Apentix/AP Biology/Unit 6: Gene Expression and Regulation/Question
AP Biologyhardmcq1 pt

In E. coli, a mutation disables DNA binding by the catabolite activator protein (CAP). Cells grow in medium containing lactose but no glucose. What best describes lac operon transcription in these cells?

A.Low basal transcription, because CAP cannot help RNA polymerase bind the promoter efficiently even though the repressor is inactive
B.Fully repressed, because lactose locks the repressor onto the operator
C.Maximal, because absence of glucose alone guarantees high transcription
D.Zero, because bacterial promoters cannot function without CAP

Explanation

Core Concept

Lactose in the medium yields allolactose, which inactivates the LacI repressor and clears the operator. High transcription, however, also needs positive control: absent glucose raises cAMP, and cAMP bound CAP normally helps RNA polymerase bind the promoter efficiently. The mutation removes that activation. With repression lifted but activation impossible, transcription settles at a low basal rate.

Correct Answer

ALow basal transcription, because CAP cannot help RNA polymerase bind the promoter efficiently even though the repressor is inactive

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