The codons in DNA specifying amino acids in proteins. 

Demonstration of the existence of “messenger” RNA. The following paper (pp. 581-85) by F. Gros et al. is also relevant.

Order of authorship in the original publication: Sarabhai, Stretton, Brenner, Bolle. Brenner (Nobel Prize 2002) and colleagues performed the first study to show co-linearity; i.e., that there is a simple congruence between the amino acid sequence of a protein and the nucleotide sequence of the gene determining that protein. Brenner and his team were working with "nonsense" mutants, called amber mutants, that terminated protein synthesis in E. coli genes.

            "The presence of random nonsense mutants would therefore yield multiple random protein fragments of different sizes. . . . [W]e now suddenly realized that since we had all these amber mutants in this gene we could give a topological proof of co-linearity. And we wouldn't have to do any protein sequencing! The only assumption we would have to make is that the protein is always read from the same end, which seemed a very reasonable one. So in 1964 we published a paper which proved that the gene and the protein were co-linear by an argument that was totally unexpected at that time. We showed that as the amber mutations moved further and further to the right in the position of the gene, we got progressively more and more of the protein made. (Brenner, My Life, p. 103). 

Brenner established  "Caenorhabditis elegans as a model organism for the investigation of animal development including neural development. Brenner chose this 1-millimeter-long soil roundworm mainly because it is simple, is easy to grow in bulk populations, and turned out to be quite convenient for genetic analysis. One of the key methods for identifying important function genes was the screen for roundworms that had some functional defect, such as being uncoordinated, leading to the identification of new sets of proteins, such as the set of UNC proteins. For this work, he shared the 2002 Nobel Prize in Physiology or Medicine with H. Robert Horvitz and John Sulston. The title of his Nobel lecture on December 2002, "Nature's Gift to Science," is a homage to this modest nematode; in it, he considered that having chosen the right organism turned out to be as important as having addressed the right problems to work on" (Wikipedia article on Sydney Brenner, accessed 03-2018). Digital facsimile from PubMedCentral at this link.

Digital facsimile from PubMedCentral at this link.