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While the single-nucleotide replacement pathway appears to facilitate the repair of most damaged bases, an alternative BER pathway is evoked when the structure of the terminal sugar phosphate is such that it can not be cleaved through the AP lyase activity of DNA polymerase, POL Beta. Under these circumstances, a short stretch of residues containing the abasic site is excised and replaced (Dianov et al., 1999). Following DNA glycosylase mediated cleavage of the damaged base, the endonuclease APE1 is recruited to the site of damage where it cleaves the 5' side of the base-free deoxyribose residue. POL Beta then displaces the DNA glycosylase and synthesizes the first replacement residue. DNA polymerase, POL Delta replaces POL Beta and mediates the synthesis of several additional residues resulting in the displacement of a DNA flap containing the abasic sugar phosphate and 3� flanking residues. The flap structure is recognized and cleaved by the flap endonuclease, FEN1 and the replacement residues are then ligated by the DNA ligase, LIG1 (Klungland and Lindahl, 1997; Matsumoto et al., 1999).
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