A Purine is a nitrogenous base consisting of a double ring structure (one 5 membered ring and one 6 membered ring) which makes up half of a rung in a DNA ladder. They are also present in RNA. Guanine and Adenine are the two types of purines you would find in DNA and RNA. Purines form hydrogen bonds with their complimentary pyrimidine. Guanine forms 3 hydrogen bonds with Cystine, and Adenine forms 2 hydrogen bonds with Thymine.