Which category of bases has two rings in its structure?

• A. Pyrimidines
• B. Purines
• C. Nucleosides
• D. Nucleotides

Answer: (B)

Bases in nucleic acids come in two families: purines and pyrimidines. Purines have two fused rings in their ring system, making them larger, while pyrimidines have a single ring. The two-ring structure of purines is exemplified by adenine and guanine, whereas cytosine, thymine, and uracil are single-ring pyrimidines. This difference helps keep the DNA and RNA helix uniform in width because purines pair with pyrimidines, balancing the overall shape of the base pairs. The terms nucleosides and nucleotides refer to the base plus a sugar (nucleosides) or base plus sugar and phosphate (nucleotides), not to the base classes themselves. Therefore, the category with two rings is purines.