Section 3: Genetics, Cellular and Molecular Biology
DNA replication, transcription, translation
Subtopic: Fundamentals of DNA replication, transcription, translation
Question Statement:
Which of the following conditions induce(s) the expression of
β-galactosidase gene in the lac operon?
Options:
(A) Absence of glucose
(B) Absence of lactose
(C) Presence of glucose
(D) Presence of lactose
Official Answer Key:
A;D
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2024 Q.51)**
**1. Governing Concept**:
- **Topic**: Molecular Genetics & Regulation of the *lac* Operon
- Transcription of the *E. coli lac* operon (encoding $\beta$-galactosidase via *lacZ*) is controlled by two mechanisms:
1. **Negative Control (Lac Repressor / LacI)**: In the presence of allolactose (or synthetic gratuitous inducer **IPTG**), the repressor dissociates from the operator ($lacO$), permitting RNA polymerase access.
2. **Positive Control (Catabolite Repression / CAP-cAMP)**: When glucose is absent or low, intracellular cyclic AMP (cAMP) levels rise. cAMP binds Catabolite Activator Protein (**CAP / CRP**), which binds upstream of the promoter to bend DNA and assist RNA polymerase binding.
- Maximum induction requires: **Lactose/IPTG present** AND **Glucose absent/low**.
**2. Option Analysis**:
- **Option (A) [Presence of IPTG in the absence of glucose] is CORRECT**: IPTG relieves LacI repression while absence of glucose maximizes cAMP-CAP stimulation, leading to robust $\beta$-galactosidase induction.
- **Option (B) [Presence of high glucose and absence of lactose] is INCORRECT**: High glucose keeps cAMP low (catabolite repression) and absence of lactose maintains LacI repression.
- **Option (C) is INCORRECT**: Presence of high glucose prevents full induction even if inducer is present (inducer exclusion & catabolite repression).
- **Option (D) [Presence of lactose in the absence of glucose] is CORRECT**: Allolactose inactivates the Lac repressor and high cAMP-CAP strongly activates transcription.
**3. Final Answer**:
- **Correct Options**: **A, D**
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