Final answer:
Purines and pyrimidines are nitrogenous bases, with purines being larger, double-ring structures and pyrimidines being smaller, single-ring structures. In DNA and RNA, a purine pairs with a pyrimidine, which is the favored form for maintaining the uniform structure of the double helix. Incorrect pairings would disrupt the helix's uniformity and are not favored.
Step-by-step explanation:
Purines and pyrimidines are the two types of nitrogenous bases found in nucleotides, the building blocks of nucleic acids like DNA and RNA. Purines, such as adenine (A) and guanine (G), have a double ring structure, while pyrimidines, such as cytosine (C) and thymine (T), have a single ring structure. A purine must always be paired with a pyrimidine to maintain the structure of DNA's double helix. For example, adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three hydrogen bonds. This correct pairing is crucial as it helps to maintain a consistent space between the two DNA strands and adds stability to the double helix due to hydrogen bonding.
Given that purines are larger than pyrimidines, a pairing between two purines would take up more space and, conversely, a pairing between two pyrimidines would take up less space when compared to the purine-pyrimidine pairing which keeps the double helix structure uniform. If such incorrect pairings occurred, the DNA structure would be akin to a staircase with steps of differing widths, which is highly unfavorable. Therefore, the pairing of one purine with one pyrimidine is the favored form in DNA and RNA structure.