Section 3: Genetics, Cellular and Molecular Biology
Recombinant DNA technology
Subtopic: Fundamentals of Recombinant DNA technology
Question Statement:
Which one of the following coenzymes is utilised by alanine racemase for the
conversion of L-Alanine to D-Alanine?
Options:
(A) Pyridoxal phosphate
(B) Thiamine pyrophosphate
(C) Tetrahydrofolate
(D) Flavin mononucleotide
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2025 Q.17)**
**1. Governing Concept**:
- **Topic**: Recombinant DNA Technology & Restriction Enzymes
- **Type II restriction endonucleases**:
- Recognize specific, typically palindromic DNA sequences of 4–8 base pairs.
- Require **$\text{Mg}^{2+}$ as a cofactor** for catalytic cleavage.
- Cleave **at specific, defined phosphodiester positions within or immediately adjacent to** their recognition site.
- Do **NOT** require ATP (unlike Type I and Type III enzymes).
- Do not possess intrinsic methylase activity (restriction and modification are carried out by separate enzymes).
**2. Option Analysis**:
- **Option (A) is CORRECT**: Cleave DNA at precise positions within or adjacent to palindromic recognition sites in an ATP-independent manner.
- **Option (B) is INCORRECT**: Cleaving hundreds of base pairs away from the recognition site characterizes Type I enzymes.
- **Option (C) is INCORRECT**: Type II restriction enzymes do not require ATP hydrolysis.
- **Option (D) is INCORRECT**: They are endonucleases, not exonucleases.
**3. Final Answer**:
- **Correct Choice**: **A**
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