Final answer:
Purine has 2) two carbon-nitrogen rings, which form a double ring structure that distinguishes them from pyrimidines, which have a single ring structure.
Step-by-step explanation:
Purine has two carbon-nitrogen rings. Adenine and guanine, which are classified as purines, have a double-ring structure consisting of a six-membered ring fused to a five-membered ring. This is in contrast to pyrimidines, such as cytosine, thymine, and uracil, which have a single carbon-nitrogen ring as their primary structure. Purines and pyrimidines are the building blocks of nucleic acids, with DNA containing adenine (A), thymine (T), guanine (G), and cytosine (C), while RNA contains adenine (A), uracil (U), guanine (G), and cytosine (C).