Basis nucleici acidi
Basis nucleici acidi (licet "nucleobasis") est compositum chemicum, nitrogenium continens, cui intus reperiuntur DNA et RNA, nucleotides, et nucleosides. Nuncupatur etiam "basis nitrogenosa" vel simplicius "basis". Facultas eorum formandi bases-pares ducit structuras helicales in DNA et RNA. Quinque bases nucleici acidi sunt adeninum (A), guaninum (G), cytosinum (C), thyminum (T) et uracilum (U). (Uracilum est solum in RNA, thyminum est solum in DNA.) Bases inter se vincula (pares basium) formant: ADN: Adeninum --- Thyminum Guaninum --- Cytosinum ARN: Adeninum --- Uracilum Guaninum --- Cytosinum Bases vinculis hydrogenicis se iungunt.