Many other organic nitrogen bases have nitrogen atoms incorporated within an aromatic ring. A nitrogen atom easily can replace a carbon in an aromatic ring, with a nitrogen lone electron pair taking the place of the fourth C-H bond. This lone pair then is the binding point for a proton when the molecule functions as a base. Four such organic bases are shown on the right. Purine and pyrimidine are the parent compounds of a family of bases of great importance in biochemistry, especially in DNA and other nucleic acids.

Clockwise from top left: