The enzyme DNA polymerase is responsible for linking the nucleotides in a new DNA strand.
Before replication of DNA occurs, the DNA double helix is first unwound, after which the two strands must be separated much like the two sides of a zipper by breaking the weak hydrogen bonds that link the paired bases.
The unwound DNA strands are held apart to expose the bases so that new nucleotide partner can hydrogen-bond to them. It is DNA polymerase that moves along the exposed strand, joining newly arrived nucleotides into a new DNA strand by linking paired bases by hydrogen bonds.
