Nucleotide

Nucleotide - the basic building block of a gene. An individual unit is composed of a phosphate group, a pentose (a sugar with a five-carbon ring) and a nitrogen base. Only nitrogen bases are different in different nucleotides. In a human DNA there are four different nitrogen bases and consequently four different nucleotides: cytosine (C), guanine (G), thymine (T) and adenosine (A).