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

Replication was allowed to take place in the presence of radioactive deoxynucleotide precursors in $E. coli$ that was a mutant for $DNA$ ligase. Newly synthesised radioactive $DNA$ was purified and strands were separated by denaturation. These were centrifuged using density gradient centrifugation. Which of the following would be a correct result?

$DNA$ multiplication is called:

$DNA$ repairing is done by

Discuss the significance of the heavy isotope of nitrogen in the Meselson and Stahl experiment.

Briefly describe the scheme of $DNA$ replication proposed by Watson and Crick.