Figure 1.

Sketches of the twin-domain model and plectoneme formation. (A) During transcription of a template that is torsionally constrained at each end, negative supercoiling accumulates upstream and positive supercoiling accumulates downstream. This RNA-polymerase-generated supercoiling can produce plectonemes, although negative supercoiling may induce DNA melting or transition to the L- or Z-form before plectonemes are formed at the buckling transition. Transcription is not the only supercoil-generating process in the cell, but it is used here as an example. (B) Unwinding DNA may produce a “soliton” in which the DNA curls about the long axis without significantly changing the tether length. Detectable tether length changes result when the curl aligns perpendicular to the direction of extension and subsequently twists to form a plectoneme.