Thymine (T) is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase.

It is actually the first pyrimidine that was successfully purified by scientists from an all natural source in 1893 – 1894. In 1900, the now accepted structure of a thymine molecule was published. Several investigators were then able to confirm the validity of the molecular structure when they synthesized the compound during 1901 to 1910.

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, thus stabilizing the nucleic acid structures.

Thymine forms a derivative called thymidine when it is combined with a sugar called deoxyribose in a glycosidic linkage. Thymidine, which is a nucleoside, can be phosphorylated with any of three phosphoric acid groups. The product of these mix are three nucleotides namely, TMP (thymidine monophosphate), TDP (thymidine diphosphate), and TTP (thymidine triphosphate).

Thymidine is a very important substance as it is involved in the biosynthesis of DNA and in the preservation as well as transfer of genetic information.

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.

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