DON FAWCETT / SCIENCE PHOTO LIBRARY DON FAWCETT / SCIENCE PHOTO LIBRARY
DNA (deoxyribonucleic acid). DNA is a polymer made up of four types of nucleotides that form a double helix: adenine, guanine, cytosine, and thymine. Unlike linear DNA, circular DNA has no ends and is the typical form of bacterial DNA and cytoplasmic DNA in animals. The closed circular form of DNA (of the polyoma virus) was discovered in 1963 by Dulbecco, Vogt, Weil, and Vinograd. Enhancement of 9A1962.
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