During DNA replication, helicase unwinds the DNA double helix, which creates two strands. One strand is positioned as 5’ to 3’, whereas the other is 3’ to 5’. DNA polymerase can only synthesize a new strand by working from a 5’ to 3’ direction, which is why the 5’ to 3’ strand becomes a leading strand since DNA polymerase can work on the strand continuously without interruptions by following the helicase. However, the 3’ to 5’ strand can be operated on by DNA polymerase in the opposite direction, which makes it a lagging strand.
The main reason is that DNA polymerase lands on the strand behind helicase and synthesizes a new strand by going in the opposite direction until it reaches either the previously synthesized strand or an RNA primer. Therefore, the lagging strand creates fragments called Okazaki fragments, which require ligase enzymes to connect to each other.