DNA is composed of two strands twisted together, also called the double helix.

Left side of image shows colored bases of DNA double helix. From top to bottom, left strand goes from 3 prime to 5 prime and right strand goes from 5 prime to 3 prime. Right side of image shows a zoom up of 2 nucleotide base combinations. The bases are connected along a strand by a negatively charged phosphate group. Strand with 5 prime at top has ribose structure drawn with oxygen atom at top and strand with 3 prime at top has ribose structure drawn with oxygen atom at bottom.

Figure 1: DNA structure. DNA forms a double-helix composed of two antiparallel strands (left) and consisting of pairs of nucleotides, formed by thymine-adenine and guanine-cytosine (right).

The double helix is said to be antiparallel because one strand runs in the 5′→3′ direction and the other runs in the 3′→5′ direction. 5' and 3', refer to the number of the carbon molecules in the sugar backbone (Figure 1). The 5' carbon has a phosphate group and the 3' carbon has a hydroxyl group.

The strands are composed of nucleotides: a sugar molecule (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogen-containing base.

A phosphodiester link connects the nucleotides with each other. The two strands are bound with weak hydrogen bonds between the complementary nucleotides. The nitrogenous bases are “inside” like rungs on a ladder.