A purine is a heterocyclic aromatic organic compound. It consists of a pyrimidine ring fused to an imidazole ring. Purines, which include substituted purines and their tautomers, are the most widely occurring nitrogen-containing heterocycle in nature.Purines and pyrimidines make up the two groups of nitrogenous bases, including the two groups of nucleotide bases.wo of the five bases in nucleic acids, adenine (2) and guanine (3), are purines. In DNA, these bases form hydrogen bonds with their complementary pyrimidines thymine and cytosine, respectively.