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Chapter 12 Notes (Summer2011)

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University of Toronto Scarborough
Biological Sciences
Rosada Silva

BIOB11Summer2011 Chapter 12 The Cell Nucleus and the Control of Gene ExpressionGene Regulation Gene Regulation o A cell only needs a fraction of its genome at any given moment o Producing unnecessary transcripts and proteins is wasteful and expensivecellular materials and energy used up o Genes are only expressed when needed and for as long as needed Bacterial Gene Regulation o Most bacterial genes are on by default o They are repressed when not needednegative regulation o To be repressed a gene needs a repressor proteina ligand that can be inducible on when signal is present or repressible off when signal is present o Genes available for expression by default are switched off by repressor proteins o Repressor proteins can regulate expression by binding to a DNA sequence operon o Repressor binding blocks the RNA polymerase from binding to the promoterrepression of operon geneexpression Components of Bacterial Operon o Operator site next to the promoter where the regulatory protein can bind to o Promoter where the RNA polymerase binds repressor protein binds there too o Structural genes code for the enzymes and are translated as a single mRNA o Regulatory gene encodes for the repressor protein Bacterial Operon o Capability of the repressor to bind the operator depends on conformation o Conformation is regulated by key compounds in the metabolic pathway being regulated o Concentration of this compound determines whether operon is active or inactive The lac Operon o Thelac operon is an inducible operon presence of lactose induces transcription of structural genes o The operon contains 3 genes z gene Beta galactosidase y gene galactoside permease and a gene thiogalactoside transacetylase o Lactose binds to the repressor changing its conformation and making it unable to bind to the operatorthe structural genes are transcribedenzymes are synthesized lactose molecules are catabolised o A repressor protein can bind to the operator and prevent transcription only in the absence of lactoselactose is an inducero As the concentration of lactose falls and eventually there is no more lactose the repressor protein binds to the operator and transcription is stoppedit no longer needs the enzyme to metabolize lactose o Repressors are a form of negative controlbut the lac operon is a form of positive controlglucose effect o When supplied with glucose the bacteria do not catabolise other compounds like lactosehigher glucose means lower cAMP 1
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