Leading strand during $DNA$ replication is formed

For the strand separation and stabilisation during $DNA$ replication,which of the following set of enzymes and proteins are required?

$E. coli$ cells were allowed to replicate in a medium containing radioactive thymidine for $5$ minutes and then transferred to a normal medium for further replication. Which of the following observations will be correct?

Which of the following serves a dual purpose in $DNA$ replication?

$^{15}N$ labeled $E. coli$ is allowed to grow in a $^{14}N$ medium. The two strands of the $DNA$ molecule of the first generation bacteria have:

Which enzyme is responsible for linking the fragments of $DNA$?