How does the absence of lactose affect lac operon expression?

How does the absence of lactose affect lac operon expression?

• A. The lac repressor is inactivated, allowing transcription.
• B. CAP-CAMP permanently enhances transcription regardless of lactose.
• C. Transcription proceeds at a basal level only.
• D. The lac repressor remains active and blocks transcription.

Answer: (D)

The key idea is how the lac operon is turned on and off by the repressor and by inducer signals. When lactose is absent, there is no allolactose to bind and inactivate the lac repressor. The active repressor stays attached to the operator, which blocks RNA polymerase from initiating transcription. So the operon remains off. CAP-CAMP plays a complementary role: it helps transcription when glucose is low, but it can only boost expression if the repressor is relieved. In the absence of lactose, the repressor remains bound, so transcription is effectively blocked despite CAP-CAMP signaling. There may be a tiny basal leak, but the normal state is repression. That’s why the correct interpretation is that the lac repressor remains active and blocks transcription.

The key idea is how the lac operon is turned on and off by the repressor and by inducer signals. When lactose is absent, there is no allolactose to bind and inactivate the lac repressor. The active repressor stays attached to the operator, which blocks RNA polymerase from initiating transcription. So the operon remains off.

CAP-CAMP plays a complementary role: it helps transcription when glucose is low, but it can only boost expression if the repressor is relieved. In the absence of lactose, the repressor remains bound, so transcription is effectively blocked despite CAP-CAMP signaling. There may be a tiny basal leak, but the normal state is repression.

That’s why the correct interpretation is that the lac repressor remains active and blocks transcription.