A purine base has two nitrogen-containing rings. Purines are larger, double-ring nitrogenous bases built from a six-membered pyrimidine ring fused to a five-membered imidazole ring.
The two DNA purine bases are adenine and guanine. Their fused-ring framework gives them a broader shape than pyrimidines, which have only one ring. In a DNA base pair, a purine pairs with a pyrimidine: adenine pairs with thymine, and guanine pairs with cytosine. This purine–pyrimidine pairing keeps the DNA double helix at a fairly consistent width.
The word “purine” can refer both to the parent chemical compound and to the larger family of substituted molecules based on that structure. Adenine and guanine are not simply free purine molecules; they are modified purine nucleobases with different chemical groups attached. Purine-derived compounds also appear in RNA and in important cellular molecules such as ATP, GTP, cAMP, and cGMP. A frequent mix-up is to count nitrogen atoms rather than rings: the purine skeleton contains four nitrogen atoms, but its defining fused structure contains two rings.