Q.What is positive super coil ? How is it released ?

DNA Supercoiling and Topoisomerases

A human cell packs about two metres of DNA into a nucleus a few micrometres wide. Even a small bacterium holds a DNA loop far longer than the cell itself. This is only possible because the double helix is twisted upon itself — a state called supercoiling — and cells have special enzymes to manage the tension this creates.

What supercoiling is

Think of a twisted telephone cord that coils into loops on itself. When the DNA double helix is over-wound or under-wound relative to its relaxed state, the strain makes the molecule coil upon itself into a supercoil. There are two directions:

• Negative supercoiling — the helix is slightly under-wound. This is the usual state of cellular DNA, because under-winding makes it easier to pull the two strands apart.
• Positive supercoiling — the helix is over-wound, coiling more tightly.

Negative supercoiling both compacts the DNA and primes it for unwinding, which is exactly what the cell needs.

Why it becomes a problem during replication and transcription

When enzymes move along the DNA to replicate or transcribe it, they must locally unwind the double helix. But unwinding one region forces the DNA ahead of it to become over-wound — building up positive supercoils that resist further unwinding, like tension piling up ahead of an untwisting rope. If not relieved, this tension would stall the whole process.

Topoisomerases — the enzymes that relieve the strain …