Lac operon: when is expression highest?
When is the lac operon transcribed at its highest rate?
Answer A: explanation for every choice
A
Correct. Lactose (as allolactose) releases the repressor, and low glucose raises cAMP so CAP activates transcription.
B
With glucose present, cAMP is low and CAP is inactive, so transcription stays low.
C
Without lactose, the repressor stays bound to the operator.
D
Both conditions block transcription: the repressor is bound and CAP is inactive.
TakeawayLactose removes the repressor. Low glucose → high cAMP → CAP activates.
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Reviewed 2026-11-05 by a medical student. Spot an error? Report it with question ID bb39.