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bio lec 12 - Exception RETROVIRUSES use RNA to synthesize...

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Exception: RETROVIRUSES use RNA to synthesize protein o All genes are not always being transcribed in all cells. CONSTITUTIVE GENES: o always being expressed in all cells o products are essential for normal cell growth and viability o AKA ‘housekeeping genes’ REGULATED GENES: o expression is controlled in response to needs of cell / organism o products are needed at specific times or in certain cells or in the presence of certain nutrients o lecture today will focus on how they know when they should be expressed Controlling Gene Expression in Prokaryotes: o Sidenote: The ‘promoter’ is the DNA sequence that RNA polymerase will bind to before it starts to transcribe o Mainly done by turning transcription on / off at the right time o A well-studied example: the lac operon . OPERON is a cluster of genes coordinately regulated by one promoter; they are only found in prokaryotes [figure 14-2] The lac operon: o Studied by Jacob and Monod (1961) o Contains structural genes that metabolize lactose in E. coli. [Structural genes code for polypeptides; structural genes are genes that are transcribed into a message and then translated into a protein] o E. Coli metabolizes lactose this way: lactose [enzyme beta-gal breaks it down into two separate sugars] glucose and galactose energy o Protein products: beta-gal, permease, transacytlase Jacob and Monod observed: o E. coli grown on media containing LACTOSE had LOTS OF BETA-GAL ACTIVITY. o E. coli grown on media containing GLUCOSE had nearly NO BETA-GAL ACTIVITY o Their results showed that E. coli only express beta-gal when they need to. Only when the substrate is available are the making what’s necessary. E. coli only express lac Z, lac Y. lac A genes when cells need protein products (in the presence of lactose) This is induction o Induction: turning on of gene expression o Inducer: environmental agent that triggers induction (in the case of lac operon, it’s lactose) o How does the presence / absence of lactose in the environment regulate gene expression inside bacterial cells? It does so by the repressor gene. The REPRESSOR GENE is not part of the operon; it’s
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