Describe the experimental proof for the semi-conservative mode of $DNA$ replication.

(N/A) The experimental proof for the semi-conservative mode of $DNA$ replication was first provided by Matthew Meselson and Franklin Stahl in $1958$ using Escherichia coli $(E. coli)$.
$(i)$ They grew $E. coli$ in a medium containing $^{15}NH_4Cl$ ($^{15}N$ is the heavy isotope of nitrogen) as the only nitrogen source for many generations. As a result,$^{15}N$ was incorporated into the newly synthesized $DNA$ and other nitrogen-containing compounds.
This heavy $DNA$ molecule could be distinguished from normal $DNA$ by centrifugation in a cesium chloride $(CsCl)$ density gradient. ($^{15}N$ is not a radioactive isotope; it can be separated from $^{14}N$ only based on density differences.)
$(ii)$ Then,they transferred the cells into a medium with normal $^{14}NH_4Cl$ and took samples at various definite time intervals as the cells multiplied. The extracted $DNA$ was centrifuged and measured for density using $CsCl$ gradients.
In the centrifuge,molecules with higher mass density sediment faster than those with lower density,allowing for the separation of hybrid,heavy,and light $DNA$ molecules.