English: This is a hypothetical, speculative structure that has been proposed for Form IV, from Biegeleisen, K, 2002, "Topologically non-linked circular duplex DNA", Bull Math Biol 64:589-609. Form IV is the alkali-denatured form of duplex circular DNA. (Circular DNA cannot be heat-denatured). Its structure is generally considered to be unknown. It is a 4-stranded structure which comes about because, at high pH, the circular duplex chromosome has become exceedingly supertwisted in the left-handed (positive) sense. This turns the formerly-circular duplex into an essentially-linear tetraplex, with 2 strands being donated from opposite sides of the former circle. An essential feature of the structure is that the phosphate groups are in the center. This file is an z-y projection of Form IV taken from a 3-d virtual model.