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. Two of the fourdeoxyribonucleotides and two of the four ribonucleotides, the respective building-blocks of DNA and RNA, are purines.