Section 3: Genetics, Cellular and Molecular Biology
CRISPR-Cas9 gene editing
Subtopic: Fundamentals of CRISPR-Cas9 gene editing
Question Statement:
The RNA primer synthesized during bacterial DNA replication is removed by
________.
Options:
(A) DNA gyrase
(B) DNA Polymerase I
(C) Primase
(D) DNA Polymerase III
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2026 Q.22)**
**1. Governing Concept**:
- **Topic**: Molecular Biology & Prokaryotic DNA Replication
- In *E. coli*, RNA primers synthesized by DnaG primase on both leading and lagging strands must be removed and replaced with deoxyribonucleotides:
- **DNA Polymerase I (Pol I)** possesses unique **$5' \to 3'$ exonuclease activity** (in addition to its $5' \to 3'$ polymerase and $3' \to 5'$ proofreading exonuclease activities).
- Through **nick translation**, Pol I simultaneously degrades the RNA primer ahead in the $5' \to 3'$ direction using its $5' \to 3'$ exonuclease while synthesizing replacement DNA behind it with its $5' \to 3'$ polymerase.
- **DNA Ligase** then seals the final phosphodiester nick.
**2. Option Analysis**:
- **Option (A) [DNA Polymerase III] is INCORRECT**: The primary replicative holoenzyme; possesses $3' \to 5'$ proofreading exonuclease but lacks $5' \to 3'$ exonuclease activity and cannot excise primers.
- **Option (B) [DNA Polymerase I] is CORRECT**: Specifically responsible for RNA primer removal via its $5' \to 3'$ exonuclease activity and filling the resulting gap.
- **Option (C) [DNA Helicase (DnaB)] is INCORRECT**: Unwinds the replication fork.
- **Option (D) [DNA Topoisomerase II (Gyrase)] is INCORRECT**: Relieves positive supercoiling ahead of the replication fork.
**3. Final Answer**:
- **Correct Choice**: **B**
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