nucleotide no͞o´klēətīd˝, nyo͞o´– [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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