Supercoiling is defined by the equation LkTwWr If topoisomer

Supercoiling is defined by the equation Lk=Tw+Wr. If topoisomerase were not active during replication, which of the following statements would be true?

a. DNA ahead of and behind polymerase has Wr=0

b. DNA ahead of and behind the polymerase has Wr < 0

c. DNA ahead of behind the polymerase has Wr > 0

d. DNA behind the polymerase has Wr <0, and DNA ahead of it has Wr >0

e. DNA behind the polymerase has Wr>0, and DNA ahead of it has Wr <0

Solution

a. DNA ahead of and behind polymerase has Wr=0

Wr = “the number of times WC duplex crosses WC duplex” (supercoiling: compaction of B-form DNA against itself; right handed supercoils are +, left handed supercoils are -)

Lko = most favorable Lk = B-form DNA with no supercoils

DNA in cells is negatively supercoiled. Twist and writhe can interconvert, but linking number cannot without breaking the phosphodiester backbone.