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Pyrimidine dimers are molecular lesions formed from thymine or cytosine bases in DNA via photochemical reactions.〔 〕 Ultraviolet light induces the formation of covalent linkages by reactions localized on the C=C double bonds.〔 〕 In dsRNA (double-stranded RNA), uracil dimers may also accumulate as a result of UV radiation. Two common UV products are cyclobutane pyrimidine dimers (CPDs, including thymine dimers) and 6,4 photoproducts. These premutagenic lesions alter the structure of DNA and consequently inhibit polymerases and arrest replication. Dimers may be repaired by photoreactivation or nucleotide excision repair, but unrepaired dimers are mutagenic. Pyrimidine dimers are the primary cause of melanomas in humans. ==Types of dimers== A cyclobutane pyrimidine dimer (CPD) contains a four membered ring arising from the coupling of the C=C double bonds of pyrimidines.〔 〕 Such dimers interfere with base pairing during DNA replication, leading to mutations. 6,4-photoproducts, or 6,4 pyrimidine-pyrimidones, occur at one third the frequency of CPDs but are more mutagenic. Spore photoproduct lyase provides another enzymatic pathway for repair of thymine photodimers. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Pyrimidine dimer」の詳細全文を読む スポンサード リンク
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