Why is the synthesis of polynucleotides directed from the 5' end to the 3' end?

The 3' end has a free hydroxyl group that allows the addition of nucleotides, while the 5' end is blocked by a phosphate group.

The synthesis is directed by the template strand, which is always read from the 5' to the 3' end.

DNA and RNA polymerases can only catalyze the formation of phosphodiester bonds between the 5' phosphate of the incoming nucleotide and the 3' hydroxyl group of the growing chain.

The directionality is due to the higher energy of the phosphate group at the 3' end.