<< To get a sense for supercoiled DNA, imagine twisting a piece of string. Let the string go, and it unwinds. Twist it enough, and it folds back on itself. The degree of twist puts stress on the string, which governs the shape it takes >>
http://www.scientificamerican.com/article/how-strange-twists-in-dna-orchestrate-life/
Rossitza N. Irobalieva, Jonathan M. Fogg, et al. Structural diversity of supercoiled DNA. Nature Communications 6, Article number: 8440 doi:10.1038/ncomms9440
http://www.nature.com/ncomms/2015/151012/ncomms9440/full/ncomms9440.html