<prompt> Pyrimidine dimers are formed due toX-raysTS-raysUV raysInfra-red rays  <response>: Pyrimidine dimers are molecular lesions formed from thymine or cytosine bases in DNA photochemical reactions. Ultraviolet light induces the formation of covalent linkages by reactions localized on the CC 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 Ref-Harpers illustrated biochemistry 30e p279 