Answer:
Because each strand acts as a template for the synthesis of a new, complementary strand.
Explanation:
This is known as semiconservative DNA replication. Leading and lagging strands are complementary, antiparallel strands (one is 3'-5' while other is 5'-3- direction) which are replicated differently. The leading strand is replicated  continuously while the lagging strand is replicated in fragments-Okazaki fragments. Replication of both strands is performed by DNA polymerase.
