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Which of the following accurately describes the status of lac operon regulation when E. coli cells...

Which of the following accurately describes the status of lac operon regulation when E. coli cells shift from using glucose to lactose as their carbon source?

A. The lac repressor is active and the CAP protein is active, so the operon is transcribed

B. The lac repressor is active and the CAP protein is inactive, so the operon is transcribed

C. The lac repressor is inactive and the CAP protein is active, so the operon is transcribed

D. The lac repressor is inactive and the CAP protein is inactive, so the operon is transcribed

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Answer #1

Answer: Choice C

Reason: Lac operon is an inducible operon which turns 'on' in the presence of substrate. Thus, if the lac repressor is active, it will bind to the operator thus masking it and preventing transcription. This turns the operon off. On the other hand, if the CAP is active, it will promote the binding of RNA polymerase strongly on the promoter thus enhancing gene expression. Thus, absence of lac repressor and presence of CAP favour the operon to express itself.

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