Section 3: Genetics, Cellular and Molecular Biology
Recombinant DNA technology
Subtopic: Fundamentals of Recombinant DNA technology
Question Statement:
Adenine can undergo a spontaneous change to hypoxanthine in a cell, leading to
a DNA base pair mismatch. The CORRECT combination of enzymes that are
involved in repairing this damage is
Options:
(A) Nuclease, DNA polymerase, DNA ligase
(B) Nuclease, DNA ligase, helicase
(C) Primase, DNA polymerase, DNA ligase
(D) Primase, helicase, DNA polymerase
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2022 Q.46)**
**1. Governing Concept**:
- **Topic**: Molecular Biology & DNA Repair Mechanisms (Base Excision Repair)
- Spontaneous oxidative deamination of Adenine converts it into **Hypoxanthine**, which aberrantly pairs with Cytosine instead of Thymine, causing an $A:T \rightarrow G:C$ transition mutation if uncorrected.
- Removal of hypoxanthine proceeds through the **Base Excision Repair (BER)** pathway:
1. **Nuclease / Glycosylase & AP Endonuclease**: AlkA/MPG (hypoxanthine-DNA glycosylase) hydrolyzes the N-glycosidic bond, creating an abasic (AP) site, followed by AP endonuclease/phosphodiesterase (endonuclease action) nicking the phosphodiester backbone.
2. **DNA Polymerase**: Inserts the correct correct deoxynucleotide (dAMP).
3. **DNA Ligase**: Seals the final phosphodiester nick.
- Thus, the repairing enzymatic triad comprises **Nuclease (AP endonuclease/exonuclease), DNA Polymerase, and DNA Ligase**.
**2. Option Analysis**:
- **Option (A) [Nuclease, DNA polymerase, DNA ligase] is CORRECT**: Standard enzymatic triad executing the excise, fill, and seal steps of excision repair.
- **Option (B) is INCORRECT**: Lacks DNA polymerase required to synthesize replacing nucleotides.
- **Option (C) is INCORRECT**: Primase synthesizes RNA primers in DNA replication, having no role in repair.
- **Option (D) is INCORRECT**: Primase is not involved in DNA mismatch or base excision repair.
**3. Final Answer**:
- **Correct Choice**: **A**
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