What is the difference between a purine and a pyrimidine?
Purines have two carbon-nitrogen rings and pyrimidines have one carbon-hydrogen ring.
Both are nitrogenous bases. However, pyrimidines contain one carbon-nitrogen ring and purines contain two carbon-nitrogen rings.
Adenine and guanine are purines, while thymine, cytosine, and uracil are pyrimidines.
In a DNA molecule, a pyrimidine base always pairs with a purine base. The pyrimidine adenine always pairs with the purine thymine, and the pyrimidine guanine always pairs with the purine cytosine. The paired nitrogen bases are held together by hydrogen bonds, which are shown in the illustration below.