DNA structure

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

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. five prime to three prime direction and the other runs in the three prime to five prime direction. Five prime and three prime, refer to the number of the carbon molecules in the sugar backbone (Figure 1). The five prime carbon has a phosphate group, and the three prime 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.