Enzymatic mechanisms of DNA repair.

DNA repair proficiency in cells is expressed by various enzymes which can recognize damaged sites arising from exogenous agents or endogenous conditions. Either a damaged base is recognized by DNA glycosylases, partially removed by hemi-DNA glycosylases acting on diadduct damage, or direct incision of the phosphodiester bond near the damaged site. Incision at those apurinic or apyrimidinic sites arising from depurination-depyrimidination or glycosylase reactions is effected by apurinic or apyrimidinic endonucleases. Excision of damaged sites is catalyzed by unique exonucleases followed by DNA polymerase catalyzed reinsertion of nucleotides. The integrity of the strands is restored by polynucleotide ligase when a juxtaposed nucleotide is properly reinserted.