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The five common nitrogenous bases fall into two structural groups: purines, with fused imidazole and pyrimidine rings, and pyrimidines, with a single six-membered ring. Their planar, aromatic structures support interactions such as π–π stacking. Tautomeric forms can affect base pairing, while the predominant physiological forms are keto for guanine, thymine, and uracil, and amino for adenine and cytosine. Bases also absorb ultraviolet light at 260 nm. A nucleoside forms when a base links to a pentose sugar through a β-N-glycosidic bond. The sugar type distinguishes ribonucleosides from deoxyribonucleosides, and conformational features influence nucleic-acid structure. Nucleoside analogs use base or sugar mimicry in efforts to inhibit viral or cancer DNA synthesis.