Gene Regulation - PDF document

Gene RegulationGene RegulationGene exp
Gene RegulationGene RegulationGene expression can be turned on, turned off, turned up or turned down!For example, as test time approaches, some of you may note that stomach acid production increases dramatically…

;. due to regulation of the genes that c
;. due to regulation of the genes that control synthesis of HCl by cells within the gastric pits of the stomach lining. Gene Regulation in ProkaryotesProkaryotes may turn genes on and off depending on metabolic demand

s and requirements for respective gene
s and requirements for respective gene products. NOTE:For prokaryotes, “turning on/off” refers almost exclusively to stimulating or repressing transcriptionGene Regulation in ProkaryotesInducible/Repressib

le Inducible/Repressible gene products
le Inducible/Repressible gene products: those produced only when specific chemical substrates are present/absent.ConstitutiveConstitutivegene products: those produced continuously, regardless of chemical substrates

present.Gene Regulation in Prokaryotes
present.Gene Regulation in ProkaryotesRegulation may be Negative: gene expression occurs unless it is shut off by a regulator moleculeorPositive: gene expression only occurs when a regulator mole turns it on Oper

onsIn prokaryotes, genes that code for
onsIn prokaryotes, genes that code for enzymes all related to a single metabolic process tend to be organized into clusters within the genome, called operonsoperons. An operonis usually controlled by a single regula

tory unit. Regulatory Elementsciscis
tory unit. Regulatory Elementsciscis--acting elementacting element: The regulatory region of the DNA that binds the molecules that influences expression of the genes in the operon. It is almost always upstream (

5’) to the genes in the operon. Tr
5’) to the genes in the operon. TransTrans--acting elementacting element: The molecule(s) that interact with the cis-element and influence expression of the genes in the operon. The lac operonThe lac opero

n contains the genes that must be expres
n contains the genes that must be expressed if the bacteria is to use the disaccharide lactose as the primary energy source. To be used as an energy source, lactose must be cleaved into glucose and galactose. The gluco

se is then available for metabolism (gly
se is then available for metabolism (glycolysis).Note: glucose is the preferred energy substrate.Negative ControlThe genes in the lacoperonare normally turned off, and only expressed when a repressor molecule is remo

ved from the regulatory region. This re
ved from the regulatory region. This repressor is removed only in the presence of lactoseThe lacOperonRegulatory RegionRepressorgeneStructural GenesLacIPOlacAlacYlacZP=PromoterO=OperatorStructural Genes

Structural genes are those that encode f
Structural genes are those that encode for the enzymes that do the metabolic work. LacZ: -galactosidase, cleaves lactose into glucose and galactoseLacY: Permease, promotes entry of lactose into cellLacA:Transacetylas

e, thought to reduce toxicity of byprod
e, thought to reduce toxicity of byproducts of lactose metabolismStructural GenesIn prokaryotes, all the structural genes within an operon are usually transcribed as a single mRNA, then the genes are independently tr

anslated by ribosomes.LacI—The Rep
anslated by ribosomes.LacI—The RepressorLacI is the regulatory molecule. When there is no lactose present in the cell,LacI binds to the Operator element and blocks binding of RNA polymerase to the Promoter elemen

t. LacIPOlacAlacYlacZXLacI—
t. LacIPOlacAlacYlacZXLacI—The RepressorWhen lactose IS present, the genes to metabolize lactose must be expressed. Lactose itself causes LacI to dissociate from the operator, which frees up the promoter

region, allowing RNA polymerase to bind,
region, allowing RNA polymerase to bind, and transcription begins. Lactose is the inducer moleculeinducer moleculefor the lac operon.Induction of the lacoperonLactoseBinding of lactose causes achange in the shape

of LacILacIPOlacAlacYlacZInducti
of LacILacIPOlacAlacYlacZInduction of the lacoperonLacIPOlacAlacYlacZLacIPOlacAlacYlacZWhat happens if you mutate LacI? LacI encodes the lac repressor, which keeps the operon shut off in the absenc

