● During `color{Violet}"replication and transcription"` a nucleic acid was copied to form another nucleic acid.

● Hence, these processes are `color{Violet}"easy to conceptualise"` on the basis of `color{Violet}"complementarity"`.

● The process of `color{Violet}"translatio"`n requires transfer of genetic information from a polymer of `color{Violet}"nucleotides"` to a polymer of `color{Violet}"amino acids"`.

● Neither does any `color{Violet}"complementarity exist"` between nucleotides and amino acids, nor could any be drawn `color{Violet}"theoretically"`.

● There existed `color{Violet}"ample evidences"`, though, to support the notion that `color{Violet}"change in nucleic acids"` (genetic material) were responsible for `color{Violet}"change in amino acids"` in proteins.

● This led to the proposition of a `color{Violet}"genetic code"` that could `color{Violet}"direct the sequence"` of `color{Violet}"amino acids"` during synthesis of proteins.

● If determining the `color{Violet}"biochemical nature"` of genetic material and the structure of DNA was very exciting, the proposition and `color{Violet}"deciphering of genetic code"` were most challenging.

● In a very true sense, it required involvement of scientists from several disciplines – `color{Violet}"physicists"`, `color{Violet}"organic chemists"`, `color{Violet}"biochemists"` and `color{Violet}"geneticists"`.

● It was `color{brown}"George Gamow"`, a physicist who argued that since there are only `color{violet}"4 bases"` and if they have to code for `color{violet}"20 amino acids"`, the code should constitute a `color{violet}"combination of bases"`.

● He suggested that in order to code for all the 20 amino acids, the code should be made up of `color{violet}"three nucleotides"`.

● This was a very bold proposition, because a `color{violet}"permutation combination"` of `4^3` (4 × 4 × 4) would generate `color{violet}"64 codons"`; generating many more codons than required.

● Providing proof that the `color{violet}"codon was a triplet"`, was a more daunting task.



● The `color{violet}"chemical method"` developed by `color{brown}"Har Gobind Khorana"` was instrumental in synthesising RNA molecules with `color{violet}"defined combinations"` of bases (homopolymers and copolymers).



● `color{brown}"Marshall Nirenberg’s"` `color{violet}"cell-free system"` for protein synthesis finally helped the code to be deciphered.

● Several `color{violet}"Ochoa enzyme"` (`color{violet}"polynucleotide phosphorylase"`) was also helpful in polymerising RNA with defined sequences in a template independent manner (enzymatic synthesis of RNA).



● Finally a `color{violet}"checker-board"` for genetic code was prepared

`star` The `color{Brown}"salient features of genetic code"` are as follows:

● The codon is `color{violet}"triplet"`. `color{violet}"61"` codons code for `color{violet}"amino acids"` and `color{violet}"3"` codons do not code for any amino acids, hence they function as `color{violet}"stop codons"`.

● `color{violet}"One codon"` codes for only one amino acid, hence, it is `color{violet}"unambiguous"` and `color{violet}"specific"`.

● Some amino acids are coded by `color{violet}"more than one codon"`, hence the code is `color{violet}"degenerate"`.

● The codon is read in mRNA in a `color{violet}"contiguous fashion"`. There are `color{violet}"no punctuations"`.

● The code is `color{violet}"nearly universal"`: for example, from bacteria to human `color{violet}"UUU"` would code for `color{violet}"Phenylalanine"` (phe).
`star` Some `color{violet}"exceptions"` to this rule have been found in `color{violet}"mitochondrial codons"`, and in some protozoans.

● `color{violet}"AUG""` has dual functions. It codes for `color{violet}"Methionine"` (met), and it also act as `color{violet}"initiator codon"`.