[thahy-meen, -min]
thymine, organic base of the pyrimidine family. Thymine was the first pyrimidine to be purified from a natural source, having been isolated from calf thymus and beef spleen in 1893-4. The accepted structure of the thymine molecule was published in 1900; this structure was confirmed when several investigators reported the synthesis of the compound during the period 1901 to 1910. Combined with the sugar deoxyribose in a glycosidic linkage, thymine forms a derivative called thymidine (a nucleoside), which in turn can be phosphorylated with from one to three phosphoric acid groups, yielding respectively the three nucleotides TMP (thymidine monophosphate), TDP (thymidine diphosphate), and TTP (thymidine triphosphate). The analogous nucleosides and nucleotides formed from thymine and ribose occur only very rarely in living systems; such is not the case with the other pyrimidines. The nucleotide derivatives of thymine do not exhibit as much activity as coenzymes, although TTP can readily donate one of its phosphate groups to adenosine diphosphate (ADP) to form adenosine triphosphate (ATP), an extremely important intermediate in the transfer of chemical energy in living systems. Since the thymine nucleotides contain only deoxyribose and not ribose, TTP is the source of thymidine only in deoxyribonucleic acid (DNA); there is no thymine in ribonucleic acid (RNA). Thymidine is significant because of its involvement in the biosynthesis of DNA and in the preservation and transfer of genetic information. See nucleic acid.

Thymine is one of the four bases in the nucleic acid of DNA that make up the letters ATGC. The others are adenine, guanine, and cytosine. Thymine (T) always pairs with adenine. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase. As the name suggests, thymine may be derived by methylation of uracil at the 5th carbon. In RNA, thymine is replaced with uracil in most cases. In DNA, thymine(T) binds to adenine (A) via two hydrogen bonds to assist in stabilizing the nucleic acid structures.

Thymine combined with deoxyribose creates the nucleoside deoxythymidine, which is synonymous with the term thymidine. Thymidine can be phosphorylated with one, two, or three phosphoric acid groups, creating, respectively, TMP, TDP, or TTP (thymidine mono-, di-, or triphosphate).

One of the common mutations of DNA involves two adjacent thymines or cytosine, which, in presence of ultraviolet light, may form thymine dimers, causing "kinks" in the DNA molecule that inhibit normal function.

Thymine could also be a target for actions of 5-fluorouracil (5-FU) in cancer treatment. 5-FU can be a metabolic analog of thymine (in DNA synthesis) or uracil (in RNA synthesis). Substitution of this analog inhibits DNA synthesis in actively-dividing cells.

Thymine bases are frequently oxidized to hydantoins over time after the death of an organism.


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