What are the contributions of George Gamow, H.G. Khorana, Marshall Nirenberg in deciphering the genetic code ?

Explanation

George Ganow, suggested that, in order to code for all the 20 Amino acids, the code should be made up of three nucleotides. This was a very bold proposition, because a permutation and combination of `4^(3)` would generate 64 codons, generating more codons than required.
H.G. Khorana developed chemical method in synthesising RNA molecules with defined combinations of bases.
Marshall Nirenberg.s cell - free system for protein synthesis finally helped the code to be deciphered. The enzyme polynuceotide phosphorylase was also helpful in polymerising RNA with defined sequences in a template independent manner (enzymatic synthesis of RNA).