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Unit 13: Prokaryotic Gene Expression Lesson 2: lac vs trp operons

Unit 13: Prokaryotic Gene Expression Lesson 2: lac vs trp operons. Please Make Sure your final protein structure questions are turned in. Start do-now silently. Not all proteins need to be made all the time. Some proteins are made infrequently (specific antibodies towards diseases)

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Unit 13: Prokaryotic Gene Expression Lesson 2: lac vs trp operons

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  1. Unit 13: Prokaryotic Gene ExpressionLesson 2: lac vs trp operons Please Make Sure your final protein structure questions are turned in. Start do-now silently

  2. Not all proteins need to be made all the time • Some proteins are made infrequently (specific antibodies towards diseases) • Some proteins are only made during certain times of an organisms development • Some proteins are being used constantly (cell respiration enzymes) • Makes sense to be able to ‘turn genes off’ when their proteins are not needed

  3. Prokaryotic cells: Proteins interact with DNA to turn genes on or off (Eukaryotic genes seem a little more complicated but they are regulated too) • Do so in response to environmental changes • Able to conserve ATP • Able to conserve amino acids Flow of info from gene to protein called gene expression

  4. This model of gene control was first proposed as a hypothesis in 1961 by Jacob and Monod, for the control of lactose (milk sugar) utilization enzymes in E. coli.

  5. Important Features of their Model • An operon consists of several DNA sequences coding for different enzymes • all enzymes involved in the same cellular process • Expression of the operon is controlled as a unit • Other DNA sequences in and near the operon control the operon’s expression • The presence or absence of the enzyme’s substrate turns on or off the controls

  6. Operon Expression • normally starts with RNA polymerase binding at the promoter region(the first nongene region of the operon) • RNA Polymerase then moves along and transcribes each gene in the operon.

  7. lac operon turned OFF • a regulatory gene is transcribed and translated into a repressor protein • repressor protein binds with the operator region of the operon • This represses (blocks) the transcription of the genes further along the operon

  8. lac operon turned ON • When the lac operon is “turned on,” the regulatory gene continues to be transcribed and translated into repressor • presence of substrate (lactose) interferes with the binding of the repressor to the operator • Expression continues until the substrate is used up and the operon turns off

  9. Not all operons function the same • lac operon used the substrate to activate gene expression • Called inducible operon • Some operons (trp) use the substrate to turn off gene expression

  10. trp operon • Same set up as lac operon (regulatory gene, promotor, operator, structural genes) • Repressor protein works opposite • Repressor protein can only bind to the operator (block transcription) when it is first bound to the amino acid trp • Called repressible operator or negative feedback

  11. Trp operon

  12. http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120080/bio26.swf::The%20Tryptophan%20Repressorhttp://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120080/bio26.swf::The%20Tryptophan%20Repressor

  13. Analysis: Describe the operon- --Without lactose. Explain why the operon is turned off. --With lactose. Explain why the operon is turned on.

  14. Analysis: Describe the operon- --Without tryptophan. Explain why the operon is turned on. --With tryptophan. Explain why the operon is turned off.

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