In the lac operon, what happens when lactose is present and glucose is absent?

• A. Transcription is activated when lactose is present and glucose is absent.
• B. Transcription occurs regardless of sugar presence.
• C. Transcription is repressed because lactose blocks the promoter.
• D. Transcription occurs only if lactose is not present.

Answer: (A)

When lactose is present, allolactose binds to the LacI repressor and inactivates it, removing the block on transcription. With glucose absent, cAMP levels rise and bind CAP; the CAP-cAMP complex then helps RNA polymerase bind to the promoter and kick off transcription. The two effects together—lifting repression and activating transcription via CAP—maximize expression of the lac genes. So transcription is activated under these conditions.