Base excision repair

Some mutagenic events lead to the changes of the base part of the nucleotide. The action of the base modifiers can sometimes be corrected by very specific DNA glycosylases that break the ribose-base glycosyl bond. This leaves the backbone of the DNA strand intact with some sites depurinated or depyrimidinated. These apurinic or apyrimidinic sites are then removed through AP endonucleases. In contrast to NER only one or two nucleotides are removed. The DNA strand is made entire again by the action of DNA polymerase I and DNA ligase through the similar mechanism as outlined in NER. The abnormal bases repaired through base excision repair BER include hypoxanthine, uracil, xanthine and alkylated bases. In some organisms it is also the mechanism used to resolve pyrimidine dimers.