The precursor dNTP (nucleoside triphosphate)
contains 3 phosphate groups attached to the 5'-carbon of deoxyribose.
As the 2 terminal phosphates are cleaved, the remaining phosphate is linked to the
3'-OH group of the chain.
Thus, chain elongation occurs in the 5' to 3'
direction by adding a nucleotide to the 3' (-OH)