Purines are characterized by containing two fused rings.

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Purines are characterized by containing two fused rings.

Purines have a bicyclic ring system: a six-membered ring fused to a five-membered ring. This two-ring, fused structure is the defining feature of purines (adenine and guanine) and explains why they are larger than the single-ring pyrimidines (cytosine, thymine, and uracil) with which they pair in nucleic acids. The fused rings provide the nitrogen-rich surface needed for hydrogen bonding patterns that help maintain the consistent width of the DNA double helix. So describing purines as containing two fused rings is accurate. A single ring would describe pyrimidines, three rings don’t occur for standard nucleobases, and purines are indeed nitrogenous bases.

Purines have a bicyclic ring system: a six-membered ring fused to a five-membered ring. This two-ring, fused structure is the defining feature of purines (adenine and guanine) and explains why they are larger than the single-ring pyrimidines (cytosine, thymine, and uracil) with which they pair in nucleic acids. The fused rings provide the nitrogen-rich surface needed for hydrogen bonding patterns that help maintain the consistent width of the DNA double helix. So describing purines as containing two fused rings is accurate. A single ring would describe pyrimidines, three rings don’t occur for standard nucleobases, and purines are indeed nitrogenous bases.

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