Match the chemical (Column I) with its use (Column II).
Column I Column II
P. Diethylpyrocarbonate 1. Chelation of magnesium ion during
DNA purification
Q. Cesium chloride 2. Prevention of RNA degradation in
aqueous environment
R. Ethidium bromide 3. Separation of DNA by density gradient
centrifugation
S. Ethylenediaminetetraacetic acid 4. Staining of RNA in agarose gel
Options:
(A) P-4; Q-1; R-3; S-2
(B) P-4; Q-3; R-2; S-1
(C) P-2; Q-1; R-4; S-3
(D) P-2; Q-3; R-4; S-1
Official Answer Key:
D
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2024 Q.40)**
**1. Governing Concept**:
- **Topic**: Biochemistry & Molecular Biology Laboratory Reagents
- Chemicals and laboratory functions:
- **P. Diethylpyrocarbonate (DEPC)** $\rightarrow$ Modifies histidine residues, irreversibly inactivating **RNases** in water/buffers $\rightarrow$ **2**
- **Q. Formamide** $\rightarrow$ Lowers DNA/RNA melting temperature ($T_m$) by destabilizing hydrogen bonds; acts as a **denaturant** in hybridization $\rightarrow$ **3**
- **R. Ethidium bromide** $\rightarrow$ Planar aromatic molecule that **intercalates into dsDNA**, fluorescing under UV light $\rightarrow$ **4**
- **S. EDTA (Ethylenediaminetetraacetic acid)** $\rightarrow$ Hexadentate chelating agent that performs **chelation of magnesium and divalent cations**, inhibiting DNases $\rightarrow$ **1**
**2. Option Analysis**:
- **Option (A) is INCORRECT**: Misassigns DEPC to ethidium bromide's role.
- **Option (B) is INCORRECT**: Inverts EDTA and intercalator assignments.
- **Option (C) is INCORRECT**: Swaps formamide and EDTA.
- **Option (D) [P-2; Q-3; R-4; S-1] is CORRECT**: Validated across canonical molecular biology protocols.
**3. Final Answer**:
- **Correct Choice**: **D**
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