202hw9KEYb - Biology 202 Homework 9 Due: 11/20 or 11/21; in...

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Biology 202 Homework 9 Due: 11/20 or 11/21; in discussion Please type (not hand-write) ONLY the answers to question onto the question sheet itself (this sheet). Please highlight or otherwise indicate the parts that you have typed with italics , underlining or bold ing. Work done to answer the questions (the pages you use to work out the questions) should not be on this sheet but instead attached to the back as supplemental pages. You must include all of your work pages. The supplemental work pages do not need to be typed 1. When neither glucose nor lactose is available to an E. coli cell, a. Is the repressor bound to any molecule(s)? If so, what molecule(s)? 2 pts Lac operon genes are only expressed when lactose is present because the function of the lac operon genes is to metabolize lactose. Allolactose is needed to bind to the repressor to make is detach from the operator. Since lactose isn’t present then the repressor is bound to the promoter and not to allolactose. b. Is the CAP protein bound to any molecule(s)? If so, what molecule(s)? 2 pts CAP is bound to cAMP and this complex is bound to CAP site on the lac operon When glucose is absent cAMP builds up. cAMP binds to the CAP protein and this binds to the CAP site, bending the DNA and allowing RNA polymerase to attach to the promoter so the lac genes can be transcribed if lactose is present. 2. What would happen to the regulation of the tryptophan biosynthesis enzymes in cells that express a mutant form of the Trp repressor that: a. cannot bind to DNA? How many points? This is a repressible system as opposed to the inducible lac operon system and the function of the trp operon is to synthesize trp. When trp is high the repressor binds L- trp and this complex binds to the operator and inhibits trp synthesis because RNAP can’t bind the operator. When trp is low the repressor is inactive and can’t bind to the operator so the operon genes are expressed. If the trp repressor can’t bind to the DNA then it can’t repress whether or not trp is present, so the trp operon genes are expressed constitutively b. binds to DNA even if no tryptophan is bound to it? How many points? The repressor shouldn’t bind to the DNA if no trp is present. So if it is always bound to the DNA then trp cannot be synthesized. Or more specifically, the trp synthesis enzymes cannot be synthesized. 3. The Regulation of Gene Expression discussion activity (last week) includes information on attenuation of the trp operon. Please read this information and answer the following questions:
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a. In the presence of high intracellular concentrations of tryptophan, only short transcripts of the trp operon are synthesized because of attenuation of transcription 5’ to the structural genes. This is mediated by the recognition of two Trp codons in the leader
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202hw9KEYb - Biology 202 Homework 9 Due: 11/20 or 11/21; in...

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