Enzyme helicase acts over the ori site and unzips (unwinds) the two strands of DNA. Unwinding creates tension in the uncoiled part by forming more supercoils. Tension is released by topoisomerases. They cause nicking of one strand of DNA (for removing coils) and resealing the same. Along with topoisomerase, bacteria possess another enzyme called DNA gyrase which can introduce negative supercoils (older worker believed that gyrase functioned both for helicase and topoisomerase). With the help of various enzymes both the strands of DNA become open for replication.