True/False: The lac repressor is active in the absence of lactose.

True/False: The lac repressor is active in the absence of lactose.

• A. False
• B. Sometimes
• C. Always
• D. True

Answer: (D)

The question tests how the lac repressor controls the lac operon. In the absence of lactose, the LacI repressor binds tightly to the operator region, blocking RNA polymerase from initiating transcription of lacZ, lacY, and lacA. This keeps the operon off. When lactose is present, some of it is converted to allolactose, which binds to LacI and changes its shape, reducing its affinity for the operator. The repressor releases, RNA polymerase can proceed, and the operon is expressed. So the lac repressor is active in the absence of lactose, making the statement true.

The question tests how the lac repressor controls the lac operon. In the absence of lactose, the LacI repressor binds tightly to the operator region, blocking RNA polymerase from initiating transcription of lacZ, lacY, and lacA. This keeps the operon off. When lactose is present, some of it is converted to allolactose, which binds to LacI and changes its shape, reducing its affinity for the operator. The repressor releases, RNA polymerase can proceed, and the operon is expressed. So the lac repressor is active in the absence of lactose, making the statement true.