1. M. Nirenberg and Matthaei synthesized artificial poly-U m-RNA. 

2. Using this synthetic poly-U m-RNA and cell free system of protein synthesis, a small polypeptide consisting of only amino acid phenylalanine was obtained. 

3. It suggested that UUU codes for phenylalanine. 

4. Dr. Har Gobind Khorana devised a technique for synthesis of artificial mRNA with repeated sequences of known nucleotides. 

5. He synthesized artificial RNA consisting of known repeated sequences of two or three nucleotides. E.g. CUC, UCU, CUC, UCU, by using synthetic DNA. 

6. This resulted in formation of polypeptide chain consisting of alternate amino acids leucine and serine. 

7. Synthetic RNA consisting of CUA, CUA, CUA, CUA repeats gave polypeptide chain with only one amino acid – leucine. 

8. Severo Ochoa established that the enzyme (polynucleotide phosphorylase) also helps in polymerizing RNA of defined sequences in a template-independent manner (i.e. enzymatic synthesis of RNA). 

9. Thus all the 64 codons in the dictionary of genetic code were deciphered.