nucleotide, organic substance that serves as a monomer in forming nucleic acids. Nucleotides consist of either a purine or a pyrimidine base, a ribose or deoxyribose, and a phosphate group. Adenosine triphosphate serves as the principle energy carrier for the cell's reactions. The most important nucleotides are those derived from the bases adenine, guanine, cytosine, thymine, and uracil.

Any of a class of organic compounds, including the structural units of nucleic acids. Each consists of a nucleoside and one or more phosphate groups. In nucleic acids, the phosphate of one nucleotide joins to the sugar of the next to form the backbone. Important nucleotides that are not part of nucleic acids include ATP, cyclic AMP (needed in glycogen breakdown), and certain coenzymes.

Learn more about nucleotide with a free trial on Britannica.com.