The powerful enzyme DNA helicase unwinds the double strands of DNA by breaking the bonds between each of the complementary bases. These bases are held together by strong hydrogen bonds, but in breaking these, DNA helicase creates two single-stranded DNA molecules.
DNA helicase attaches itself to the DNA molecule at what is called the origin of replication in order to begin the process of DNA replication. The helicase acts as a sort of wedge that "unzips" the double-stranded DNA molecule into a Y-shaped fork known as the replication fork. By the time the helicase has reached the end of the DNA strand, moving the replication fork along as it goes, there are two single strands of DNA, and replication is underway.