# DBT JRF 2020 — Question 140

Exam: DBT JRF 2020 · Section: B · Topic: Molecular Biology · Difficulty: Medium

## Question

DNA glycosylases are DNA repair enzymes involved in

## Options

- **A.** DNA replication
- **B.** negative supercoiling of DNA
- **C.** SOS response
- **D.** base excision repair

## Answer

Option D

## Explanation

Base Excision Repair (BER) corrects small, non-helix-distorting DNA base lesions: oxidised bases (8-oxoguanine), deaminated bases (uracil from cytosine deamination), alkylated bases, and abasic (apurinic/apyrimidinic) sites. Mechanism: (1) DNA GLYCOSYLASE recognises and cleaves the N-glycosidic bond between the damaged base and deoxyribose, releasing the damaged base and creating an abasic (AP) site; (2) AP endonuclease (APE1) nicks the backbone 5' of the AP site; (3) DNA polymerase $\beta$ removes the AP site and inserts the correct nucleotide; (4) XRCC1/DNA ligase III seals the nick. DNA glycosylases are substrate-specific: Uracil-DNA glycosylase (UNG); OGG1 (8-oxoguanine); Nth1 (thymine glycol). BER is distinct from Nucleotide Excision Repair (NER), which corrects helix-distorting bulky lesions (UV-induced CPDs and 6-4PPs) using a 12–27 nucleotide oligonucleotide removal.

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