UV-Induced Reactions of Direct and Sensitized DNA Damage in Model and Cellular Systems

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UV radiation from the sun in the B-region (UVB, 290–320 nm) and A-region (UVA, 320–400 nm) is a key genotoxic environmental factor that causes DNA damage, which threatens the integrity of the genome in living cells. If DNA defects remain unrepaired, they can cause potentially carcinogenic mutations. UV-induced damage in DNA involves direct or photosensitized reactions, and the chemical nature and quantity of photoproducts depend on the wavelength of UV radiation. UVB photons, by direct absorption by DNA bases, cause the formation of two main types of pyrimidine dimeric photoproducts. Under the action of UVA, mainly thymine dimers are formed in DNA, and only of one type; they can be formed either as a result of direct photon absorption or by transferring energy to thymine bases from sensitizing molecules. In addition, UVA effectively induces sensitized reactions of oxidative DNA damage. This review presents current knowledge about the fundamental mechanisms of photochemical reactions in DNA and discusses the role of various DNA photoproducts in the initiation of genotoxic processes. When considering oxidative reactions in DNA, attention is paid to the properties of pterins, riboflavin, protoporphyrin IX, and melanin as potential cellular photosensitizers for the generation of reactive oxygen species involved in these reactions.