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Bio325 sample final exam key 07

Bio325 sample final exam key 07 - 1 a(3 pts Recall that the...

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1 1) a. (3 pts) Recall that the lac operon in E. coli consists of the LacZ, LacY, and LacA genes, and that the transcription of the lac operon is controlled by the lac repressor, which is encoded by LacI. Briefly explain how the lac repressor affects transcription of the lac operon: The lac repressor physically prevents RNA polymerase from binding to the promoter. b. (3 pts) How are the effect of the lac repressor alleviated by the addition of lactose? The lactose inducer stimulates a conformational change in the lac repressor that prevents it from binding to the DNA. 2) The diagrams below show the levels of LacZ, LacY, and LacA enzymes in E. coli strain A (left), strain A + plasmid containing an additional copy of the lac operon and lac repressor genes (middle), and the plasmid in a lac - E. coli strain (right). The diagrams show how the levels of each enzyme change after the addition of lactose to the growth media. a. (4 pts) What is the genotype of strain A? [Use these designations for genotype: I + , I - , I s , P - , P + ,O + ,O c , Z + , Z - , Y + , Y - , A + , A - .] __________________________________________________________________ b. (4 pts) What is the genotype of the lac genes on the plasmid? __________________________________________________________________ c. (6 pts) Strain B has the genotype: I s P - O + Z + Y + A + Draw the induction graph for lacZ, Y, and A for strain B alone (left) and for strain B with the plasmid (right): I - P + O + Z - Y + A + I + P + O c Z + Y - A - time [enzyme] Strain B only time [enzyme] Strain B + plasmid Z, Y, A Y, A Z
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2 3) (6 pts) Briefly describe 2 ways in which a transcriptional repressor can inhibit the activity of a transcriptional activator: 1. It can compete for the same binding site on the DNA 2. It can bind to the activator and block the activation domain (or block its DNA- binding surface) 4) (3 pts) In eukaryotic cells transcriptional activators bind to enhancer sequences on the DNA. How does this stimulate transcription?
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