The 3' end of the DNA strand acts as a nucleophile that forms a covalent bond with an atom of the phosphorus of the alpha phosphate group of the dNTP that combines with the DNA.The reaction involving elongation of the chain of the DNA is catalyzed by DNA polymerase and involves the attack by a nucleophile on the 3' hydroxyl group of primer on phosphorus atom found in the innermost part of deoxynucleoside triphosphate.