Biology GENETIC CODE

### KEY TOPICS

star theories of Genetic Code
star Scientists involved
star Salient features of Genetic Code

### THEORIES FOR GENETIC CODE

● 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".

### SCEINTISTS INVOLVED

● 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

### SALIENT FEATURES OF GENETIC CODE

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".