Initial pairing errors occur at a frequency of about one in ten thousand, while errorsin a complete DNA molecule are only about one in one billion. In bacteria, the initial base-pairing errors are usually corrected by DNA polymerase, which checks each newly added nucleotide against its template. If the polymerase detects an error, it removes the incorrect nucleotide and replaces it. DNA repair Mechanisms for repairing DNA help maintain the integrity of the encoded genetic information. Accidental change in DNA can result from exporure to reactive chemicals, radioactivity, X-rays and ultravi olet light.
Cells continuously monitor such accidental changes and repair most.
Excision repair . The damaged segment is excised by one repair enzyme and the remaining gap is filled in by base-pairing nucleotides with the undamaged strand. DNA polymerase and DNA ligase are enzymes that catalyze the filling-in process.