In the lac operon of E. coli, a loss-of-function mutation in the lacI gene leads to constitutive expression of the operon. Which molecular mechanism explains this constitutive expression?
- A The repressor cannot bind allolactose
- B CAP cannot bind cAMP
- C The repressor cannot bind the operator ✓
- D RNA polymerase cannot bind the promoter
Correct answer: C. The repressor cannot bind the operator
Explanation
The lacI gene encodes the lac repressor protein. A loss-of-function mutation prevents the repressor from binding the operator region, so RNA polymerase transcribes the operon constitutively regardless of lactose presence. When functional, the repressor binds the operator and blocks transcription; allolactose binds the repressor and causes its dissociation, allowing transcription to proceed.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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