This circle is not supercoiled and is called a relaxed circle. In forming this underwound circle we have changed the linking number from 25 to 23. This DNA circle is unstable because the DNA duplex has been disrupted. The complete duplex can be reformed if the circle then twists into a supercoil as shown below. We now need two additional topological terms to describe our supercoiled DNA molecule.
Twist is the number of complete revolutions that one strand makes about the duplex axis.
Writhe is the number of times the helix axis crosses over itself and is a measure of supercoiling. The usual value of Wr is negative, which means the supercoil is right-handed. Importance Practical - gel separation. Packing of DNA - Supercoiled DNA is more compact. I on the highly supercoiled DNA of a virus.