Each codon occurring in tRNA specifies one amino acid. Also in 1961 Nirenberg and Matthaei mixed, in vitro, ribosomes (separated from sonically disrupted cells by high speed differential centrifugation) with a suspension of cytoplasmic material from the same sort of cells.
The mixture contained, among other materials, enzymes, amino acids, tRNA, ribosomes and energy- transfer coenzymes such as ATP and GTP. Here were found all, except one, of the many elements necessary for the synthesis of polypeptides. The one missing ingredient was mRNA.
Now Nirenberg and Matthaei had synthesized a simple form of mRNA in which the entire chain was made up of only one kind of nucleotide, uridylic acid (U). This historic polynucleotide is now often called “poly-U.” Be it noted that, although in RNA uracil replaces thymine, uracil and thymine and equivalent in respect to pairing.
When U was added to the cellular “puree” just described, the cellular elements in the puree cooperated as they do in a living cell to form a polypeptide chain.
But since there was only one kind of triplet or codon, UUU in the synthetic mRNA, the polypeptide formed consisted of only one kind of amino acid, phenylalanine.
It was thus revealed that the codon UUU meant phenylalanine; the first genetic code word ever to be deciphered! Of course, many eager researchers were at work deciphering other code words the huge mass of data resulting from these studies fills many long shelves in libraries and carries the names of scores of brilliant scientists.
Suffice it to say here that, in the same or similar ways, codons have been found since then for all the amino acids that occur in natural proteins. AAA means lysine, AGA means arginine, UUU means phenylalanine, and UUA means leucine, and so on.
It was found also that several acids have more than one codon, for example, leucine is specified by CUC, CUU, UUA and UUG. The code is therefore said to be degenerate in that each amino acid is not restricted to only one codon. This is true in vitro, but the code appears not to act degenerately in vivo.