A Half DNA ladder - A template for copying the Whole

Because of the obligatory pairing of adenine-to-thymine and guanine-to-cytosine, Watson and proposed that one half of the DNA ladder serves as a template for recreating the other half during DNA replication.

By 1958, two lines of evidence can be together to provide proof of this hypothesis.

First, an enzyme was discovered-DNA polymerize-that adds complementary nucleotides to the template provided by a half DNA molecule.

Second, an ingenious experiment used nitrogen isotopes to fellows the construction of new DNA molecules during successive generation of bacteria, This showed that one strand of each NA molecule is passed along unchanged to reach two daughter cells.

This "conserved " strand acts as the template for DNA polymerizes to synthesize a 2nd complementary strand, which completers each new DNA molecule.