DNA. Artwork of a molecule of deoxyribonucleic acid (DNA) (blue, background) and nucleotide base pairs. DNA is composed of two strands twisted into a double helix. Each strand consists of a sugar- phosphate backbone attached to nucleotide bases. There are four different bases: adenine, cytosine, guanine and thymine. The bases always pair in the same way; adenine with thymine (top left and right respectively) and guanine with cytosine (bottom left and right respectively). Hydrogen bonds (short grey lines) hold the nucleotide bases together. DNA contains sections called genes, which encode the body's genetic information.