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Illustration of a DNA (deoxyribonucleic acid) molecule (yellow). DNA is made of two helical strands, the phosphate-deoxyribose backbone, that are connected via four bases (adenine, cytosine, guanine, thymine) which build pairs. Adenine can only pair with thymine, and cytosine can only pair with guanine. In the background are structural formulas of the bases. (KEYSTONE/SCIENCE PHOTO LIBRARY/SIMONE ALEXOWSKI / SCIENCE PHOTO LIBRARY)