6.1.2—Purines (adenine, guanine)

A purine is a nitrogenous base with a fused double-ring structure. In the nucleic acids studied here, the two purines are adenine (A) and guanine (G).

• Where they occur: Adenine and guanine can be the nitrogenous base in nucleotides that contribute to both DNA and RNA. The sugar and phosphate are the other nucleotide components; the base name alone is not a complete nucleotide.
• How the class is used: In a DNA or RNA sequence, A and G identify purine bases. Their complementary partners are pyrimidines: A pairs with T in DNA or U in RNA, while G pairs with C.
• Necessary contrast: Pyrimidines have a single-ring structure. The pyrimidines in this course are cytosine (C), thymine (T) and uracil (U). Purine/pyrimidine is a structural classification, not a synonym for a whole nucleotide or a rule that names the partner without checking the actual base.

The double-ring feature lets a learner classify A and G from the base category, while the surrounding sugar/phosphate context determines whether that base is part of a DNA or RNA nucleotide. Complementary pairing then uses the actual base identity, with one purine pairing with one pyrimidine to maintain the regular width of a nucleic-acid strand pair.

Purine does not mean “the whole nucleotide” and does not mean “a base that can be identified only by its partner”. Learn the two purines directly: adenine and guanine. Do not treat thymine as an RNA purine or uracil as a DNA purine.