The correct answer is (A) Base-excision repair.

DNA glycosylases initiate base-excision repair (BER) by recognizing and removing damaged or modified bases, such as those from oxidation, alkylation, or deamination, creating an abasic site for further processing.​

Option Analysis

Base-Excision Repair (A)

DNA glycosylases act first in BER by cleaving the N-glycosidic bond between the damaged base and deoxyribose sugar, generating an apurinic/apyrimidinic (AP) site. AP endonuclease then incises the backbone, DNA polymerase fills the gap, and ligase seals it, making BER the primary pathway relying on these enzymes.​

Direct Repair (B)

Direct repair reverses specific damage without excision or strand breaks, using enzymes like photolyase for UV dimers or O6-alkylguanine alkyltransferase for alkylated bases, without involving DNA glycosylases.​

Mismatch Repair (C)

Mismatch repair corrects replication errors like base mismatches or small loops via MutS/MutL recognition, helicase unwinding, exonuclease degradation, and polymerase resynthesis, bypassing glycosylases which primarily handle base damage rather than replication mismatches.​

Nucleotide-Excision Repair (D)

Nucleotide-excision repair removes bulky helix-distorting lesions (e.g., UV adducts) through damage recognition by XPC/XPA, dual incisions by XPG/XPF-ERCC1, and patch resynthesis, without DNA glycosylases that target single-base alterations.​

Base-excision repair (BER) is the DNA repair system that requires DNA glycosylases to remove damaged bases like oxidized or alkylated nucleotides, essential for genome stability in CSIR NET Life Sciences.​

Role of DNA Glycosylases

DNA glycosylases scan DNA for subtle base modifications from endogenous (e.g., ROS) or exogenous sources, flipping the damaged base into an active site for cleavage and AP site formation. Examples include uracil-DNA glycosylase (UNG) for cytosine deamination and OGG1 for 8-oxoguanine, preventing mutations.​

BER Pathway Steps

• Glycosylase excises the base, creating an AP site.​

• AP endonuclease (APE1) nicks the strand.​

• Polymerase β inserts the correct nucleotide; ligase seals the nick.​

Short-patch BER replaces one nucleotide; long-patch handles larger gaps.​

Comparison of DNA Repair Systems

BER targets non-bulky damage, distinguishing it for CSIR NET questions on DNA repair systems requiring DNA glycosylases.​