Spontaneous hydrolytic deamination converts a cytosine in a resting cell's DNA to uracil. If this lesion is repaired before the cell divides, which enzyme performs the first step of the repair pathway?

• A AP endonuclease
• B Excinuclease ABC (UvrABC)
• C DNA polymerase beta
• D Uracil-DNA glycosylase ✓

Explanation

Deamination of cytosine yields uracil paired with guanine, a subtle lesion handled by base excision repair. Uracil-DNA glycosylase flips the uracil out and cleaves the N-glycosidic bond, leaving an apurinic/apyrimidinic (AP) site; AP endonuclease incises the backbone next, followed by phosphodiesterase, polymerase beta fill-in and ligase. UvrABC excinuclease belongs to nucleotide excision repair, which handles bulky helix-distorting adducts rather than single altered bases.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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