Photoreactions in deoxyribonucleic acids DNA

The energy-rich UV light portion of the terrestrial solar spectrum (2 280-400 nm) is harmful to most organisms and can even cause skin cancer in humans (basal and squamous cell carcinoma, melanoma). This is mainly due to 

Analogous photoproducts may form between any types of adjacent pyrimidines, T-T, T-C, C-T, and C-C, except that the (6-4) photoproduct does not form at C-T sites. Adenine-thymine heterodimers (see Chart 8.4) have also been detected [29, 30]. 

The UV-induced generation of cyclobutane dimers is greatly dependent on double-helix conformational factors. In dormant spores of various bacillus species, for example, a group of small, acid-soluble proteins specifically bind to DNA, thereby enforcing a particular conformation that is unfavorable for the formation of harmful cyclobutane-type lesions. As a consequence, these dormant spores are much more resistant to UV radiation than the corresponding grooving cells, in which DNA strands reassume conformations favorable for the formation of cyclobutane-type lesions [31]. 

Notably, photodimers of the cyclobutane type are cleaved by irradiation with far-UV light (240 nm) with a quantum yield of almost unity by way of the so-called [2+2] cycloreversion reaction. In living cells, dimer lesions can be repaired by the nucleotide excision repair pathway, which is based on the excision of a small piece of DNA around the lesion. Lesions not removed from the genome lead to cell death or mutagenesis. 

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