What is the role of the enzyme ligase in replication?
Ligase fills in missing nucleotides on the lagging stand of DNA during replication.
Since the leading stand in DNA is being read and added to in the same direction as the Helicase complex is moving, there are no gaps.
But the lagging stand is developed in the opposite direction. This means replication on this side has to start and stop, leaving gaps between the finished DNA (Okazaki Fragments). Ligase catilizes the addition of the proper nucleotides to fill in the gaps between the Okazaki fragments thus completing the new double helix.