Section 3: Genetics, Cellular and Molecular Biology
DNA replication, transcription, translation
Subtopic: Fundamentals of DNA replication, transcription, translation
Question Statement:
Correctly match the Coenzyme with its respective involvement in a specific
Reaction type.
Coenzyme Reaction type
P. Thiamine pyrophosphate 1. Acyl group transfer
Q. Tetrahydrofolate 2. Transfer of one carbon group
R. Flavin adenine dinucleotide 3. Transfer of methyl group
S. 5′-Deoxyadenosyl cobalamin 4. Oxidation-reduction
5. Aldehyde transfer
Options:
(A) P-5; Q-2; R-4; S-3
(B) P-5; Q-1; R-2; S-3
(C) P-1; Q-2; R-4; S-5
(D) P-5; Q-3; R-1; S-2
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2025 Q.41)**
**1. Governing Concept**:
- **Topic**: Molecular Biology & DNA Topoisomerases
- DNA topoisomerases manage DNA topological states and relieve torsional strain:
- **Type I Topoisomerases**: Cleave **one single strand** of the DNA double helix, pass the unbroken strand through the break (or allow controlled rotation), and religate the nick without requiring ATP. Change linking number in steps of 1 ($\Delta Lk = \pm 1$).
- **Type II Topoisomerases** (e.g. bacterial DNA gyrase, eukaryotic Topo II$\alpha$): Cleave **both strands** of the DNA duplex, pass another intact double-stranded DNA segment through the gate, and reseal the double-strand break using **ATP hydrolysis**. Change linking number in steps of 2 ($\Delta Lk = \pm 2$).
**2. Option Analysis**:
- **Option (A) is INCORRECT**: Type I topoisomerases cleave a single strand and do not hydrolyze ATP.
- **Option (B) is CORRECT**: Type II topoisomerases introduce transient double-strand breaks, pass an intact duplex through, religate both strands, and depend on ATP hydrolysis.
- **Option (C) is INCORRECT**: DNA helicase unwinds strands, while topoisomerase relieves supercoiling.
- **Option (D) is INCORRECT**: Type II enzymes change the linking number by $\pm 2$, not $\pm 1$.
**3. Final Answer**:
- **Correct Choice**: **B**
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