A
nucleotide is an organic
molecule consisting of a nitrogenous heterocyclic
base (a
purine or a
pyrimidine), a
pentose sugar (
deoxyribose in
DNA or
ribose in
RNA), and a
phosphate or polyphosphate group. (A
nucleoside is similar, except that it contains only the sugar and base, without a phosphate.)
Nucleotides are the monomers of nucleic acids and also play important roles in cellular energy transport and transformations (notably ATP and NAD+/NADH) and in enzyme regulation (see for example, protein kinase).
