What is the asymmetry of DNA replication?

The leading strand and lagging strand that are unwound by the DNA helicase at the replication fork run in opposite directions, but the new DNA sequences are synthesized in only one direction, i.e., 5’ to 3’ direction, because the enzymes involved in DNA replication can only work in the 5’ to 3’ direction. Therefore, these two template strands are replicated in different ways, resulting in the asymmetry of DNA replication.

During DNA replication, the leading strand can be continuously replicated by the polymerase since its template strand is in the 3’ to 5’ direction. However, the replication of the lagging stand is not so straightforward. It cannot be created in a continuous manner due to the 5’ to 3’ directionality of its template strand. Polymerases need to work backwards from the replication fork, creating periodic breaks in the process of replicating the lagging strand. DNA fragments, rather than continuous DNA sequence as in the leading strand, are generated in the lagging strand. These fragments, known as Okazaki fragments, are then connected into a single, continuous strand by the DNA ligase. In this way, the entire replication process is completed and it is considered asymmetric because of the difference in replicating these two strands.