What are the types of DNA repair mechanisms?

DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. Depending on the type of damage inflicted on the DNA molecule, a variety of repair mechanisms are adopted to restore the lost information, including direct reversal repair, single-strand repair, double-strand break repair, and translesion synthesis.

• Direct reversal repair: It is a mechanism of repair where specialized proteins eliminate the DNA damage by chemically reversing it. It is the simplest and most energy efficient form of DNA repair, which does not require a refence template. Moreover, it does not involve the process of breaking the phosphodiester backbone of the DNA.
• Single-strand repair: When only one of the two strands of a double helix has a defect, the other strand can be used as a template to guide the correction of the damaged strand. Three types of mechanisms are commonly employed to repair the single-strand damage, including Base excision repair (BER), Nucleotide excision repair (NER) and Mismatch repair.
• Double-strand break repair: When both strands in the double helix are severed, DNA molecule forms double-strand breaks. Three mechanism exist to repair these double-strand breaks: non-homologous end joining (NHEJ), microhomology-mediated end joining (MMEJ), and homologous recombination (HR).
• Translesion synthesis: When cells have no access to a template to recover the original information, they use translesion synthesis as a last resort, which allows the DNA replication machinery to replicate past DNA lesions such as thymine dimers or AP sites.