AP site

An AP site refers to a location within DNA or RNA that is missing a nucleobase, either a purine (adenine or guanine) or a pyrimidine (cytosine or thymine/uracil). This creates an abasic site, also known as an AP site (apurinic/apyrimidinic site). These sites arise spontaneously through the hydrolysis of the N-glycosidic bond linking the base to the sugar-phosphate backbone, or can be generated by DNA glycosylase enzymes as part of the base excision repair (BER) pathway. AP sites are common DNA lesions and are mutagenic and cytotoxic if left unrepaired. The presence of an AP site can block DNA replication and transcription. Specific enzymes, such as AP endonucleases, are responsible for cleaving the phosphodiester backbone near AP sites, initiating further steps in DNA repair.