In the lac operon, addition of glucose along with lactose to the growth medium results in low expression of the operon despite the presence of lactose. The molecular basis of this effect is:

• A Glucose binds the operator and physically blocks RNA polymerase
• B Glucose causes dissociation of the repressor from the operator
• C Low cAMP levels prevent CAP-cAMP complex formation, reducing RNA polymerase binding at the promoter ✓
• D Glucose activates rho-dependent termination within the lac genes

Explanation

This is catabolite repression. Glucose lowers intracellular cAMP, so the catabolite activator protein (CAP) cannot form the CAP-cAMP complex that binds upstream of the lac promoter and recruits RNA polymerase. Without this positive activation, transcription remains low even though lactose-bound repressor has left the operator. Glucose itself does not bind DNA, does not act on the repressor, and termination here is not Rho-mediated, so the other mechanisms do not apply.

Reference: Lehninger Principles of Biochemistry, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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