Question

what is supercoiling and what is it controlled by

Answer 1

DNA supercoiling refers to the over- or under-winding of a DNA strand, and is an expression of the strain on that strand. Supercoiling is important in a number of biological processes, such as compacting DNA, and by regulating access to the genetic code, DNA supercoiling strongly affects DNA metabolism and possibly gene expression. In a "relaxed" double-helical segment of B-DNA, the two strands twist around the helical axis once every 10.4–10.5 base pairs of sequence. Adding or subtracting twists, as some enzymes can do, imposes strain. If a DNA segment under twist strain were closed into a circle by joining its two ends and then allowed to move freely, the circular DNA would contort into a new shape, such as a simple figure-eight. Such a contortion is a supercoil. Supercoiling allows for the compact packing of circular DNA.

Types of Supercoiling

• Positive Supercoiling : It is the left-handed, coiling of DNA thus winding occurs in the counterclockwise direction. This process is also known as the "overwinding" of DNA.
• Negative Supercoiling : Negative supercoiling is the right-handed coiling of DNA thus winding occurs in the clockwise direction. It is also known as the "underwinding" of DNA.

Control of Supercoiling

Two DNA topoisomerases control the level of negative supercoiling in bacterial cells. DNA gyrase introduces supercoils, and DNA topoisomerase I prevents supercoiling from reaching unacceptably high levels.

Positive and negative supercoils require two different topoisomerases. This prevents the distortion of DNA by the specificity of the topoisomerases.The two classes of topoisomerases are Type I and Type II. Type I stimulates the relaxation of supercoiled DNA and Type II uses the energy from ATP hydrolysis to add negative supercoils to DNA. Both of these classes of topoisomerases have important roles in DNA transcription, DNA replication, and recombinant DNA. Topoisomerase form loops (unwinded regions of the double helix) of negative supercoils. If the DNA lacks superhelical tension, there is no unwinding of supercoils.