Thymine (C5H6N2O2, molecular weight: 126.12) is one of the four nucleic acid bases found in DNA. Thymine has a methyl group (−CH3) at the 5′-position of the pyrimidine ring. In double-stranded DNA, thymine pairs with adenine via Watson-Crick base pairing with two hydrogen bonds. The half-life of thymine in water is 56 years at 100 °C and 20 × 108 years at 0 °C at pH 7. Thymine has a UV absorption maximum at 265 nm and may dimerize when it is irradiated with ultraviolet light.