Chapter 12 7th edition.ppt - Translation Overall process...

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Translation: Overall process Site of protein synthesis = ribosome mRNA & tRNA bound to ribosome Insure proper order of AAs in polypeptide chain AAs must first be activated … involves tRNA Aminoacyl-tRNA synthetases Product = AA—covalent bond—tRNA (aminoacyl-tRNA)
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Translation: 3 steps Step 1 – chain initiation 1 st aminoacyl-tRNA bound to mRNA at site of polypeptide synthesis Aminoacyl-tRNA forms complex with ribosome and mRNA Binding sites for 1 st and 2 nd aminoacyl-tRNAs brought together Step 2 – chain elongation Peptide bond(s) formed Step 3 – chain termination
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The genetic code a “triplet, nonoverlapping, commaless, degenerate, universal code” Triplet Sequence of 3 bases (codon) specifies an AA 64 possible codons (4 3 ) Nonoverlapping Bases are not shared between consecutive codons mRNA moves along 3 bases at a time
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The genetic code a “triplet, nonoverlapping, commaless, degenerate, universal code” Commaless No intervening bases exist between codons Degenerate More than one triplet can encode the same AA 64 codons = 61 code for 20 AAs & 3 code stop signals Trp & Met have only 1 codon each, while Leu, Arg, Ser each have 6 Acts as a buffer against deleterious mutations For 8 AAs (L, V, S, P, T, A, G, R), the 3 rd base is irrelevant Universal All organisms Some exceptions Mitochondria
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Codon—anticodon pairing and wobble mRNA codon pairs with tRNA anticodon Many tRNAs can recognize more than 1 codon Due to variations in H bonding These variations are allowed when wobble position is I, G, U Variations not allowed with A or C Base in wobble position of anticodon can base-pair with several bases in codon (not just A-T, G-C) U can pair with A or G G can pair with C or U *
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