A nitrogen-containing organic base composed of a single heterocyclic ring (see diagram), synthesized mainly from amino acids. Two pyrimidines, * cytosine and * thymine , are constituents of DNA, while * uracil replaces thymine in RNA. Thiamin (vitamin B1) is also partly constructed from a pyrimidine. ![Pyrimidine. Pyrimidine.](http://botanydictionary.org/images/stories/botany/botany-dictionary-58.jpg) Pyrimidine.
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