nucleotide

A complex organic molecule, the basic building block of nucleic acids. A nucleotide has three components:

• a pyrimidine or purine base;
• a 5-carbon sugar; and
• phosphoric acid

In RNA, the sugar is ribose, hence the name ribonucleic acid. In DNA, the closely related sugar deoxyribose occurs (deoxyribonucleic acid). The purines for both are adenine and guanine; the pyrimidines are cytosine and uracil in RNA, cytosine and thymine in DNA.