The enzyme needed is topoisomerase. Topoisomerases are enzymes whose function is to regulate the unwinding of the DNA double helix. They ensure there is no overwinding or underwinding of the helix. During the replication and transcription of DNA, the double helix becomes overwound ahead of the replication fork. If left to go on, the tension created would eventually hinder the ability of DNA and RNA polymerase enzymes to continue down the strand of DNA. Topoimerases I and II enzymes correct this, ensuring the polymerase enzymes do their work smoothly.