Plectonemically interwound

The energy source for the process is provided by cleavage of ATP. DNA gyrase untwists relaxed circular dsDNA one turn at a time and reseals the cut ends. A reverse gyrase from certain bacteria twists the relaxed DNA more tightly. In both cases the change causes the DNA to form superhelical turns.67193-197 These may be either solenoidal or plectonemically interwound (as a twisted thread Fig. 5-18). 

Figure 5-18 Two forms of supercoiling of a DNA duplex (A) Solenoidal. (B) Plectonemically interwound. These are both negatively supercoiled, as can be deduced from the arrows at the nodes, hut the solenoid is left-handed and the plectonemic form right-handed. From Wasserman and Cozzarelli.187...

Figure 5-19 Topological equivalence of toroidal (solenoidal) and plectonemically interwound forms of a circular DNA. These two forms have a constant value of the linking number Lk (or a), the twist Tw, and writhing number Wr.

A single double-helical DNA can be further coiled in space, forming a higher-ordered structure called a supercoil. A supercoil can be either solenoidal (or toroidal) when the DNA wraps helically about a circular superhelix axis or plectonemic (or interwound) when the DNA winds helically up and back down about a superhelix axis (Figs. 1.14A and 1.14B). 

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