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These reactive chemical species can reach DNA by diffusion and the bimolecular reaction damages the DNA (oxidative stress). Unlike direct DNA damage which causes sunburn, indirect DNA damage does not result in any warning signal or pain in the human body. The bimolecular reactions that cause the indirect DNA damage are illustrated in the figure:
Photolyase is particularly important in repairing UV induced damage in plants. The photolyase mechanism is no longer working in humans and other placental mammals who instead rely on the less efficient nucleotide excision repair mechanism , although they do retain many cryptochromes . [ 11 ]
Nucleotide excision repair (NER) is a particularly important excision mechanism that removes DNA damage induced by ultraviolet light (UV). UV DNA damage results in bulky DNA adducts — these adducts are mostly thymine dimers and 6,4-photoproducts. Recognition of the damage leads to removal of a short single-stranded DNA segment that contains ...
In human cells, oxidative DNA damage occurs about 10,000 times a day and DNA double-strand breaks occur about 10 to 50 times a cell cycle in somatic replicating cells (see DNA damage (naturally occurring)). The selective advantage of DNA repair is to allow the cell to survive in the face of DNA damage.
Melanoma cells are commonly defective in postreplication repair of DNA damages that are in the form of cyclobutane pyrimidine dimers, a type of damage caused by ultraviolet radiation. [ 11 ] [ 12 ] A particular repair process that appears to be defective in melanoma cells is homologous recombinational repair. [ 12 ]
This process of absorption works to reduce the risk of DNA damage and the formation of pyrimidine dimers. UVA light makes up 95% of the UV light that reaches earth, whereas UVB light makes up only about 5%. UVB light is the form of UV light that is responsible for tanning and burning. Sunscreens work to protect from both UVA and UVB rays.
UV-DDB is not only responsible for the repair of damaged DNA, it can also function by acting as a damage sensor. [5] In base excision repair, UV-DDB galvanizes OGG1 and APE 1 activities. [5] During DNA damage, proteins OGG1 and APE 1 encounter difficulty in repairing the lesions in a DNA wrapped nucleosome.
UV light, specifically non-ionizing shorter-wavelength radiation such as UVC and UVB, causes direct DNA damage by initiating a synthesis reaction between two thymine molecules. The resulting dimer is very stable. Although they can be removed through excision repairs, when UV damage is extensive, the entire DNA molecule breaks down and the cell ...