Purines are heterocyclic organic compounds. They contain two rings fused together: a six-membered and a five-membered ring. Each ring contains two "hetero" (non-carbon) atoms, all nitrogens (blue). Adenine has one substituent on the six-member ring, an amino group (NH2) in the 6 position. Guanine has the amino group at position 2 and an oxo-group (oxygen atom, red or orange) at position 6 on its six-member ring. The R attached to one nitrogen in the five-member ring shows where the sugar is attached. Some hydrogen atoms are not shown for the sake of clarity.