Final answer:
In a cell strain with impaired joining of Okazaki fragments, the enzyme DNA ligase is most likely to be mutated since it is responsible for ligating the fragments during DNA replication.
Step-by-step explanation:
If you isolate a cell strain where the joining of Okazaki fragments is impaired, it is likely that there is a mutation in DNA ligase. This is the enzyme responsible at the replication fork for ligating, or joining, the short DNA segments formed on the lagging strand during DNA replication. Since Okazaki's hypothesis showed that short DNA fragments accumulated in E. coli cells infected with ligase-deficient mutants, we can infer that DNA ligase is the enzyme that would most likely be mutated if Okazaki fragment joining is impaired.
Specifically, DNA ligase acts to seal the nicks between Okazaki fragments, creating a continuous DNA strand. The mutation in this enzyme would result in the accumulation of these fragments and possibly lead to replication issues within the cell.