mid 25 - Prokaryotic Translation Chapter 4; pp 119-131 Most...

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Prokaryotic Translation Chapter 4; pp 119-131
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Aminoacyl-tRNA Synthetases: Twenty Flavors • Most cells make 20 different aminoacyl-tRNA synthetases, for each type of amino acid. • For instance, serine has six different codons that specify serine, so seryl-tRNA synthetase must recognize six tRNA molecules with six different anticodons which are different from one another. How do you think these enzymes recognize their correct tRNAs? • Many of these enzymes recognize their tRNA molecules using the anticodon. BUT? • So, tRNA molecules are also recognized using segments on the acceptor end and bases elsewhere in the molecule, particularly nucleotide 73 (discriminator base). acceptor end
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Aminoacyl-tRNA Synthetase (ARS) •Each aminoacyl tRNA synthetase recognizes one particular amino acid and all tRNAs that recognize codons for that aa •An amino acid is covalently linked to tRNAs at the 2’ or 3’ OH by aminoacyl-tRNA synthetases (an ATP requiring reaction). • ARS can proofread and remove incorrect aa; < 1/5000 error • How does an aminoacyl-tRNA synthetase work?
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Experiment: tRNA specifies the amino acid incorporation • Fritz and Lipmann chemically altered an amino acid after it had already been attached to its specific tRNA • Found that alanine is incorporated into sites normally occupied by cysteine residues • Amino acids are “illiterate.” Question revisited: Amino acid or tRNA reads the mRNA codons?
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Nonstandard Base Pairing Often Occurs Between Codons and Anticodons • If perfect Watson-Crick base pairing was needed between codons and anticodons, cells would have to contain exactly 61 different tRNA species. However many cells contain fewer than 61 tRNAs. • 30-40 different tRNAs have been identified in bacterial cells. • Explanation: A single tRNA anticodon is able to recognize more than one codon corresponding to a given amino acid. How can 61 different codons be read when cells have less than 61 different tRNAs? • This broader recognition can occur because of nonstandard pairing between bases in the “wobble” position: the 3rd base (3’) in an mRNA codon and the corresponding first base (5’) in the tRNA anticodon. How is this possible?
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Wobble at the third position • One tRNA can recognize more than one codon - wobble at the third position • Very strict Watson-Crick pairing for the first two positions; more flexible at the third position
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• Crick established certain “wobble rules” that dictate which nucleotides can and cannot form new hydrogen-bonded associations through wobble. Ionsine is one of the rare bases found in tRNA, often in the anticodon. Wobble at the third position Codon - Anticodon Pairings Allowed by the Wobble Rules 5’ end of anticodon (1 st ) 3’end of codon (3 rd ) G U or C C G only A U only U A or G I U, C, or A
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1. Mismatch : U in the wobble position (position 1, or the most 5’ end) in the anticodon of tRNA can base-pair with A or G in the wobble codon (position 3) of mRNA. G in the
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mid 25 - Prokaryotic Translation Chapter 4; pp 119-131 Most...

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