Identify two structural features of purines and pyrimidines. purines contain four ring nitrogen atoms. contain only two ring nitrogen atoms. contain two heterocyclic rings. contain only three ring nitrogen atoms. contain one heterocyclic ring. pyrimidines contain only two ring nitrogen atoms. contain two heterocyclic rings. contain four ring nitrogen atoms. contain one heterocyclic ring. contain only three ring nitrogen atoms. classify each molecule as a purine or a pyrimidine. purines pyrimidines answer bank