Damaged DNA molecule, computer artwork. DNA (deoxyribonucleic acid) is composed of two strands twisted into a double helix. Each strand consists of a sugar-phosphate backbone (large spheres) attached to nucleotide bases (small spheres). 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.