Final answer:
DNA polymerase is unidirectional and can only add nucleotides in the 5' to 3' direction due to the requirement for a free 3'-OH group, dictating how the leading and lagging strands are synthesized during DNA replication.
Step-by-step explanation:
A good mechanistic explanation for why DNA polymerase is unidirectional is due to its structural constraints. DNA polymerase requires a free 3'-OH group present on the pre-existing nucleotide chain to which it can add new nucleotides.
It catalyzes the formation of a phosphodiester bond specifically between the 3'-OH end of the growing chain and the 5' phosphate of the incoming nucleotide, thus the new strand can only grow in the 5' to 3' direction.
The anti-parallel nature of DNA strands means that while one strand (the leading strand) can be synthesized continuously towards the replication fork, the other strand (the lagging strand) must be synthesized in short stretches of DNA known as Okazaki fragments because synthesis must always initiate from a free 3'-OH group available at a primer and then proceed away from the replication fork. These fragments are later joined together to form a continuous strand.