Which of the following enzymes is responsible for the synthesis of $DNA$ from $RNA$?

For how long should bacteria be cultured in a $^{14}N$ medium after being transferred from a $^{15}N$ medium so that the extracted $DNA$ consists of equal amounts of hybrid $DNA$ and light $DNA$?

An $E. coli$ cell with light $DNA$ is incubated in a culture medium containing a heavy isotope of nitrogen $(^{15}N)$. After $60$ minutes, find out the percentage of light $DNA$. (in $\%$)

Given below are the two statements.
Statement $I$: In prokaryotes,there is only one replicon,however,in eukaryotes,there are several replicons in tandem.
Statement $II$: The two separated strands in a replicon of $DNA$ are prevented from rejoining by helicase enzyme.
In the light of the above statements,choose the correct answer from the options given below.

Okazaki segments are formed during

When $DNA$ replication starts,