dna topoisomerase - chintoo

[ArchivalUser]

what is the difference in DNA topoisomerase 1 (dna gyrase) and 2? can someone explain in simple terms I am confused all about negative and positive supercoiling?
thanks

[ArchivalUser]

Positive supercoiling of DNA occurs when the right-handed, double-helical conformation of DNA is twisted even tighter (twisted in a right-handed fashion) until the helix begins to distort and "knot." Negative supercoiling involves twisting against the helical conformation (twisting in a left-handed fashion), which preferentially underwinds and "straightens" the helix at low twisting stress, and knots the DNA into negative supercoils at high twisting stress. DNA can be negatively suprecoiled (right handed) or positively supercoiled (left handed).Negative Supercoiling results from underwinding or unwinding , where as positive supercoiling results from tighter winding or overwinding of DNA double helix.
Type 1 Topoisomerases catalyze the relaxation of negative supercoils in DNA by increasing its linking number in increaments of one turn. The exposure of a negatively supercoiled DNA to nicking -closing enzyme sequentially increases its linking number until the supercoil is entirely relaxed.
Topoisomerase 2 (Gyrases) catalyze the stepwise negative supercoiling of DNA with the concomitant hydrolysis of an ATP to ADP and Pi. In the absence of ATP, DNA gyrase relaxes negatively supercoiled DNA but at relatively slow rate.

[ArchivalUser]

Thanks very much good luck

[ArchivalUser]

hi payamsalehi...can u tell me why negative supercoils need to be relaxed...i just dont understand it...can u please also tell me how cessation of dna synthesis damages a cell...i mean,i know it wont replicate but how does this apply to chemotherapy?thanks