Section 3: Genetics, Cellular and Molecular Biology
DNA replication, transcription, translation
Subtopic: Fundamentals of DNA replication, transcription, translation
Question Statement:
Which one of the following techniques relies on the spin angular momentum of a photon?
(A) CD spectroscopy (B) Fluorescence spectroscopy
(C) IR spectroscopy (D) Raman spectroscopy
Options:
(A) CD spectroscopy
(B) Fluorescence spectroscopy
(C) IR spectroscopy
(D) Raman spectroscopy
Official Answer Key:
D
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2016 Q.13)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Molecular Genetics (*lac* Operon Transcriptional Regulation)
**2. Detailed Step-by-Step Analysis**:
- The *lac* operon is controlled by two key regulatory proteins:
1. **Lac repressor (LacI)**:
- In the presence of **lactose**, allolactose (inducer) binds LacI, altering its conformation and preventing it from binding the operator.
- In the **absence of lactose**, LacI is active and **bound to the operator**.
2. **Catabolite Activator Protein (CAP / CRP)**:
- Intracellular cyclic AMP (cAMP) levels are inversely related to glucose:
- In the presence of glucose, adenylate cyclase is inhibited $\implies$ low cAMP $\implies$ CAP remains inactive and unbound.
- In the **absence of glucose**, adenylate cyclase is active $\implies$ high cAMP $\implies$ cAMP-CAP complex forms and **binds to the CAP site** (upstream of the promoter) to stimulate transcription.
- The question asks for conditions where: **Lactose is absent** and **Glucose is absent**:
- **Lactose absent**: Lac repressor remains **bound to the operator**.
- **Glucose absent**: cAMP levels are high, so cAMP-CAP is **bound to the CAP site**.
- Therefore, both regulatory proteins are bound: **CAP is bound, and Lac repressor is bound**.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: In the absence of lactose, the repressor cannot be unbound.
- **Option (B) [INCORRECT]**: Both proteins are bound under these conditions.
- **Option (C) [INCORRECT]**: CAP is bound because glucose is absent.
- **Option (D) [CORRECT]**: In the absence of both glucose and lactose, both CAP and the Lac repressor are simultaneously bound to their respective DNA regulatory elements.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **D**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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