Question

question 34
the lactose repressor ______.
a is inactivated by binding lactose
b requires lactose for its transcription
c is transcribed with the structural lac genes
d is activated by binding lactose

Explanation:

To solve this, we analyze each option based on the lac operon concept in biology (Natural Science - Biology subfield):

• Option A: The lactose repressor (a protein) is active when not bound to lactose (it represses transcription of lac structural genes). When lactose (or allolactose, its isomer) binds to the repressor, the repressor undergoes a conformational change and becomes inactive, so it can no longer bind to the operator. This matches the mechanism.
• Option B: The repressor is encoded by the lacI gene, which is constitutively transcribed (doesn’t require lactose for transcription). Eliminate B.
• Option C: The repressor is transcribed from the lacI gene, which is separate from the structural lac genes (lacZ, lacY, lacA) that are transcribed as a polycistronic mRNA. Eliminate C.
• Option D: Binding lactose inactivates the repressor, not activates it. The repressor is active (represses) without lactose. Eliminate D.

Answer:

A. is inactivated by binding lactose