Changes in the DNA are reflected in the protein product. There is a correlation between what happens to the genetic sequence and the resulting protein sequence.
So, there must be some sense to the way the DNA nucleotides are arranged. DNA is made up of four different nucleotides: adenine (A), cytosine (C), guanine (G), and thymine (T).
James Watson and Francis Crick explain how they solved the structure of DNA. Erwin Chargaff explain how he measured the levels of each of the four nitrogenous bases.
Seymour Benzer was able to map mutations within one phage gene and correlate the mutations with protein changes. This showed that the DNA sequence of a gene was aligned with protein sequence.