A nucleotide is the basic building block of nucleic acids. RNA and DNA are polymers made of long chains of nucleotides. A nucleotide consists of a sugar molecule (either ribose in RNA or deoxyribose in DNA) attached to a phosphate group and a nitrogen-containing base. The bases used in DNA are adenine (A), cytosine (C), guanine (G), and thymine (T). In RNA, the base uracil (U) takes the place of thymine.
Nucleotides are the units and the chemicals that are strung together to make nucleic acids, most notably RNA and DNA. And both of those are long chains of repeating nucleotides. There's an A, C, G, and T in DNA, and in RNA there's the same three nucleotides as DNA, and then the T is replaced with a uracil. The nucleotide is the basic building block of these molecules, and is essentially are assembled by the cell one at a time and then strung together by the process of either replication, in the form of DNA, or what we call transcription when you're making RNA.- Lawrence C. Brody, Ph.D.