AP site

AP site or apurinic/apyrimidinic site is a location in DNA that has neither a purine nor a pyrimidine base, due to the loss of a base in a nucleotide. AP sites are critical intermediates in the process of DNA repair, specifically in the base excision repair (BER) pathway. These sites are formed spontaneously due to the instability of certain nucleotides or as a result of DNA damage caused by external factors such as ultraviolet (UV) radiation or certain chemicals. The repair of AP sites is crucial for maintaining the integrity of the genome and preventing mutations that could lead to diseases such as cancer.

Formation of AP Sites

AP sites can be formed by two main mechanisms:

1. Spontaneous Depurination: The glycosidic bond between the base and the sugar in the DNA can spontaneously hydrolyze, leading to the loss of a purine base (adenine or guanine).
2. DNA Damage: Exposure to certain chemicals or radiation can damage the bases, leading to their removal by cellular enzymes that recognize the damaged bases.

Repair of AP Sites

The primary mechanism for repairing AP sites is the base excision repair (BER) pathway. This process involves several steps:

1. Recognition: Enzymes known as DNA glycosylases recognize and remove the damaged base, creating an AP site.
2. Cleavage: An enzyme called AP endonuclease cleaves the phosphodiester bond near the AP site, creating a nick in the DNA.
3. Resynthesis: DNA polymerase adds the correct nucleotide to fill the gap.
4. Ligation: DNA ligase seals the nick, completing the repair process.

Importance of AP Site Repair

The repair of AP sites is vital for preventing mutations that could lead to the development of diseases. Unrepaired AP sites can result in breaks in the DNA strand or can mispair during DNA replication, leading to mutations. The efficiency and accuracy of the BER pathway are crucial for maintaining genomic stability and preventing the accumulation of mutations that could lead to cancer or other genetic diseases.

See Also

• DNA repair
• Mutation
• Genome
• Cancer
• DNA replication