Watson and Crick observed that the two strands of $DNA$ are anti-parallel and complementary to each other with respect to their base sequences. This type of arrangement in $DNA$ molecule led to the hypothesis that $DNA$ replication is semiconservative. It means that the double stranded $DNA$ molecule separates and then, each of the separated strand acts as a template for the synthesis of a new complementary strand. As a result, each $DNA$ molecule would have one parental strand and a newly synthesized daughter strand.

since only one parental strand is conserved in each daughter molecule, it is known as semi-conservative mode of replication.