Plectonemic and solenoidal supercoiling. (a) Electron micrograph of plectonemically supercoiled plasmid DNA. (b) Plectonemic supercoiling consists of extended right-handed coils. (c) Solenoidal supercoiling of the same DNA molecule depicted in (b), drawn to scale. Solenoidal negative supercoiling consists of tight left-handed turns about an imaginary tubelike structure. Solenoidal supercoiling provides a much greater degree of compaction.