What distinguishes purines from pyrimidines?

Purines and pyrimidines are two categories of nitrogenous bases found in nucleic acids, such as DNA and RNA. The key distinction between them lies in their structural composition. Purines, which include adenine and guanine, are characterized by having a double-ring structure. This two-ring formation makes purines larger and bulkier compared to pyrimidines.

Pyrimidines, on the other hand, include cytosine, thymine (found in DNA), and uracil (found in RNA). These bases have a single-ring structure, making them smaller than purines. The variation in ring structure and the resultant size differences are what fundamentally differentiate these two categories of nitrogenous bases. This distinction is crucial for understanding the pairing rules in DNA and RNA, as well as their respective roles in genetic coding and regulation.

Thus, the statement that purines are larger with two rings whereas pyrimidines have one ring accurately captures this essential difference.