gene regulation

GENE REGULATIONGENE REGULATION

QuickTime™ and a decompressor

are needed to see this picture.


are needed to see this picture.VS.

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!



Consider the difference:Consider the difference:

Prokaryote genes are regulated via. Transcriptional control

Prokaryote genes are regulated via. Transcriptional control



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

decompressorare needed to see this picture.

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



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



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)



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



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





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



These three genes, turns out, are linked by a common control

mechanism





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…



The lactose is cleaved by enzyme galactosidase…

Another enzyme then converts galactose to

glucose



And still a third enzyme (function not clear) is needed to complete the process

for glucose to move across the cell membrane



E. Coli growing on glucose produces very little galactosidase…

NO NEED!!



However, when grown on lactose, there are SEVERAL

THOUSAND galactosidase molecules



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


RNARNA polymerasepolymerase can bind to can bind to promotorpromotor


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…


They are dependent on your whims of what you decide to eat for their

nutrients!


They need tryptophan (an amino acid) to survive!!


What if you don’t eat anything

with tryptophan in it??


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.

gene regulation

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