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CHAPTER 15 R EGULATION OF G ENE ACTIVITY AND G ENE M UTATIONS Chapter Outline 15.1 Prokaryotic Regulation A. The Operon Model 1. Bacteria do not require the same enzymes all the time; they produce just those enzymes needed at the moment. 2. In 1961, French microbiologists Francois Jacob and Jacques Monod proposed the operon model to explain regulation of gene expression in prokaryotes; they received a Nobel prize for this. a. In the operon model, several genes code for an enzyme in the same metabolic pathway and are located in a sequence on a chromosome; expression of structural genes is controlled by the same regulatory genes. b. An operon is the structural and regulatory genes that function as a single unit; it includes the following: 1) A regulator gene located outside the operon codes for a repressor protein molecule that controls whether the operon is active or not. 2) A promotor is the sequence of DNA where RNA polymerase attaches when a gene is transcribed. 3) An operator is a short sequence of DNA where an active repressor binds, preventing RNA polymerase from attaching to the promotor and transcription therefore does not occur. 4) Structural genes are one to several genes coding for enzymes of a metabolic pathway that are transcribed as a unit. B. The trp Operon 1. Jacob and Monod found some operons in E. coli usually exist in the on rather than the off condition. 2. This prokaryotic cell ( E. coli ) produces five enzymes as part of the metabolic pathway to synthesize the amino acid tryptophan. 3. If tryptophan is already present in medium, these enzymes are not needed and the operon is turned off by the following method. a. In the trp operon, the regulator codes for a repressor that usually is unable to attach to the operator. b. The repressor has a binding site for tryptophan (if tryptophan is present, it binds to the repressor). c. This changes the shape of the repressor that now binds to the operator. 4. The entire unit is called a repressible operon ; tryptophan is the corepressor . 5. Repressible operons are involved in anabolic pathways that synthesize substances needed by cells. C. The lac Operon 1.
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