Figure 1.

Principal DNA repair mechanisms employed by cellular organisms. (A) Direct repair of UV-induced pyrimidine dimer by a photolyase; (B) AP endonuclease-catalyzed cleavage of DNA at the 5’-side of an abasic site; (C) AP lyase cleaves DNA at the 3’-side of an abasic site; (D) 3’-repair phosphodiesterase and 3’-phosphatase remove a 3’-blocking moiety at DNA strand break generated by DNA glycosylases and free radicals; (E) RNAse H-like endoribonuclease cleaves a DNA-RNA duplex at the 5’-side of an RNA nucleotide; (F) NIR AP endonuclease cleaves DNA at the 5’-side of a 2’-deoxyuridine residue; (G) Uracil–DNA glycosylase removes uracil from DNA by hydrolysis of the N-glycosidic bond that links the base to the deoxyribosephosphate backbone, leaving an abasic site as the end product; (H) Nth-like bi-functional DNA glycosylases/AP lyases excise a pyrimidine hydrate in DNA and then cleave phosphodiester backbone at 3’-side of a remaining AP site; (I) Endonuclease V (Nfi) incises DNA at the second phosphodiester bond 3’ to a deaminated adenosine (hypoxanthine). (J) NER multi-protein complex removes a pyrimidine dimer in DNA by dual incision mechanism, in which the lesion is excised in the form of a short damaged nucleotides-containing single-stranded DNA oligonucleotide. N, deoxyribonucleotide; N with the red backbone, ribonucleotide; T^h, thymine hydrate; U, uracil; Hx, hypoxanthine; T = T, UV induced pyrimidine dimers.