Final answer:
DNA synthesis proceeds in the 5' to 3' direction because DNA polymerase can only add nucleotides to the 3' end of a strand. The leading strand is synthesized continuously towards the replication fork, while the lagging strand is synthesized in Okazaki fragments due to the anti-parallel structure of DNA.
Step-by-step explanation:
DNA synthesis proceeds in the 5' to 3' direction due to the enzymatic limitations of DNA polymerase, which can only add new nucleotides to the free 3' hydroxyl end of the growing nucleotide chain. During DNA replication, one strand known as the leading strand is synthesized continuously towards the replication fork because its template strand is oriented in the 3' to 5' direction. This allows DNA polymerase to synthesize this strand in one piece.
In contrast, the lagging strand has its template oriented in the 5' to 3' direction, which means that as the fork opens, replication must proceed away from the fork. This is accomplished through the synthesis of short fragments of DNA called Okazaki fragments. Each fragment begins with an RNA primer and is later joined together to form the continuous lagging strand.
The anti-parallel nature of the DNA strands poses a challenge for the replication process, especially for the synthesis of the lagging strand, and requires these intermittent steps to ensure that both strands are accurately replicated.