Final answer:
The enzyme that joins the fragments of the lagging strand is DNA ligase, which is likely to be mutated in a cell strain with impaired fragment joining. The correct option is b. DNA ligase
Step-by-step explanation:
The enzyme responsible for joining the small fragments of the lagging strand together into a continuous strand is called DNA ligase. This enzyme is crucial during DNA replication because it seals the nicks between the Okazaki fragments, which are short sequences of DNA. When examining a cell strain with impaired joining of Okazaki fragments, the mutant enzyme at the replication fork is most likely DNA ligase.
Conversely, DNA polymerase III is mainly responsible for the addition of nucleotides to a growing DNA strand, while primase synthesizes RNA primers necessary for DNA polymerase to initiate synthesis. Meanwhile, helicase unwinds the DNA helix, and topoisomerase alleviates the supercoiling that occurs during replication.
It is also important to note that the ends of linear chromosomes are maintained by telomerase, which adds repetitive nucleotide sequences to the telomeres, ensuring the integrity and lengthening of chromosomes. The correct option is b. DNA ligase