The GATE Grind

GATE 2022 BT – Question 46

Genetics, Cellular and Molecular Biology · Molecular Biology: Replication, transcription, splicing, translation, non-coding and micro RNA, RNA interference, DNA damage and repair · 2 marks · Multiple choice

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

  1. Nuclease, DNA polymerase, DNA ligase
  2. Nuclease, DNA ligase, helicase
  3. Primase, DNA polymerase, DNA ligase
  4. Primase, helicase, DNA polymerase

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Show answer and explanation

Correct answer: (A) Nuclease, DNA polymerase, DNA ligase

Explanation

**The lesion.** Adenine deaminates spontaneously to **hypoxanthine**, which pairs with cytosine instead of thymine, creating a mismatch. This damaged base is fixed by **base excision repair (BER)**.

**Steps of BER and the enzymes used:**
1. A DNA glycosylase removes the damaged base, leaving an abasic site.
2. An AP endonuclease (a **nuclease**) cuts the backbone at the abasic site.
3. **DNA polymerase** fills the gap with the correct nucleotide.
4. **DNA ligase** seals the nick.

So the combination is **nuclease, DNA polymerase and DNA ligase** (A). Primase and helicase are used in replication, not in this repair.