(a)
(i) According to the one of the first models proposed for the genetic code by physicist George Gamow, each amino acid in a polypeptide was encoded by three sequential nucleotides. The code words, or codons, for amino acids were nucleotide triplets.
(ii) In 1968, the Nobel Prize in Physiology or Medicine was awarded jointly to Robert W. Holley, Har Gobind Khorana and Marshall W. Nirenberg for their work on the interpretation of the genetic code and its function in protein synthesis, showing how the order of nucleotides in nucleic acids, which carry the genetic code of the cell, encode the proteins synthesized by the cell . They established the biological language or genetic code common to all living organisms, is spelled out in three-letter words: each set of three nucleotides codes for a specific amino acid.
(iii) Nirenberg and Mathaei used a synthetic poly (U) RNA and deciphered the genetic code by translating this as polyphenylalanine.
(iv) Over the course of several years, Marshall Nirenberg, Har Khorana and Severo Ochoa and their colleagues elucidated the genetic code - showing how nucleic acids with their 4-letter alphabet determine the order of the 20 kinds of amino acids in proteins. In 1959, Nobel Prize for the discovery of RNA polymerase was given to Ochoa.
(b) The genetic code is the set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins (amino acid sequences) by living cells.
• They are unambiguous means that one codon specifies only one amino acid and not any other.
• They are universal means that the genetic code is applicable universally, i.e., a codon specifies the same amino acid from a virus to a tree or human being.
• They are degenerate means some amino acids often have more than one codon. Only methionine and tryptophan have singlet triplet codons. All other amino acids are specified by two to six codons.
