nucleobase

A nucleobase, or nitrogenous base, is a nitrogen-containing heterocyclic molecule that forms the core of a nucleotide, the basic unit of nucleic acids. In a nucleotide, the nucleobase is attached to a five-carbon sugar (deoxyribose in DNA or ribose in RNA) and one or more phosphate groups. Nucleobases are grouped into two families: purines, which have a two-ring structure (adenine and guanine), and pyrimidines, which have a single-ring structure (cytosine, thymine, and uracil).

In DNA, the canonical bases are adenine (A), thymine (T), cytosine (C), and guanine (G). In RNA, thymine is replaced by uracil (U), giving A, U, C, and G. Base pairing follows specific hydrogen-bonding rules: A pairs with T (or U in RNA) via two hydrogen bonds, and G pairs with C via three hydrogen bonds. This pairing underpins the replication and transcription of genetic information and the folding of RNA into functional structures.

Nucleobases have distinct structural and chemical properties that influence genetic stability and information encoding. Modifications to bases occur in both DNA and RNA, such as methylation or ribonucleoside alterations, affecting replication fidelity, gene expression, and RNA function. Noncanonical bases, including inosine in tRNA and various modified nucleosides in RNA, contribute to decoding accuracy and structural diversity.

Beyond their genetic role, nucleobases participate in metabolism and signaling, and their biosynthesis can proceed via de novo pathways or salvage pathways that recycle free bases and nucleosides.