e of lactose.What happens if you mutate
e of lactose.What happens if you mutate LacI?Inactivation of LacIwould be called a constitutive constitutive mutationmutation, because the genes of thelacoperonwould be on all the time even if there is no lactose

present (removed repression).Positive C
present (removed repression).Positive Control of the lacOperonA further increase in transcription of the lac operon occurs if a molecule called catabolite-activating protein(CAP)(CAP)also binds the promoter region.La

cIPOlacAlacYlacZCAP facilitates th
cIPOlacAlacYlacZCAP facilitates the binding of RNA polymerase,and therefore increases transcriptionPositive ControlRemember, glucose is the preferred substrate. CAP exists in the state that will bind the promot

er ONLY when glucose is absent. LacIP
er ONLY when glucose is absent. LacIPOlacAlacYlacZThis is the form CAP takes when there is no glucosePositive ControlWhen glucose is present, CAP exists in a state that will NOT bind the promoter of the lacoper

on. LacIPOlacAlacYlacZXThis is th
on. LacIPOlacAlacYlacZXThis is the shape CAP takes when glucose ispresent. It cannot bind the promoter in this shapeXRegulation of thelacOperonSo, transcription is regulated as follows:OffOffwhen lactose is

absent (repressed)ActiveActivewhen l
absent (repressed)ActiveActivewhen lactose is present as well as glucose (de-repressed)Really activeReally activewhen lactose is present but glucose is absent (activated)Gene Regulation in EukaryotesDifferences

between Prokaryotes and Eukaryotes1. D
between Prokaryotes and Eukaryotes1. DNA is a lot more complicated in eukaryotes—there’s a lot more of it and it’s complexed with proteins to form chromatin2. Genetic information is carried on multiple

chromosomes3. Transcription and transl
chromosomes3. Transcription and translation are physically separatedDifferences between Prokaryotes and Eukaryotes (cont.)4. Eukaryotic mRNA is processed prior to translation5. Eukaryotic mRNA is much more stable (

not as easily degraded)Gene expression
not as easily degraded)Gene expression can be controlled at the level of translation!6. Different cell types express different genesChromatin RemodelingChemical alteration of the histone proteins of chromatin facili

tates or inhibits access of RNA polymera
tates or inhibits access of RNA polymerases to DNA promoters.Recruitment of Co-activatorsRemember enhancer elementsenhancer elements? These are binding sites for molecules that influence formation of the RNA polymer

ase initiation complex. Enhancer eleme
ase initiation complex. Enhancer elements may have DNA sequences for both positive and negative regulators of transcription. EnhancersThe presence or absence of regulators is determined by the cell’s environmen

t, metabolic state, developmental state
t, metabolic state, developmental state and/or the presence or absence of signal molecules. The net effect of all the information available, summed up by the regulators present, dictates the transcription efficiency of

RNA polymerase from a given promoter.
RNA polymerase from a given promoter. DNA MethylationChemical modification of DNA by adding or removing methyl (-CH3) groups from the DNA bases, usually cytosine. The presence of the methyl group alters the shape of

DNA, which influences the binding of pr
DNA, which influences the binding of proteins to the methylatedDNA. DNA MethylationTypically, increased methylation decreases transcription efficiency. In mammalian females, one X chromosome is inactivated (only one

of the X chromosomes is used to drive t
of the X chromosomes is used to drive transcription). The inactivated X chromosome has much more methylation than the active chromosome. Post-Transcriptional RegulationAlternative Splicing: Exon 1Exon2Exon3Exon4

Exon51.Exon 1Exon2Exon3Exon4Exon5
Exon51.Exon 1Exon2Exon3Exon4Exon52.Exon 1Exon2Exon4Exon5Post-Transcriptional ControlRNA Stability1.Stability sequences2.Instability sequences3.Translation efficiency—increased translation increases