Order of DNA replication/ Topoisomerase removes stress on the DNA strand above the replication fork. DNA replicates in the S phase of the cell cycle before mitosis begins. DNA polymerase III attaches free-floating complementary nucleotides in small sections of the lagging strand, callled Okazaki fragments. Helicase unwinds the DNA strand at the replication fork. DNA ligase stitches the Okazaki fragments together to make a complete strand of DNA. DNA polymerase III attaches free-floating nucleotides in the 3' to the 5' direction along the leading strand. Helicase rewinds the DNA at the replication fork. RNA primase creates an RNA primer on the lagging strand, starting at the 3' end.