A nucleotide is a an organic molecule that serves as a subunit (or monomer) of nucleic acids (DNA and RNA). The nucleotide itself is composed of a nitrogenous base, a five-carbon sugar, and a minimum of one phosphate group. The function of a nucleotide is to carry packets of energy within the cell in the form of nucleoside triphosphates. Nucleotides play a major role in cellular metabolism.