Uracil in DNA is removed by
Cytosine deamination spontaneously generates uracil in DNA which pairs with adenine during replication causing C:G→T:A transition if unrepaired. Uracil-DNA glycosylase exemplifies BER initiation by flipping uracil out of helix into catalytic pocket, hydrolyzing N-glycosidic bond releasing free uracil and creating AP site. Pocket discriminates against thymine by methyl group exclusion and normal purines by size. Other glycosylases target 8-oxoguanine, alkyladenine, but uracil removal specifically handled by UNG family highly conserved. Subsequent AP endonuclease cuts, Pol I inserts cytosine, ligase seals, preventing mutagenesis.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 12: Uracil Glycosylase Removal of Uracil