Baseexcision repair excises modified bases and then replaces the entire nucleotide

recognize hypoxanthine, 3-methyladenine, 7-methylguanine, and other modified bases.

After the base has been removed, an enzyme called AP (apurinic or apyrimidinic) endonuclease cuts the phospho-diester bond, and other enzymes remove the deoxyribose sugar (Figure 17.29b). DNA polymerase then adds new nucleotides to the exposed 3'-OH group (Figure 17.29c), replacing a section of nucleotides on the damaged strand. The nick in the phosphodiester backbone is sealed by DNA ligase (Figure 17.29d), and the original intact sequence is restored (< Figure 17.29e).

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