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Pyrimidine dimers are molecular lesions formed from thymine or cytosine bases in DNA via photochemical reactions. Ultraviolet light (UV) induces the formation of covalent linkages between consecutive bases along the nucleotide chain in the vicinity of their carbon–carbon double bonds. The dimerization reaction can also occur among pyrimidine bases in dsRNA (double-stranded RNA)—uracil or cytosine. Two common UV products are cyclobutane pyrimidine dimers (CPDs) and 6–4 photoproducts. These premutagenic lesions alter the structure and possibly the base-pairing. Up to 50–100 such reactions per second might occur in a skin cell during exposure to sunlight, but are usually corrected within seconds by photolyase reactivation or nucleotide excision repair. Uncorrected lesions can inhibit polymerases, cause misreading during transcription or replication, or lead to arrest of replication. Pyrimidine dimers are the primary cause[not verified in body] of melanomas in humans.
A cyclobutane pyrimidine dimer (CPD) contains a four membered ring arising from the coupling of the two double-bonded carbons of each of the pyrimidines. Such dimers interfere with base pairing during DNA replication, leading to mutations.
A 6–4 photoproduct (6–4 pyrimidine–pyrimidone or 6–4 pyrimidine–pyrimidinone) is an alternate dimer consisting of a single covalent bond between the carbon at the 6 position of one ring and carbon at the 4 position of the ring on the next base. This type of conversion occurs at one third the frequency of CPDs but is more mutagenic. Spore photoproduct lyase provides another enzymatic pathway for repair of thymine photodimers.
Translesion polymerases frequently introduce mutations at pyrimidine dimers, both in prokaryotes (SOS mutagenesis) and in eukaryotes. Although the thymine-thymine CPDs (thymine dimers) are the most frequent lesions caused by UV light, translesion polymerases are biased toward introduction of As, so that TT dimers are often replicated correctly. On the other hand, any C involved in CPDs is prone to be deaminated, inducing a C to T transition.
Pyrimidine dimers introduce local conformational changes in the DNA structure, which allow recognition of the lesion by repair enzymes. In most organisms (excluding placental mammals such as humans) they can be repaired by photoreactivation. Photoreactivation is a repair process in which photolyase enzymes directly reverse CPDs via photochemical reactions. Lesions on the DNA strand are recognized by these enzymes, followed by the absorption of light wavelengths >300 nm (i.e. fluorescent and sunlight). This absorption enables the photochemical reactions to occur, which results in the elimination of the pyrimidine dimer, returning it to its original state.
Nucleotide excision repair is a more general mechanism for repair of lesions. This process excises the CPD and synthesizes new DNA to replace the surrounding region in the molecule. Xeroderma pigmentosum is a genetic disease in humans in which the nucleotide excision repair process is lacking, resulting in skin discolouration and multiple tumours on exposure to UV light. Unrepaired pyrimidine dimers in humans may lead to melanoma.