Localised unwinding of a circular DNA molecule, which occurs during DNA replication, induces torsional stress into the remaining portion of the molecule because the ends of the strands are not free to rotate. As a result, the DNA molecule twists back on itself, like a twisted rubber band, forming supercoils 
The supercoiling of DNA is a form of compaction of circular DNA duplex, as its segments are twisted around each other. In the relaxed state, the polynucleotide strands form a right-handed helix, in a pattern of ten nucleotide pair per turn. A underwound circular DNA duplex resists the cuts made by DNA topoismerase ezymes, by twisting opposite to the direction of underwinding, forming supercoils 
- ↑ Lodish,H.,Berk,A.,Kaiser,C.A.,Krieger,M.,Bretscher,A.,Ploegh,H., and Amon,A. (2013) Molecular Cell Biology,7th Edition, International Edition.
- ↑ Hartl, D. L. (2012) Genetics: analysis of genes and genomes, 8th edition, Jones &amp; Bartlett Learning. Chapter 7, page 229-30.
- ↑ Hartl, D. L. (2012) Genetics: analysis of genes and genomes, 8th edition, Jones and; Bartlett Learning. Chapter 7, page 229-30.