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| Genetic Code |
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| The complex process by which the information in RNA is decoded into a polypeptide is one of the exciting discoveries the genetic code. |
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| With only four biochemical letters (A, G, C, U) a one letter code could not unambiguously encode 20 amino acids. A two letter code could encode only 16 amino acids. So, a triplet code based on 3 biochemical letters or nucleotide bases could make 4 x 4 x 4 = 64 codons. This will be required to code for 20 or so different amino acids. |
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| The discovery of the genetic code became possible through the contribution of many scientists like Francis Crick, Seveno Ochoa, Maxell Ninenberg, Hargobind Khorana and J. H. Malther in the 1960s. Ninemberg and Khorana shared the Nobel Prize in 1968. |
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 The genetic code is a triplet code: Three adjacent bases, termed as codon, specify one amino acid |
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 Non-overlapping: Adjacent codons do not overlap. |
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 No punctuation: The genetic code is comma less. |
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 The genetic code is universal i.e., a given codon specifies the same amino acid in all protein synthesising organisms. |
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 The genetic code is degenerate: it lacks specificity and one amino acid often has more than one code triplet. |
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 Each codon codes for only one amino acid, none for more than one. |
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 Three of the 64 codons, names UAA, UAG and UGA do not specify any amino acid but signal the end of the message. They are called nonsense or terminator codons. |
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 The codons AUG and GUG are called the initiation or start codons as they begin the synthesis of polypeptide. |
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| fig. 22.9 Standard Genetic Code |
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| In the above figure, the sequence of nucleotides in the triplet codons of RNA is indicated; each triplet specifies a particular amino acid. |
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