Under “physiological” conditions (a 0.1 molar solution of NaCl), a DNA molecule takes on the form of a disordered coil with a radius of gyration of several micrometers; if any lengths of the molecule come within 1 nm of one other, they strongly repel. But under different conditions—in a highly dilute aqueous solution that also contains a small concentration of polyvalent cations—the same DNA molecule condenses into a tightly packed, circumferentially wound torus. Figure 1a shows just such a DNA torus. Its average radius is about 50 nm, and the distance between the axes of neighboring, parallel portions of the molecule is only slightly larger than its diameter.

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