Purines are 9-membered double rings which include Guanine and Adenine. A purine has an imidazole ring joined to a pyrimidine ring at 4 and 5 positions. Purines have nitrogen at 1,3,7 and 9 positions. Pyrimidines are 6-membered rings and have nitrogen at 1 and 3 position. Thymine, uracil, and cytosine come under pyrimidine.