During base excision repair of a deaminated cytosine, which enzyme catalyzes the first step of the pathway?
- A AP endonuclease
- B Uracil DNA glycosylase followed by AP endonuclease ✓
- C DNA glycosylase
- D Excinuclease ABC
Correct answer: B. Uracil DNA glycosylase followed by AP endonuclease
Explanation
Deamination of cytosine produces uracil paired with guanine. Uracil DNA glycosylase first cleaves the N-glycosidic bond, releasing uracil and leaving an apurinic or apyrimidinic (AP) site. AP endonuclease then cuts the backbone at the AP site, after which exonuclease removal, gap filling by DNA polymerase, and ligation complete the repair. Excinuclease ABC is the prokaryotic nucleotide excision repair complex used for bulky lesions such as pyrimidine dimers, not small base changes.
Reference: Lippincott's Illustrated Reviews: Biochemistry, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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