The tertiary structure of DNA molecule is made up of the two strands of DNA wind around each other. DNA double helix can be arranged in space, in a tertiary arrangement of strands.
• Linking Number( Lk) in a covalently closed circular DNA, where the two strands cannot be separated will result in a constant number of turns in a given molecule. Lk of DNA is an integral composed of two components:
1)Twist (Tw): number of helical turns of DNA strand
2) Writhe (Wr): number of supercoiled turns in DNA
Normally, DNA has Lk of about 25, meaning it is underwound. However, DNA can also be supercoiled with two "underwindings" which is made up of negative supercoils. This is muck like the two "turns- worth" of a single stranded DNA and no supercoils. This kinds of interconversion of helical and superhelical turns in important in gene transcription and regulation.