Cracking Genetic Code

In the early 1960s geneticists were racing to crack the genetic code. More specifically, they were trying to determine what sequences of RNA coded for what amino acids.
Nirenberg and Matthaei were the first to make it to the finish line. In 1961 Nirenberg and his colleagues completed the first sequencing of the code. Their procedure is as follows:
First, a homopolymer, in this case, poly(U) mRNA (a chain of Uracils)
was added to a test tube containing a cell-free translation system, 1 radioactively labeled amino acid, and 19 unlabeled amino acids.
The test tube was incubated at 37 degrees Celsius
Translation, the process of translating the sequence of an mRNA molecule to a sequence of amino acids during protein synthesis, took place
The protein was filtered, and the filter was checked for radioactivity
The procedure was repeated in 20 tubes, with each tube containing a different labeled amino acid
The tube in which the protein was radioactively labeled contained newly synthesized protein with the amino acid specified by the homopolymer. In this case UUU specified for the amino acid phenylalanine.
If the protein is radioactive, then the labeled amino acid must be in the protein and the codon codes for that amino acid. In other experiments, AAA encoded lysine, and CCC encoded proline. However, there were a few problems that needed to still be resolved. The first method only specified amino acids produced from sequenced codons of the same three letters. Codons GGG did not work.
In 1964 Nirenberg and Leder finished the puzzle by conducting new experiments that utilized tRNAs to complete the universal genetic code.
First, very short mRNAs with known codons were synthesized and added to a mixture of ribosomes and tRNAs attached to amino acids. The ribosome bound the mRNA and the tRNAs specified by the mRNA
The mixture was then passed through a nitrocellulose filter. The tRNAs paired with ribosome-bound mRNA stuck to the filter, whereas unbound tRNAs passed through it.
The filter was assayed to determine which amino acid was bound. When an mRNA with GUU was added, the tRNAs on the filter were bound to valine; therefore, the codon GUU specifies valine. Many other codons were determined by using this method
This method identified 61 codons
The coding dictionary codes for 64 possible codon triplets and 20 amino acids, mRNA strands read from 5’ to 3’, and there are specific start and stop codons