DNA polymerase is a form of template directed DNA synthesis, with the polymerase using the existing DNA strand (unzipped by the helicase) to determine which complementary nucleotides to incorporate. This helps maintain the integrity of the genome by ensuring a template strand is always present when DNA is being replicated – this semi-conservative replication was first demonstrated by Messolsohn and Stahl.
The precursor molecules used by the DNA polymerase include dNTPs (dATP, dTTP, dGTP, dCTP) to bind to the daughter strand during elongation, with pyrophosphate (PPi) being released on catalysis. A water molecule is also released, as this is an SN2 reaction – the water molecule is formed from an OH and H being cleaved from the phosphates in the backbone during nucleotide incorporation. The template DNA molecule is also necessary to allow the polymerase to have something to replicate.