Both in prokaryotic and eukaryotic cells, there are repair enzyme systems to deal with DNA damage, which is caused rather frequently. The changes in DNA leading to damage are broadly divided into two general classes : (i) Single base changes,
are corrected through DNA replication leading to changes in DNA sequences; this may be caused by conversion of one base to another (e.g. deamination of 5-methylcytosine to thymine). (ii) Structural distortions
may result from a single strand nick, removal of a base or introduction of a covalent link between bases of same or different strands (e.g. formation of thymine dimer due to UV-see Mutations: 3. Molecular Level (Mechanism)
A cell may have several systems to deal with DNA damage. These systems include the following : (i) Direct repair
involves reversal of the damage; for instance, photoreactivation of pyrimidine dimer involves removal of covalent bonds giving the original structure (no fresh DNA synthesis is involved), (ii) Excision repair
involves recognition of a damaged or altered base(s) followed by excision of a sequence including damaged bases. This is achieved by an endonuclease, or an exonuclease. A new stretch of DNA is then synthesized to replace the excised material, (iii) Mismatch repair
involves correction of mismatches or pairing between bases which are not complementary. Mismatches may arise either (a) during replication or (b) due to base conversion (e.g. deamination) and are corrected by a process described as error correction during DNA replication. Error correction involves a process that distinguishes between 'new' and 'old' strands (due to methylation), so that the error in the newly synthesized strand is corrected.