Section 3: Genetics, Cellular and Molecular Biology
DNA replication, transcription, translation
Subtopic: Fundamentals of DNA replication, transcription, translation
Question Statement:
Which of the following statement(s) is(are) CORRECT regarding the lac operon
in E. coli when grown in the presence of glucose and lactose?
Options:
(A) At low glucose level, the operon is activated
(B) At high glucose level, the operon is activated to enable the utilization of lactose
(C) The lac repressor binds to operator region inactivating the operon
(D) Binding of lactose to the lac repressor induces the operon
Official Answer Key:
A;C
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2022 Q.51)**
**1. Governing Concept**:
- **Topic**: Molecular Genetics & Operon Regulation
- In the wild-type *E. coli lac* operon:
- Lactose is transported by LacY (permease) and converted to **allolactose** by LacZ ($\beta$-galactosidase).
- Allolactose acts as the physiological inducer, binding the LacI repressor and causing it to dissociate from the operator ($lacO$).
- Mutants with defective operator ($lacO^c$) or defective repressor ($lacI^-$) show **constitutive** expression of the operon even in the absence of inducer.
**2. Option Analysis**:
- **Option (A) is CORRECT**: Allolactose is the true natural inducer produced from lactose via side transglycosylation activity of $\beta$-galactosidase.
- **Option (B) is INCORRECT**: LacI acts as a trans-acting repressor, not a cis-acting operator.
- **Option (C) is CORRECT**: Operator-constitutive mutations ($lacO^c$) result in continuous, un-repressed transcription of the structural genes in the absence of inducer.
- **Option (D) is INCORRECT**: cAMP-CAP exerts positive control, not negative repression.
**3. Final Answer**:
- **Correct Options**: **A, C**
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