“Marshall Nirenberg, Har Gobind Khorana and Robert Holley deciphered the mystery of genetic code and its role in translation. Khorana synthesized artificial DNA composed of specific sequences using combinations of 2, 3 or 4 nucleotides. He added the poly-mRNA obtained from this artificial DNA to a cell-free extract and analysed its translation by incorporation of radioactive amino acids. Comparing nucleotide sequences to the peptides produced in these reactions he deduced codon assignments of amino acids. For example, polyUC (with possible codons UCU CUC or CUC UCU) gave a mixture of poly serine and leucine while polyUUC (with possible codons UUC, UCU, CUU) gave a mixture of poly phenylalanine, serine and leucine. He discovered that in these experiments UCU and serine are common to both experiments; so UCU triplet codes for serine.
Codons are non-overlapping, so any insertion or deletion will disrupt the triplet order causing a frame shift. Substitution of nucleotides may result in missense (coding for a different amino acid) or silent (no change in amino acid) mutations. Both frame shift and substitution can result in a nonsense (stop codon-UGA, UAG or UAA) mutation. Since genetic code is degenerate organisms have a preference for a specific set of codons for each amino acid. The abundance of corresponding tRNA, aminoacyl tRNA synthetase and amino acids influence this codon bias.
Scientists are now trying to expand our genetic code by introducing novel or unnatural amino acids. For this, the tRNA is recoded so that its anticodon (the region that binds to specific codons – it has to form hydrogen bonds with the specific codon and hence is in the opposite direction) can recognise a stop codon or a four-base codon. The amino acyl tRNA synthetases are also engineered to recognise and load these modified tRNAs with unnatural amino acids such as m-acetyl phenylalanine, carboxy glutamic acid and hydroxy proline instead of the standard amino acids.”
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