Final answer:
DNA ligase is the enzyme that seals the gaps between Okazaki fragments on the lagging strand during DNA replication. It creates phosphodiester bonds between the fragments, completing the DNA backbone.
Step-by-step explanation:
The enzyme that joins the nick (gap between Okazaki fragments) found on the lagging strand is DNA ligase. This enzyme is responsible for sealing the breaks in the sugar-phosphate backbone of DNA.
When synthesizing the lagging strand, DNA polymerase creates Okazaki fragments, and after the RNA primers are replaced by DNA, DNA ligase catalyzes the formation of a phosphodiester bond between the fragments, finalizing the replication process. Therefore, if you isolate a cell strain with impaired joining of Okazaki fragments, the most likely mutated enzyme at the replication fork is DNA ligase.