Answer:
DNA ligase is an enzyme which seals gaps in the sugar - phosphate backbone of DNA molecules.
This is necessary in the replication of DNA as the lagging strand is synthesised in short fragments by DNA polymerase. The gaps between the fragments are then sealed by DNA ligase.
DNA ligase is also very important in genetic engineering. When a plasmid (circular strand of DNA) is cut open and an extra section of DNA inserted, it is DNA ligase which seals the two DNA molecules together.