Supercoiling of DNA

• Supercoiling can be demonstrated  by tightly grasping one end of a helical telephone cord while twisting the other end
• As the two strands of the double helix are separated, a problem is encountered, namely, the appearance of positive supercoils (also called supertwists).

Functions  of Supercoils:

1. Supercoiling promotes packing of DNA into compact structures
2. Helps  to  generate regions  with  broken  hydrogen bonds  which  facilitate  DNA  strand  separation and
3. facilitate  replication,  repair  and recombination  of the  DNA .

• If the cord is twisted in the direction of tightening  the coils, the cord will wrap around itself in space to form positive supercoils.
• If the cord is twisted in the direction of loosening the coils, the cord will wrap around itself in the opposite direction to form negative supercoils.

Enzymes called DNA topoisomerases, which are responsible for removing supercoils in the helix.

Topoisomerase functions:

1. Nicking Resealing Enzyme
2. Enzymes  that  can  relax  or  insert  supercoils
3. Enzymes  that  relieve  torsional  strains  in  the  DNA.

Topoisomerases  can  be of two  types:  Type  I  and  Type  II

Topoisomerase Type I

1. These enzymes reversibly cut one strand of the double helix. They have both nuclease (strand-cutting) and ligase (strand-resealing) activities.
2. They do not require ATP, but  store  energy from the phosphodiester bond they cleave, reusing the energy to reseal the strand.
3. Each time a transient “nick” is created in one DNA strand.
4. Type I topoisomerase relax negative supercoils.

Topoisomerase Type II

1. These enzymes bind tightly to the DNA double helix and make transient breaks in both strands.
2. The enzyme then causes a second stretch of the DNA double helix to pass through the break and, finally, reseals the break.
3. both negative and positive supercoils can be relieved by this ATP-requiring process.
4. DNA gyrase, a Type II topoisomerase found in bacteria and plants.
5. Type II DNA topoisomerases are also required in both prokaryotes and eukaryotes for the separation of interlocked molecules of DNA.

Anticancer agents, such as etoposide, target human topoisomerase II

Bacterial DNA gyrase is a unique target of a group of anti  microbial agents called quinolones, for example, ciprofloxacin

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