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. The structure shown here may therefore be regarded as hypothetical, but not purely so, as there is no likely alternative. 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. By default, the essential feature of the structure must be that the phosphate groups are in the center, since no other alternative is readily imaginable.
