Which is the main enzyme for $DNA$ replication?

Which organism was used as experimental material to study $DNA$ replication?

Match the columns.

Column - $I$	Column - $II$
$(a)$ Helicase	$(1)$ Joining of nucleotides
$(b)$ Gyrase	$(2)$ Opening of $DNA$
$(c)$ Primase	$(3)$ Unwinding of $DNA$
$(d)$ $DNA$ Polymerase $III$	$(4)$ $RNA$ Priming

The experimental proof for semiconservative replication of $DNA$ was first shown in a:

Who used heavy nitrogen $N^{15}$ for proving the semiconservative way of $DNA$ replication?

If a normal $DNA$ molecule replicates in an $N^{15}$ medium for consecutive generations,what is the percentage of light $DNA$ in the $4^{th}$ generation (in $\%$)?