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Lac Operon.docx

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Biology 1002B
Tom Haffie

Lecture 14: Lac Operon Which Ribosome Has Been Translating The Longest?  Many polymerases on many genes  A – translating the longest Which of these signals in DNA does DNA understand?  Promoter o Double stranded structures o Interpreted by the cell as DNA  SD Box o Understood by the cell as RNA Operon  Genes that code for subunits of enzymes are together on bacterial chromosome  Single promoter can serve all those genes, together called – transcription unit  Transcription gives rise to one long mRNA molecule  Key advantage for grouping genes – single on and off “switch” can control the whole cluster of genes “coordinate control”  Switch is a segment of DNA called an operator o Positioned within promoter or between promoter & enzyme-coding genes  Operator controls access of RNA polymerase to genes  Operon o Together –operator, promoter and genes they control o Entire stretch of RNA required  Inducible Operon o Is usually off but can be stimulated (induced) when a specific small molecule interacts with a regulatory protein o Ex. Lac operon  Repressible Operon o Transcription is usually on but can be inhibited (repressed) when a specific small molecule binds allosterically to a regulatory protein o Ex. Trp operon  Operator o Sequence of nucleotides near start of an operon to which an active repressor can attach o Binding of the repressor prevents RNA polymerase from attaching to promoter and transcribing genes  Repressor o A protein that inhibits gene transcription o In prokaryotes, repressors bind to the DNA in or near the promoter o In eukaryotes, repressors may bind to control elements within enhances, to activators, or to other proteins in a way that blocks activators from binding to DNA  Corepressor o A small molecule that binds to a bacterial repressor protein and changes its shape, allowing it to switch an operon off Lactose Metabolism  Regulated by the lac Operon in E. Coli  E. Coli had the ability to grow on lactose, degrade lactose and use energy o Galactosidease breaks down lactose into:  Galactose & Glucose  Fed into glycolysis o Galactosidease side reaction – creates isomer of galactose, allolactose  Galactosidease activity is low  Inducible (wildtype phenotype): Add lactose to system – galactosidease rises  Mutant types (lack inducibility) -
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