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involves a mechanism in repairing the damage done during DNA polymerase proofreading. During the new strand synthesis, DNA polymerase sometimes goes wrong in inserting the nucleotides. The role of DNA polymerase also involves proofreading the mismatched bases and acts in 3’to 5’ direction. In this way, DNA polymerase corrects the wrong bases inserted into the DNA. The exonuclease proofreading activity helps in reducing the mutation rates in the cell. UV induced pyrimidine dimers get repaired through. The process of repairing pyrimidine dimers is known as photoreactivation. The process involves reverting them to their original form using near-UV light with a wavelength of 320-370 nm. An enzyme known as photolyase gets activated in this process. Photolyase is a product of phr gene. It gets activated by the light and helps in reverting the dimers to their original form. Chemical agents such as alkylating agents mostly induce damage to the DNA.also involves certain enzymes. Alkylating agents transfer the alkyl groups to various bases. For example, E. coli cells employ an enzyme known as an O-methylguanine methyltransferase for repairing alkylation damage. An ADA gene encodes this enzyme. It removes the methyl group and changes the base to its original form.