by Rich Feldenberg
This blocks normal base pairing on to the other DNA strand of the double helix, and results in a mutation. Fortunately there are cellular repair mechanisms that can find and fix these errors, but some errors escape detection and cause major harm. Some melanomas are thought to be due to thyimine dimers caused by the effect of UV sunlight.
Thymine dimers are actually a more specific form of what is called pyrimidine dimers. The bases thymine and cytosine (T and C) are pyrimidines. Two pyrimidines can dimerize under the same conditions leading to the same sort of DNA mutations. You could have T-T dimers (thymine dimers), but also T-C, and C-C leading to the same problems. So, remember to use sunblock and be careful about exposure to the sun!!