Plectonemic Structure

Plectonemic is any tertiary structure in a polymeric molecule with supercoil strands in a regular and simple form, such as supercoiled DNA.

This type of supercoiling – form shown by DNA in vivo – doesn’t produce sufficient compaction to package DNA in the cell.

In the origin of plectonemic structures there are two interwound helical filaments whose geometry is characterized by a superhelical filament angle and radius. Due to this, the superhelical angle and the twisting moment in the filaments control the action of some enzymes like topoisomerases, RNA polymerase, helycases and DNA polymerases.

Another type of supercoiling is the solenoidal form which is shown in underwound DNA. Both structures are forms of negative supercoiling and are interconvertible, though they’re two different structures.

The main advantage of plectonemic structure is it’s stability in solution, although solenoid forms can be stabilized when present in high levels in tumor cells.

Fig.1 A plectonemic helix with radius r, pitch p and opening angle a.

Image source:
Job Ubbink, Theo Odjik, Electrostatic-Undulating Theory of  Plectonemically Supercoiled DNA, Biophysical Journal, Volume 76, Issue 5 (May 1999), pp. 2502-2519, Faculty of Chemical Engineering and Materials, Delft University of Technology, 2600 GA Delft, the Netherlands