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?
By letting the bacteria continue to grow in a 14N medium, the DNA extracted after each generation showed a progressively lower amount of hybrid DNA and a higher amount of lighter DNA. The table shows 2nd generation DNA daughter molecules that would be expected if the DNA did indeed unzip and expose complementary nucleotide bases.
Diagram taken from Original paper
from Proc Natl Acad Sci U S A. 1958 July 15; 44(7): 671–682