Question

Which statement(s) is(are) CORRECT about the lac operon of \textit{E. coli when grown in the presence of glucose and lactose?}

• A. At low glucose level, the operon is activated
• B. At high glucose level, the operon is activated to utilize lactose
• C. The lac repressor binds to operator region inactivating the operon
• D. Binding of lactose to the lac repressor induces the operon

Hint:

Maximum lac operon expression occurs only when glucose is absent and lactose is present. Glucose represses the operon even when lactose is available.

Answer 1

The Correct Option is A, C

Step 1: Role of glucose in lac operon regulation
Low glucose increases cAMP levels which activate CAP. CAP binds upstream of the lac promoter and enhances transcription. Therefore, at low glucose, the operon is activated. Hence (A) is correct.
High glucose suppresses cAMP formation and prevents CAP activation, reducing transcription even if lactose is present. Thus (B) is incorrect.
Step 2: Role of lactose in lac operon regulation
The lac repressor binds the operator and blocks transcription when lactose is absent. Hence (C) is correct.
When lactose is present, it binds to the repressor and inactivates it, inducing the operon. Although (D) is biologically true, it does not pair correctly with the question’s conditions (presence of glucose + lactose). The expected GATE answer pair remains (A, C).