Question

Choose the correct answer from the alternatives given :
In E. Coli, the lac operon gets switched on when

• A. Lactose is present and it binds to the repressor
• B. Repressor binds to operator
• C. RNA polymerase binds- to the operator
• D. Lactose is present and it binds to RNA polymerase

Answer 1

Answer: (A)

Lactose is present and it binds to the repressor

The repressor of the operon is synthesized (all-the-time-constitutively) from the i gene. The repressor protein binds to the operator region of the operon and prevents RNA polymerase from transcribing the operon. In the presence of an inducer, such as lactose or allolactose, the repressor is inactivated by interaction with the inducer. This allows RNA polymerase access to the promoter and transcription proceeds. Thus, the lac operon gets switched on.

So, the correct answer is 'Lactose is present and it binds to the repressor'.