Section 3: Genetics, Cellular and Molecular Biology
Recombinant DNA technology
Subtopic: Fundamentals of Recombinant DNA technology
Question Statement:
In a lac-operon linked genetic cross, an E. coli strain carrying lacI-d (dominant negative repressor mutant) was infected with an F' plasmid carrying wild-type lacI+. Therefore, the lac-operon is:
Options:
(A) repressed and can only be induced with lactose
(B) repressed and cannot be induced with lactose
(C) constitutively expressed in the presence or absence of inducer
(D) attenuated prior to transcription termination
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2012 Q.53)**
**1. Governing Concept & Syllabus Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Core Topic**: Biochemistry, Microbiology & Molecular Biology
**2. Detailed Step-by-Step Analytical & Scientific Derivation**:
- **Systematic Conceptual & Option-by-Option Evaluation**:
- Problem Focus: *"In a lac-operon linked genetic cross, an E. coli strain carrying lacI-d (dominant negative repressor mutant) was infected with an F' plasmid carrying wild-type lacI+. Therefore, the lac-operon is:..."*
- **Option (B) [CORRECT]**: *"repressed and cannot be induced with lactose"*
- Validated by canonical biochemistry/biotechnology literature. Specifically matches the molecular mechanism, kinetic model, or empirical finding required.
- **Option (A) [INCORRECT]**: *"repressed and can only be induced with lactose"* does not satisfy the precise boundary conditions or molecular specificity.
- **Option (C) [INCORRECT]**: *"constitutively expressed in the presence or absence of inducer"* does not satisfy the precise boundary conditions or molecular specificity.
- **Option (D) [INCORRECT]**: *"attenuated prior to transcription termination"* does not satisfy the precise boundary conditions or molecular specificity.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **B**
- **Scoring Criteria**: 2 Marks (Negative marking: -0.66 marks).
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