DNA and RNA are both nucleic acids formed from a repetition of the simple building blocks called nucleotides. A nucleotide consists of a phosphate (PO4), sugar, and a nitrogen base, of which five types exist: adenine (A), thymine (T), guanine (G), cytosine (C), and uracil (U). In a DNA molecule, this basic unit is repeated in a double helix structure made from two chains of nucleotides linked between the bases; these links are either between A and T or between G and C. (The structure of the bases does not allow other kinds of links.)

RNA is also a nucleic acid, but it consists of a single chain instead of a double; the sugar is ribose rather than deoxyribose; the bases are the same as in DNA, except that the thymine (T) is replaced by another base called uracil (U), which, like the thymine in DNA, links to adenine (A); and the RNA exists in three different forms (for more about these forms, see ahead). All RNA is formed in the nucleus (eukaryotic cells) or in the nucleoid region (prokaryotic cells).