The enzyme DNA helicase unzips the DNA double helix to allow it to replicate. DNA helicase has a rotation speed of 10,000 rotations per minute and moves ahead of the replication fork. It continuously unwinds the DNA strands to allow DNA polymerase to attached new nucleotide strands.
DNA helicase is a hexameric ring structure of six subunits. It unzips the double helix by breaking down the hydrogen bonds holding the DNA strands together. After being unzipped, the strand with the 3' -OH group becomes the leading strand, which can be replicated immediately. The strand with the 5' -OH group is the lagging strand, and it is replicated later in a process known as discontinuous replication.