In fact, the units of DNA that reside in the nucleus of eukaryotic cells, and DNA pieces as people typically think of them, are not single molecules. Rather, they are pairs of molecules, which entwine like vines to form a "double helix" (top half of the illustration at the right).

Each vine-like molecule, or strand of DNA, is a chemically linked chain of nucleotides, which each consist of a deoxyribose sugar, a phosphate, and one of four varieties of "aromatic" bases. Because DNA strands are composed of these nucleotide subunits, they are polymers.

The diversity of the bases means that four distinct kinds of nucleotide exist, which are commonly referred to by the identity of their base. These are adenine (A), thymine (T), cytosine (C), and guanine (G).

In a DNA double helix, two polynucleotide strands come together through complementary pairing of the bases, which occurs by hydrogen bonding. Each base forms hydrogen bonds readily to only one other—A to T and C to G—so that the identity of the base on one strand dictates what base must face it on the opposing strand. Thus the entire nucleotide sequence of each strand is complementary to that of the other, and when separated, each may act as a template with which to replicate the other from free nucleotides (middle and lower half of the illustration at the right).

Because pairing causes the nucleotide bases to face the helical axis, the sugar and phosphate groups of the nucleotides run along the outside, and the two chains they form are sometimes called the "backbones" of the helix. In fact, it is chemical bonds between the phosphates and the sugars that link one nucleotide to the next in the DNA strand.