A nucleotide is composed of a phosphate group, a nitrogen-containing base, and a five-carbon sugar amino group. A nucleotide is composed of a phosphate group, a nitrogen-containing base, and a five-carbon sugar amino group. A nucleotide is the building block or structural component of DNA and RNA. It consists of a base , that is one from adenine, thymine, guanine, and cytosine. and a molecule of sugar and one of phosphoric acid.