Question

Repair pathways repairs Photolyase Repair pathways repairs Nucleotide Excision repair system

Answer 1

Methyltransferase
Repair - O6-Methyl guanine
Direct repair of O6 methylguanine is carried out by O6-methylguanine-DNA methyltransferase it catalyzes transfer of
the methyl group of O6-methylguanine to one of its own Cys residues.

Photalase
Repair - Pyrimidine dimer
Photolyases use energy derived from absorbed light to reverse the damage. Photolyases generally contain two cofactors that serve as light-absorbing agents, or chromophores. One of the chromophores is always FADH The excited flavin (FADH-) donates an electron to the pyrimidine dimer.

Nucleotide-excision repair
Repair - DNA lesion that cause large structural changes pyrimidine dimers.
An excinuclease binds to DNA at the site of a bulky lesion and cleaves the damaged DNA strand on either side of the lesion. Then DNA segment of 13 nucleotides (13 mer) or 29 nucleotides (29 mer) is removed by helicase. The gap is filled by DNA polymerase and nick is sealed with DNA ligase.

Ap endonuclease repair
Repair - Abnormal bases - uracil, hypoxanthine, xanthine, Alkalyated bases.
DNA glycosylase recognizes a damaged base and cleaves between the base and deoxyribose in the backbone. An AP endonuclease cleaves the phosphodiester backbone near the AP site. DNA polymerase I initiates repair synthesis from the free 3 hydroxyl at the nick removes with its 5'-3' exonuclease activity portion of the damaged strand and replacing it with undamaged DNA. The gap is filled by DNA polymerase and nick is sealed with DNA ligase.