How do purines differ from pyrimidines?

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Diagram of a DNA polynucleotide chain showing linked deoxyribose sugars, phosphate groups, and nitrogenous bases (adenine, guanine, cytosine, thymine), with an inset comparing ribose and uracil in RNA. — Nucleic acid A portion of the polynucleotide chain of deoxyribonucleic acid (DNA), with an inset showing the corresponding pentose sugar and pyrimidine base in ribonucleic acid (RNA).

In the context of nucleic acids, purines and pyrimidines are the two categories of nitrogen-containing bases that serve as essential building blocks for DNA and RNA. They differ primarily in their chemical structure and classification:12

Purines: This category includes adenine (A) and guanine (G). Structurally, purines are polycyclic compounds.134
Pyrimidines: This category includes cytosine (C), thymine (T), and uracil (U). Pyrimidines are characterized as six-membered ring compounds.124

In both DNA and RNA, these bases form base pairs through hydrogen bonding, which is fundamental to the structure of these molecules. While all nucleic acids contain adenine, guanine, and cytosine, they differ in their fourth base: DNA contains thymine, whereas RNA contains uracil.312