Adenine and guanine are the two purine bases of deoxyribonucleic acid. In a nucleotide, the purine base is attached to a sugar, deoxyribose, and a phosphate group. Deoxyribonucleotides are joined through phosphodiester bonds to make the polynucleotide DNA.
A purine is composed of two connected nitrogenous rings. Adenine has one amine functional group attached to the larger of the rings. Guanine has a double-bonded oxygen along with one amine functional group attached to its ring. In DNA, a purine can form base pairs with a pyramidine, the other nitrogenous base, through hydrogen bonds. Adenine hydrogen bonds to thymine, and guanine hydrogen bonds to cytosine.