DNA strand. Computer artwork showing the double helix structure of a number of DNA (deoxyribonucleic acid) molecules. 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 bases: adenine, cytosine, guanine and thymine, which are joined together by hydrogen bonds. DNA contains sections called genes that encode the body's genetic information.