Purines and pyrimidines are two types of nitrogenous bases that play a critical role in the structure of DNA and RNA. They differ in both their chemical structure and the rules governing their pairing.

Purines are the larger nitrogenous bases, characterized by their two fused carbon-nitrogen rings. The two primary purines found in both DNA and RNA are adenine (A) and guanine (G). In contrast, pyrimidines are smaller bases, consisting of a single carbon-nitrogen ring. The pyrimidines found in DNA include cytosine (C) and thymine (T), while in RNA, uracil (U) replaces thymine.

In DNA, the pairing between purines and pyrimidines occurs through hydrogen bonding. Specifically, adenine pairs with thymine, forming two hydrogen bonds, while guanine pairs with cytosine, forming three hydrogen bonds. This specific base pairing is essential for the processes of DNA replication and transcription.

In RNA, purines and pyrimidines also engage in hydrogen bonding, with some notable differences. Adenine pairs with uracil, also forming two hydrogen bonds, while guanine continues to pair with cytosine, maintaining the three hydrogen bonds. Additionally, the single-stranded nature of RNA allows it to adopt complex secondary structures, enabling further base pairing interactions beyond the standard pairings.

Grasping the distinctions between purines and pyrimidines, along with their respective pairing rules, is vital for understanding the intricate structure and function of both DNA and RNA.