Left - diagram of base excision repair - click to enlarge image.
NTPs = ribonucleoside triphosphates
dNMP = deoxyribonucleoside monophosphate
First, the altered base is excised by a specific DNA glycosylase, which breaks the beta N-glycosidic bond and creates an AP, or abasic site. This site is identical to that generated by spontaneous depyrimidination or depurination. Six DNA glycosylases have been identified in humans – each excises an overlapping subset of either spontaneously formed (such as hypoxanthine), oxidized (such as 8-oxo-7,8-dihydroguanine), alkylated (such as 3-methyladenine), or mismatched (for example, T:G) bases.
Next, the terminal sugar-phosphate is removed by an AP endonuclease (Ape1), leaving a 3’-OH terminal and an abnormal 5'-abasic terminus. The resulting gap is refilled by the 5’-deoxyribose-phosphodiesterase action of a DNA polymerase I (DNA polymerase beta in mammals), then the strands are re-ligated by DNA Ligase I or a complex of XRCC1 and LigIII.
An alternative BER pathway corrects errors involving more than one nucleotide. The Fen1 protein excises the long-patch structure that is produced by DNA polymerase strand displacement. This "long-patch" repair process is divided into two subpathways: a PCNA-stimulated, Pol-beta-directed pathway and a PCNA-dependent, Pol-delta/epsilon -directed pathway.
link to table - human DNA repair genes : diagram>BER
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