The first step of DNA replication is the unwinding of the double helix structure of the DNA molecule. This is accomplished by an enzyme called helicase, which breaks the hydrogen bonds between the complementary base pairs and separates the two strands of DNA. This creates a replication fork, with one strand serving as a template for the synthesis of a new complementary strand. The process of unwinding the double helix and separating the strands is called denaturation.