Final answer:
The enzyme required for forming a new phosphodiester bond in DNA cloning using plasmids and for joining Okazaki fragments during DNA replication is DNA ligase.
Step-by-step explanation:
The enzyme required to form a new phosphodiester bond when inserting DNA into a plasmid is DNA ligase. This enzyme plays a crucial role in the DNA replication and repair processes by facilitating the joining of Okazaki fragments and sealing breaks in the DNA strand. If the joining together of Okazaki fragments is impaired in a cell strain, it is likely that a mutation has occurred in DNA ligase, as this enzyme is responsible for sealing these fragments during replication. DNA ligase catalyzes the formation of a phosphodiester linkage between the 3' OH and 5' phosphate ends of the DNA, thus completing the formation of the continuous DNA strand.