Q.17 Match the type of DNA repair mechanism in Group I with the enzyme(s) involved
in Group II
Group I                                                       Group II
P) Mismatch repair                        1) DNA glycosylase
Q) Base excision repair               2) UvrA, UvrB, UvrC and UvrD
R) Nucleotide excision repair    3) RecA
S) Double strand break repair   4) MutL, MutS and MutH
(A) P–4, Q–1, R–3, S–2
(B) P–4, Q–2, R–1, S–3
(C) P–4, Q–1, R–2, S–3
(D) P–2, Q–1, R–4, S–3

The correct answer is option (C): P-4, Q-1, R-2, S-3.​

Matching Explanation

Mismatch repair (P) corrects replication errors using MutS (mismatch recognition), MutL (connects components), and MutH (nicks unmethylated strand).​
Base excision repair (Q) removes damaged bases via DNA glycosylase, creating an abasic site for further processing.​
Nucleotide excision repair (R) excises bulky lesions with UvrA (damage recognition), UvrB (helix opening), UvrC (incision), and UvrD (helix unwinding).​
Double strand break repair (S) involves RecA for strand invasion and homologous recombination in bacteria.​

Option Analysis

Option A (P-4, Q-1, R-3, S-2) fails as R mismatches RecA (recombination protein) and S mismatches Uvr proteins (NER-specific).​
Option B (P-4, Q-2, R-1, S-3) fails as Q mismatches Uvr proteins and R mismatches glycosylase (BER-specific).​
Option C (P-4, Q-1, R-2, S-3) matches correctly: Mut proteins for mismatch, glycosylase for BER, UvrABC/D for NER, RecA for DSB repair.​
Option D (P-2, R-4, Q-1, S-3) fails as P mismatches Uvr proteins.​

DNA repair mechanisms matching enzymes is a key CSIR NET Life Sciences topic, testing knowledge of mismatch repair (MutL MutS MutH), base excision repair (DNA glycosylase), nucleotide excision repair (UvrA UvrB UvrC UvrD), and double strand break repair (RecA). These prokaryotic pathways maintain genomic stability by correcting replication errors, oxidative damage, UV lesions, and breaks.​

Core Pathways

• Mismatch Repair: MutS detects mismatches, MutL links to MutH for strand-specific nicking at hemi-methylated GATC sites, followed by excision and resynthesis. Essential for replication fidelity.​

• Base Excision Repair: DNA glycosylase removes altered bases (e.g., uracil from cytosine deamination), AP endonuclease incises, polymerase fills the gap.​

• Nucleotide Excision Repair: UvrA2B recognizes helix-distorting damage, UvrC dual-incises, UvrD unwinds the oligonucleotide for replacement. Targets UV dimers.​

• Double Strand Break Repair: RecA forms nucleoprotein filaments for homologous recombination, invading sister duplexes post-RecBCD processing.​

CSIR NET PYQ Insights

This matching question appears in exams, with option C correct per standard prokaryotic assignments. Mutations in these enzymes link to mutator phenotypes and cancer models. Practice integrates concepts across molecular biology units.​