During replication and transcription a nucleic acid was copied to form another nucleic acid.

Hence, these processes are easy to conceptualise on the basis of complementarity.

The process of translation requires transfer of genetic information from a polymer of nucleotides to synthesise a polymer of amino acids.

Neither does any complementarity exist between nucleotides and amino acids, nor coulds any be drawn theoretically.

There existed ample evidences though, to support the notion that change in nucleic acids (genetic material) were responsible for change in amino acids in proteins.

This led to the proposition of genetic code that could direct the sequence of amino acids during synthesis of proteins.

Historical background or contribution of various scientist:

George Gamow : In $1954$, George Gamow, a physicist proposed that in order to code for all the $20$ amino acids the code should be made up of three nucleotides (triplet code).

This can be explained as if one base coded for one amino acid, only four amino acids.

If a sequence of two bases coded for one amino acid, the four basis could specify only $16[4 \times 4]$ amino acids, which is also inadequate.

But if a sequence of three bases coded for one amino acid, the four bases would specify $64[4 \times 4 \times 4]$ amino acids.

But this generated many more codons than required.

In $1960 \mathrm{~s}$ the proof that genetic codes are infect, came from the researches of the following scientists :

$(i)$ Har Gobind Khurana : Developed a chemical method for the synthesis of $RNA$ molecule with defined base combinations (homopolymers and copolymers)

$(ii)$ Marshall Nirenberg : Put forward a cell free system for protein synthesis that helped in deciphering the code.

$(iii)$ Severo Ochoa: Showed that the polynucleotide phosphorylase also helped in polymerising $RNA$ with defined sequences in a template independent manner (enzymatic $RNA$ synthesis).