Final answer:
The enzyme likely to be mutated when the joining of Okazaki fragments is impaired is DNA ligase. If the issue is related to the unwinding of the DNA double helix, the enzyme impaired is likely DNA helicase.
Step-by-step explanation:
Impaired Joining of Okazaki Fragments
When observing a cell strain in which the joining together of Okazaki fragments is impaired, it is likely that a mutation has occurred in an enzyme at the replication fork.
The enzyme most likely to be mutated in this case is DNA ligase. DNA ligase is responsible for joining the Okazaki fragments on the lagging strand during DNA replication.
Its failure would lead to an inability to form continuous strands, resulting in an impaired DNA replication process. However, if the observation is focused on a tightly bound DNA double helix that will not unwind, the enzyme likely to have lost its function is DNA helicase.
Helicase unwinds the DNA double helix at the replication fork by breaking the hydrogen bonds between the nucleotide pairs.
This process is essential for allowing the replication machinery to access the single-stranded DNA for replication.