DNA molecule. Computer artwork of a molecule of deoxyribonucleic acid (DNA). DNA is composed of two strands twisted into a double helix. Each strand consists of an outer sugar-phosphate backbone (blue) with nucleotide bases attached (ball and stick structures). There are four different bases: guanine, cytosine, thymine and adenine. Guanine always pairs with cytosine and adenine always pairs with thymine. DNA contains sections, called genes, which encode an organism's genetic information. Genes determine each cell's structure, function and behaviour.