Final answer:
Purines, which include adenine and guanine, have a double ring structure with nine members, while pyrimidines, which include cytosine, thymine, and uracil, have a single six-membered ring structure.
Step-by-step explanation:
The members that make up the backbone for purines and pyrimidines are part of the structure of nucleotides, which are the foundational building blocks of nucleic acids such as DNA and RNA. The purines, which include adenine (A) and guanine (G), have a double ring structure consisting of a six-membered ring fused to a five-membered ring, accounting for a total of nine members. Conversely, pyrimidines, which include cytosine (C), thymine (T), and uracil (U), consist of a single six-membered ring structure, resulting in six members.
In DNA, adenine pairs with thymine and guanine pairs with cytosine through hydrogen bonds, while in RNA, uracil replaces thymine. The sugar-phosphate backbone in both DNA and RNA is composed of a pentose sugar and a phosphate group. However, it's important to note the numbering convention: the atoms of the pentose sugar ring are numbered with primed numbers (1', 2', 3', 4', 5'), while the atoms of the purine or pyrimidine ring are numbered with unprimed numbers.