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

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

4. 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

5. 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

6. 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.

7. 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

8. 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.

9. 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

11. 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

13. 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

14. 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

15. Trp operon

17. 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

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

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