Thymine

Thymine, symbol T, is one of the four nucleobases in DNA. It is a pyrimidine base and, chemically, 5-methyluracil with the formula C5H6N2O2. In DNA, thymine pairs with adenine via two hydrogen bonds, contributing to the stability and fidelity of the double helix. Thymine is not used in RNA, where uracil replaces it.

In cells, thymine exists in the form of thymine nucleotides, most notably deoxythymidine monophosphate (dTMP) and its derivatives. The synthesis of dTMP from deoxyuridine monophosphate (dUMP) is catalyzed by thymidylate synthase, using methylene-tetrahydrofolate as the methyl donor. This reaction is a key control point for DNA synthesis and a target of anticancer drugs such as 5-fluorouracil, which inhibits thymidylate synthase, and drugs that disrupt folate metabolism.

Thymine is found in the DNA of almost all organisms and is absent from RNA. Its presence supports genome stability and accurate DNA replication. DNA can incur damage that involves thymine, such as UV-induced thymine dimers, which are repaired by nucleotide excision repair and related pathways to maintain genetic integrity.