In the lac operon, what happens in the presence of an inducer?

The correct answer is Option (2) → The repressor is inactivated

The figure shows, in its lower part (in presence of inducer), the inducer joining the repressor to give an inactive repressor. The inactive repressor cannot hold the operator, so RNA polymerase transcribes z, y and a into lac mRNA, and the three enzymes are made.

Option (1) The repressor is activated — Incorrect. The repressor is already active on its own and binds the operator when no inducer is present. The inducer does the opposite and switches it off.

Option (2) The repressor is inactivated — Correct. The inducer interacts with the repressor and inactivates it. The operator is set free and transcription of the operon proceeds.

Option (3) RNA polymerase is inhibited — Incorrect. RNA polymerase is not inhibited by the inducer. Once the repressor is inactivated, RNA polymerase gets access to the promoter and starts transcription.

Option (4) The operator region is blocked — Incorrect. The operator is blocked only when the active repressor sits on it, which happens in the absence of the inducer. In the presence of the inducer the operator is free.