A nitrogenous base that has a double ring structure made up of a five-atom ring attached by one side to a six-atom ring. The two rings contain a total of five carbon and four nitrogen atoms. Biologically significant purines include the bases adenine and guanine.
When bound to a ribose or deoxyribose sugar and a phosphate molecule, these bases form two of the five nucleotide building blocks of DNA* and RNA
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