the steps of DNA replication * A portion of the double helix is unwound by a helicase * A molecule of a DNA polymerase binds to one strand of the DNA and begins moving along it in the 3′ to 5′ direction, using it as a template for assembling a leading strand of nucleotides and reforming a double helix. In prokaryotes, this molecule is called DNA polymerase III and in eukaryotes, this molecule is called DNA polymerase delta (δ). *Because DNA synthesis can only occur 5′ to 3′, a molecule of a second DNA polymerase (α, in eukaryotes) binds to the other template strand as the double helix opens. This molecule must synthesize discontinuous segments of polynucleotides (called Okazaki fragments). Another enzyme, DNA ligase then stitches these together into the lagging strand.