Chapter 18 Notes

Chapter 18 Notes - Chapter 18 Regulation of Gene Expression...

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Chapter 18 Regulation of Gene Expression 18.1 Bacteria often respond to environmental change by regulating transcription 1. First, cells can adjust the activity of enzymes already present. (feedback inhibition). Second, cells can adjust the production level or certain enzymes. 2. The basic mechanism for this control of gene expression in bacteria, described as the operon model , was discovered in 1961 by Francois Jacob and Jacques Monod. 3. Operons: The Basic Concept a. The switch is a segment of DNA called an operator. It’s positioned within the promoter or between the promoter and the enzyme-coding genes, the operator controls the access of RNA polymerase to the genes. b. The operon is the operator, promoter, and the genes they control. c. The operator can be switched off by the protein called the trp repressor. It binfs to the operator and blocks attachment of RNA polymerase to the promoter, preventing transcription of the genes. d. The trp repressor is the product of a regulatory gene called trpR , which is located some distance from the operon it controls and has its own promoter. e. The trp operon is not switched off permanently because the binding of repressors to operators is reversible and the trp repressor, like most regulatory proteins, is an allosteric protein, with two alternative shapes, active and inactive. f. The trp repressor is synthesized in an inactive form with little affinity for the trp operator. g. Tryptophan functions in this system as a corepressor , a small molecule that cooperates with a repressor protein to switch an operon off. 4. Repressible and Inducible Operons: Two Types of Negative Gene Regulation a. The trp operon is said to be a repressor operon because its transcription is usually on but can be inhibited when specific small molecule binds allosterically to a regulatory protein. b. An inducible operon is usally off but can be stimulated when a specific small molecule interacts with a regulatory protein. c. The gene for B-galactosidase is part of the lac operon, which includes two other genes coding for enzymes that function in lactose utlization. d. The regulatory gene, lacl , located outside the operon, codes for an allosteric repressor protein that can switch off the lac operon by binding to the operator. e. The lac operon, is active by itself, binding to the operator and switching the lac operon off. In this case, a specific small molecule, called an inducer , inactivates the repressor. f. Inducible enzymes are induced by a chemical signal while repressible enzymes generally function in anabolic pathways, which synthesize essential end products from raw materials. g. Gene regulation is only positive when a regulatory protein interacts directly with the genome to switch transcription on. 5.
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This note was uploaded on 02/07/2012 for the course BIO 1711 taught by Professor Burleson during the Spring '11 term at North Texas.

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Chapter 18 Notes - Chapter 18 Regulation of Gene Expression...

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