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EXCISION REPAIR
Mechanism for the repair of environmental hurt to one strand of DNA (loss of purines due to thermal fluctuations, formation of pyrimidine dimers by UV irradiation).
The site of hurt is recognised, excised by an endonuclease, the correct sequence is copied from the complementary strand by a polymerase and the ends of this correct sequence are joined to the rest of the strand by a ligase. The term is sometimes restricted to bacterial systems where the polymerasealso acts as endonuclease.