The genetic code is a map of the correspondence between codons on mRNA and amino acids on proteins, this correspondence is a function of complementary base pairing between codons on mRNA and bases on the anticodon loop on tRNA, coupled with the binding of specific amino acids to the amino acid acceptor arm of tRNA, the fact that scientists were able to, with painstaking experimentation, decipher what on mRNA corresponded to which amino acid at the end of tRNA was crucial to the advent of molecular biology and its offspring, biotechnology. It enabled us to sequence proteins and produce those proteins by synthesizing DNA which made mRNA which made that specific protein, it enabled us to sequence DNA and mRNA and to predict , and to synthesize, the protein products that would result from the translation of the mRNA in question, in short, it became a foundation for much of genetic engineering.
The experiments themselves were beautiful, at least as much as beautiful can be in the world of biogeekdom…
Brenner and Crick had demonstrated using frameshift mutations that the code was triplet, so when the experiments that are described below were undertaken that was already known.
The method I shall elaborate on here is also known as the filter-assay method, and since a picture is worth a thousand words, let me introduce one here…
Note that they had a simpler assay too, and the image is self explanatory in this case…
Going back to the filter assay method, which is the one that I intend to explain, the experiment was carried out as follows,
 They chemically synthesized mRNA codons/triplets.
 They mixed mRNA triplets, aminoacyl tRNA and ribosomes.
 It so happens that complementary mRNA and tRNA bind to each other, and all the elements in  form a complex which includes the amino acid bound to the tRNA that is bound to the mRNA.
 These complexes are large and do not pass through a nitrocellulose filter, but unbound aminoacyl tRNA does.
 By identifying which amino acid was retained on the filter sheet, it was possible to identify which amino acid the originally synthesized codon corresponded to.
This is how one of the major breakthroughs in the history of molecular biology came to be.
All of this is explained in more detail in this Nature Scitable article http://www.nature.com/scitable/topicpage/nucleic-acids-to-amino-acids-dna-specifies-935 , I recommend Scitable as a resource for learning biology, by the way 😉
You can find an autobiographical account of the quest to decipher the genetic code by Marshall Nirenberg here
That is it until the next post.