DNA elongation. 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) end.