Purines and pyrimidines are biochemical compounds that the body uses for a number of functions. Most notably, DNA and RNA -- the molecules responsible for the genetic code and protein synthesis -- contain purine and pyrimidine derivatives, known as nucleobases. The sequence of nucleobases in DNA encodes the genetic information passed down from one generation to the next.
A pyrimidine is a six-atom ring in which four of the atoms are carbon and two are nitrogen. Its chemical formula is C4H4N2. The ring contains a mix of single and double bonds, making it an aromatic compound. The nucleobase pyrimidines in DNA are cytosine and thymine, and those in RNA are cytosine and uracil. Pyrimidines occur widely throughout nature, including the compounds Vitamin B1, uric acid and barbiturates.
A purine consists of a pyrimidine ring fused to another type of ring called imidazole. Purine's chemical formula is C5H4N4. The purine nucleobases adenine and guanine appear in DNA and RNA. Purines also occur in caffeine, ATP -- the principal energy-storage molecule -- and many other compounds. In double-stranded DNA, a purine from one strand always binds to a pyrimidine at the same location on the sister strand. Adenine binds only with thiamine, and guanine to cytosine. When DNA transcribes RNA, uracil is used instead of thiamine on the RNA strand.