Final answer:
New phosphodiester linkages always form at the 3' end of the existing polymer.
Step-by-step explanation:
A polynucleotide chain is formed by the joining of nucleotides through phosphodiester bonds. The phosphate group attached to the 5' carbon of one nucleotide forms a second ester linkage with the hydroxyl group of the 3' carbon of the next nucleotide, creating a 5'-3' phosphodiester bond. Therefore, new phosphodiester linkages always form at the 3' end of the existing polymer.