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This image may not be used in educational posters .Illustration of a double stranded DNA (deoxyribonucleic acid) molecule. 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 (A), cytosine (C), guanine (G), and thymine (T), which are joined together by hydrogen bonds. DNA contains sections called genes that encode the body's genetic information. (KEYSTONE/SCIENCE PHOTO LIBRARY/RUSSELL KIGHTLEY)