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: Torsional strain, again is it talking about torsional stress?