What is strain? Why does DNA coil upon itself?

[Tyto alba]

I have been reading Lehninger's Principles of Biochemistry to understand DNA supercoiling (pg-930).

Supercoiling is the twisting of a DNA upon itself and this happens when the two helically wound strands of DNA are either underwound or overwound. The book says that this act of underwinding or overwinding generates a structural strain which is then accommodated by coiling of the DNA upon itself (supercoiling).

Here the word strain has been used in the way stress of mechanical physics would be used. I wonder if it means stress.

Besides I would like to know how underwinding or overwinding cause the DNA to coil upon itself.

Excerpts from the book:

DNA supercoiling is generally a manifestation of structural strain.

Most DNA are negatively supercoiled (which means that the supercoiling is left handed). In almost every instance, the strain is a result of underwinding of DNA double helix.

Supercoiling is not a random process; the path of the supercoiling is largely prescribed by the torsional strain imparted to the DNA...

Torsional strain, again is it talking about torsional stress?

 

[DaveC426913]

I can't supply a scientific answer, but anyone who is familair with a telephone cord knows what it will do if it is continually wound or unwound.

Or an overwound elastic band. This is a YouTube video entitled DNA topology:

 

[BillTre]

Wikipedia says: In continuum mechanics, stress is a physical quantity that expresses the internal forces that neighboring particles of a continuous material exert on each other, while strain is the measure of the deformation of the material.

Double stranded DNA is only 2 molecules across, so it might make more sense to use the monomer to monomer deformation rather than forces. Overall under- or over-coiling might be more directly computed.

Some of the extra DNA turns might be used in the coiling of the DNA around nucleosomes.
In addition, there are unusual forms of DNA with different twist properties. These could add to or relieve supercoiling.