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

They are linked by covalent bonds.

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.