In addition to the double helical structure of $DNA$, Watson and Crick also proposed a scheme of $DNA$ replication.

To quote their original statement that is as follows :

$(i)$ It has not escaped our notice that the specific pairing copying mechanism for the genetic material (Watson and Crick, $1953$).

$(ii)$ The scheme suggested that the two strands would separately and act as template for the synthesis of new complementary strands.

$(iii)$ After the completion of replication, each $DNA$ molecule would have one parental and one newly synthesise strand.

$(iv)$ This scheme was termed as semiconservative $DNA$ replication.

It is proven that $DNA$ replicates semiconservatively.

It was shown first in Escherichia coli and subsequently in higher organisms, such as plants and human cells.

Mathew Meselson and Franklin Stahl performed the following experiment in $1958:$

$(i)$ They grew E. coli in a medium containing ${ }^{15} \mathrm{NH}_{4} \mathrm{Cl}\left[{ }^{15} \mathrm{~N}\right.$ is the heavy isotope of nitrogen] as the only nitrogen source for many generations.

The result was that ${ }^{15} \mathrm{~N}$ was incorporated into newly synthesised $DNA$ (as well as other nitrogen containing compounds).