Thymine dimer

A thymine dimer is the covalent bonding of two adjacent thymine residues within a DNA molecule, often catalyzed by ultraviolet radiation or chemical mutagenic agents. It is an example of a more general type of DNA damage known as pyrimidine dimers which as the name suggests can occur between any adjacent pair of pyrimidine bases (such as between 2 cytosines or a cytosine and uracil). Excision repair enzymes and the DNA repair system can often recognize and repair this type of damage by the large kink in the DNA that it causes. In many organisms (excluding placental mammals such as humans), DNA photolyases can repair the damage directly by cleaving the dimer.

Unrepaired or mis-repaired thymine dimers and the resultant mutations can contribute to the development of skin cancers.