Excision repair

Excision repair is a term applied to several DNA repair mechanisms.

Excision repair mechanisms that remove the damaged nucleotide replacing it with an undamaged nucleotide complementary to the nucleotide in the undamaged DNA strand. These include:
 * Base excision repair (BER), which repairs damage due to a single nucleotide caused by oxidation, alkylation, hydrolysis, or deamination;
 * Nucleotide excision repair (NER), which repairs damage affecting 2&minus;30 nucleotide-length strands. These include bulky, helix distorting damage, such as thymine dimerization and other types of cyclobutyl dimerization caused by UV light as well as single-strand breaks. A specialized form of NER known as Transcription-Coupled Repair (TCR) deploys high-priority NER repair enzymes to genes that are being actively transcribed;
 * Mismatch repair (MMR), which corrects errors of DNA replication and recombination that result in mispaired nucleotides.