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Chapter 15

Chapter 15 - Control of Gene Expression.docx

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BIOL 204
Dr.Roland Treu

CHAPTER 15 CONTROL OF GENE EXPRESSIONHuman egg cell almost completely inactive metabolically when released from ovaryAt first glance might think most efficient for each differentiated cell type to retain only genes needed to carry out its specific functionoIn fact structuralfunctional differences in cell types result from presenceabsence of products resulting from expression of genes rather than from actual genes themselvesProducts of geneshousekeeping genesexpressed in nearly all cells whereas products of other genes may be found only in certain cell types under particular env conditionsoEx all cells contain genes coding for enzymes needed for basic cellular metabolism that is Krebs cycle as well as genes coding for various hemoglobin polypeptidesWhile Krebs cycle gene products found in all genes particular Hb found only in those cells that give rise to RBC in fetusnewbornadultBeyond transcription expression of gene products is subject to further controls affecting processing of RNA possible translation into protein and activitylifespan of product itself151 Regulation of Gene Expression in ProkaryotesTranscription and translation closely related in proks in ways that reflect prok life historiesEx E ColioCan find itself in intestinal tract of cow one minute and then in treated municipal water supply soon afteroSugars such as lactose might be more available in water env and genes coding for enzymes needed to metabolize this energy source need to be turned onoOther nutrients ie AA tryptophan may also be available in water TF genes coding for enzymes needed to manufacture AA from scratch need to be turned offoVersatile and responsive control system allows bacterium to make most efficient use of particular array of nutrients available at any given time151a The Operon is a Unit of TranscriptionWhen env in which bacterium lives changes some metabolic processes stopped and others startedFor each metabolic process few too many genes involved and regulation must be coordinatedoEx 3 genes encode proteins for metabolism of lactose by E ColioIn absence of lactose those 3 genes are not expressed and vvoThat is control of these genes is at transcription level1961 JacobMonod proposed operon model for control of expression of genes for lactose metabolism in E ColiOperoncluster of prok genes and DNA sequences involved in their regulationoEach operon which can contain several to many genes is transcribed as a unit from promoter into single mRNA and aar mRNA contains codes for several proteinsoCluster of genes transcribed into single mRNAtranscription unitoRibosome translates mRNA from one end to other sequentially making each protein encoded in mRNAOperatorregulatory DNA sequence in operon short segment to which regulatory protein bindsoEncoded by gene separate from operon that protein controlsoSome operons controlled by regulatory protein called repressor which when active prevents genes of operon from being expressedoOthers controlled by activator which when active stimulates expression of genesMany operons controlled by 1 regulatory mechanism andof repressorsactivators control 1 operonoResult complex network of superimposed controls that provides regulation of transcription allowing almost instantaneous responses to changing env conditions151b The lac Operon for Lactose Metabolismlac operon for lactose metabolism transcribed when inducer inactivates repressorJacobMonod genetic studies showed that for lactose metabolism3 genes involvedlacZ lacY and lacAThese 3 genes are adjacent to one another on chromo in order ZYAGenes transcribed as a unit into single mRNA starting w lacZ gene promoter for transcription unit upstream of lacZlacZ gene encodes the enzyme galactosidase which catalyzes conversion of disaccharide sugar lactose into the monosaccharide sugars glucose and galactoseThese sugars then metabolized by other enzymes producing energy for celllacY gene encodes permease enzyme that transports lactose actively into cell and lacA gene encodes transacetylase enzyme function of which is unknownCalled cluster of genes and adjacent structures that control their expression lac operonoCoined operon from key DNA sequence that regulates transcription of operonthe operator bc it controls operation of genes adjacent to itShowed lac operon controlled by regulatory protein Lac repressoroEncoded by regulatory gene lacI nearby but separate from lac operon and synthesized in active formoLactose absent from medium active Lac repressor binds to the operatorblocking the RNA polymerase from binding to the promoteraar transcription cannot occuroLactose present in medium lac operon turned on and all 3 enzymes synthesized rapidlyLactose enters cell and galactosidase molecules already present convert some of it ti allolactose isomer of lactose and inducer for lac operon turns on 3 genes in operonw repressor out of way RNA polymerase able to bind to promoter and it transcribes three genesLac operon called inducible operon bc inducer molecule increases its expressionWhen lactose used up regulatory system again switches lac operon offoThat is absence of lactose means no allolactose inducer molecules to inactivate repressoroAgainactive repressor binds to operator blocking transcription of operonControls aided by fact that bacterial mRNAs very shortlived 3 min avgoQuick turnover permits cytoplasm to be cleared quickly of mRNAs transcribed from operonoEnzymes themselves also have short lifetimes and quickly degraded
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