Every living cell contains a minuscule amount of an extraordinary substance called DNA (for deoxyribonucleic acid) that carries in it the traits of heredity. When its home cell begins to divide, the DNA performs wondrously: its complicated molecules, ordinarily like two ropes twisted together, untwist and separate. Each rope attracts bits and pieces from fluid around it and forms a new double helix like the original one. Apportioned between the halves of the dividing cells, the duplicated DNA molecules determine whether the new individuals will be men or muskrats, pine trees...
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