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Copyright The Columbia University Press. thymine (thÄ«Â´mÄn), 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 1893â4. 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.
To look at it in another way: Thymine(uracil in RNA) and cytosine are pyrimidines. Pyrimidines are single-ringed organic base that will only attach to purines, or adenine and guanine by hydrogen bond. Because cytosine and thymine are both pyrimidines, it cannot form a hydrogen bond with each other.