Pyrimidine dimers and their effect on the DNA duplex. (a) One type of reaction caused by UV light results in a cyclobutane ring, which involves C-5 and C-6 of adjacent pyrimidine (in this case, thymine) bases. An alternative reaction results in a 6-4 photoproduct that links C-6 and C-4 of adjacent pyrimidines. (b) Formation of a pyrimidine dimer introduces a bend or kink in the DNA.