Aside from elucidating DNA’s structure on April 25, 1953 in Nature, Watson and Crick also wrote,

If the sequence of bases on one DNA chain is given, then the sequence on the other chain is automatically determined.

They realized that this is how genetic messages are copied with so much fidelity when chromosomes duplicate during cell division. The DNA molecule unzips to form two separate strands, each of which serves as a template for the synthesis of a new strand.
But as obvious as the mechanism seemed to the two theorists, the evidence only came in 1957-58 from the experiments of Meselson and Stahl. How did they use radioactive isotopes in an experiment that supported the idea that DNA unzips and doubles each strand?

They used a centrifuging technique that allowed them to separate DNA containing a heavier isotope of nitrogen(¹⁵N) from DNA assembled with a lighter one.(¹⁴N). In a technique that reminds us of a recent 5-chlorouracil experiment ( see R.Oiley’s article in science 2.0) they grewE.coli bacteria for many generations in a special medium. Using ammonium chloride(NH₄Cl)tagged with ¹⁵N  they ensured that each DNA strand would only have bases containing the heavier isotope . They then switched to a medium that had only the lighter isotope and also let it replicate for one generation. After extracting DNA from the bacteria, they observed that the DNA was a hybrid containing both isotopes. I’ve used a kind of Punnett square to reveal the possible combinations: