thymine thī´mēn [key], 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.
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