactose) binds allosterically to allosterically to thethe repressorrepressor
inducerinducer
withwith lactoselactose presentpresent - - shape change, and the DNA shape change, and the DNA no longer recognizes the no longer recognizes the repressorrepressor
inducerinducer
withwith lactoselactose presentpresent - - RNA RNA polymerase actively polymerase actively transcribestranscribes the structural the structural genesgenes
inducerinducer
inducerinducerWhen enough Lactose has been digested, the RepressorRepressor binds again to the OperatorOperator and switches the Operon "off”…
no digestive enzymes made.
inducerinducer
Repressor usually controls Repressor usually controls by keeping the operon off!…. by keeping the operon off!…. The presence of an The presence of an inducerinducer inactivates the repressor, inactivates the repressor, permitting the genes to be permitting the genes to be transcribed.transcribed.
usually involved in genes usually involved in genes that BREAK DOWN molecules to that BREAK DOWN molecules to
provide energyprovide energy
Lets do another example…
Pix of intestine here
Think of the poor
bacteria that live in your colon…
Pix of intestine here
They are dependent on your whims of what you decide to eat for their
nutrients!
Pix of intestine here
They need tryptophan (an amino acid) to survive!!
Pix of intestine here
What if you don’t eat anything
with tryptophan in it??
Pix of intestine here
THEY STILL NEED the
tryptophan
A single promoter serves ALL FIVE genesTranscription w/ give rise to one LONG mRNA that codes for all five enzymes in the pathway
Turns out…there are five genes that code for the enzymes that produce tryptophan.
usually ON
ex. “trp Operon”
It is the normal state for cell to make
tryptophan
ex. “trp Operon”
If tryptophan accumulates b/c you are eating it… the bacteria will shut off its own synthesis of tryptophan, by not making an enzyme
Tryptophan present
transcription usually on!!
Tryptophan absent - usual
Repressor usually controls by Repressor usually controls by keeping the operon on!…. The keeping the operon on!…. The presence of an presence of an inducerinducer activates activates the repressor, inhibiting the the repressor, inhibiting the genes from being transcribed.genes from being transcribed.
usually involved in genes usually involved in genes that PUT TOGETHER molecules that PUT TOGETHER molecules
to provide energyto provide energy
Summary
• Know Operon theory !• Know the difference between repressible and inducible enzymes.
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