During DNA replication, double stranded parental DNA need to be separated by helicase to produce two single stranded DNA which are used as as template (leading and lagging template) for DNA synthesis by DNA polymerase.
However, this unwinding activity by helicase accumulates a number of positive supertwisting in front of replication fork. A saturated level of this positive superstwisting will inhibit further elongation because helicase is unable to unwind the positive supercoil-template. To solve this problem, in E. coli cell, a topoisomerase II known as gyrase, will work to remove this stress by introducing negative supercoil. Therefore, gyrase is very important which without the replisome can not go further.