Replication of $DNA$ occurs in which direction?

The experimental system used in studies of the discovery of semi-conservative replication of $DNA$ was:

$DNA$ polymerase enzyme is responsible for the synthesis of

$DNA$ polymerase enzyme is required for the synthesis of .......

$DNA$ polymerase links nucleotides by forming which type of bond?

Bacteria were grown in a medium containing a heavy isotope of nitrogen $(^{15}N)$ for many generations,and all their $DNA$ contained only heavy nitrogen. $A$ bacterium of this type was transferred to a normal medium and allowed to duplicate. After two divisions,the distribution of heavy $DNA$ is likely to be: