Helicases and topoisomerases are enzymes that have their major functions during not only replication, but also during transcription. The deoxyribonucleic acid (DNA) molecule would be a highly coiled molecule. In order for replication and transcription, it has to be uncoiled to make it available for the enzymes involved in the process. To make sure, the uncoiled enzymes helicases and topoisomerases are used. While helicase is used in a linear DNA molecule, topoisomerase is used in the case of a circular DNA molecule.
Helicases are involved in the breakage of the hydrogen bonds that exists between the two strands in a linear DNA molecule. This would enable both ribonucleic acid (RNA) polymerase and DNA polymerase to initiate their process of transcription and replication respectively. The topoisomerases are involved only in the circular DNA molecules.
In a circular molecule of DNA, the DNA would already be under stress due to coiling. During the process of replication, the DNA strands would be unwound, which would add further pressure to the other end of the DNA. In order to relax the supercoiling of the DNA, the topoisomerases restricts the DNA molecule, unwinds them or removes the supercoiling and relegates them. This would ensure that the DNA molecule is not under much pressure. These are the functions of both helicases and topoisomerase enzymes.