AP site

An AP site (meaning apurinic and apyrimidinic) is a location in DNA that does not have either a purine or pyrimidine base, usually due to DNA damage. If left unrepaired AP sites can lead to mutation during semiconservative replication as a random nucleotide base will be inserted into the strand synthesised opposite them.

AP sites are repaired by AP endonucleases. These remove a stretch of one or more nucleotides on the 5' side of the AP site, then fresh nucleotide bases are inserted using the intact DNA strand as a template. If only one nucleotide is being repaired, then the so-called "short patch repair" mechanism is employed where DNA Ligase II, DNA Polβ and XRCC1 are used. If more than one nuceotide is being replaced, then "long patch" machinery is employed in which DNA Polδ or Polε as well as PCNA, Fen-1 and DNA ligase I bridge the gap. Some of these factors are also involved in DNA replication (such as PCNA)).