The main mutagenic effects of ultraviolet light are to produce chemical cross-links between adjacent thymines in a DNA strand. These could be repaired by:

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The cross-links produced by UV light between adjacent thymines are called thymine dimers. These thymine dimers are repaired by nucleotide excision repair (NER), which specializes in removing bulky DNA lesions such as thymine dimers. DNA ligase is an enzyme that seals breaks in the sugar-phosphate backbone of DNA, but it does not repair thymine dimers. Mismatch repair and base excision repair are mechanisms that specialize in repairing errors that occur during DNA replication or damage caused by chemical agents, but they are not efficient at repairing thymine dimers.

The Main Mutagenic Effects of Ultraviolet Light Are to Produce