A thymine dimer is a covalent linked between adjacent two thymine bases within a strand of DNA. Thymine dimers arise from ultraviolet-light induced formation of covalent bonds, as shown in Figure 25.9. Cells take several approaches to repairing thymine dimers, including photoreactivation (Figure 25.10), nucleotide excision repair (Figure 25.12), base excision repair (Figure 25.13), and recombinational repair (Figure 25.15).