nucleotide (nōˈklēətĪdˌ, nyōˈ–) [key], 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.
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