Biology 3 - 1 Translation(continued-If tRNA molecules carrying the correct amino acids are the key to reading the genetic code the critical

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Translation (continued) -If tRNA molecules carrying the correct amino acids are the key to reading the genetic code, the critical question becomes: How are specific amino acids attached to specific tRNA molecules? -Translation of the genetic code requires: 1) tRNA molecules, and 2) aminoacyl-tRNA synthetases, which are enzymes that “charge” a tRNA with a particular amino acid. -The CCA terminus of the tRNA molecule is “charged” with a particular amino acid which binds to either the 3’ or 2’ Carbon of the ribose and form ester linkages, which we have to break in order to form a polypeptide chain. The term is called “charging” because amino acids are positively charged (the amino group), which adds a +1 charge to the tRNA molecule. -Different aminoacyl-tRNA synthetases recognize and charge different tRNAs and this specificity comes from the recognition of different anticodon loops and acceptor stems of tRNAs, and in some cases, additional regions. -Aminoacyl-tRNA synthetases translate the genetic code with high fidelity through two different proofreading schemes: 1) amino acid activation, in which amino acids that are two big are excluded from the activation site (so sometimes amino acids that are smaller than the right one can enter), and 2) editing at discrete sites, in which amino acids that are smaller than the right amino acid which are incorrectly placed in the activation site will be removed. -Aminoacyl-tRNA synthetases have three binding sites: 1) tRNA binding site, 2) Amino acid sites, which are specific for specific amino acids, and 3) binding site for ATP, because tRNA charging requires energy. -The charging cycle occurs in different steps: 1) a specific amino acid with an ATP molecule bind to the aminoacyl-tRNA synthetase in the two binding sites, and the two phosphate groups are removed, leaving an “activated” amino acid with a bound AMP group, 2) a tRNA specific for the specific amino acid binds to the tRNA binding site, and 3) the AMP group dissociates and the amino acid binds to the tRNA, and the charged tRNA molecule is dissociated from the aminoacyl-tRNA synthetase. -Ribosomes are required for translation and can be visible as small spheres in a eukaryotic cell’s cytoplasm. -In prokaryotes, transcription and translation are coupled, and while transcription is occurring, multiple ribosomes (polysomes) bind to the mRNA and translate it. -Ribosomes read the instructions for protein synthesis from mRNA molecules in the 5’ à 3’ direction (so that translation can occur shortly after transcription) and the amino acid chain is polymerized from the N- terminus to the C-terminus. -In E. Coli, ribosomes contain two subunits, the large 50S subunit and the small 30S subunit. They also contain three aminoacyl-tRNA sites: 1) E (Exit) site, in which a tRNA that has been added to the A site and had its amino acid polymerized in the growing polypeptide chain exits, 2) P (Peptydil) site, in which a newly synthesized polypeptide chain is attached to the Peptidyl tRNA, and 3) A (Aminoacyl) site, where
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This note was uploaded on 04/30/2008 for the course BIO 2960 taught by Professor Staff during the Spring '08 term at Washington University in St. Louis.

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Biology 3 - 1 Translation(continued-If tRNA molecules carrying the correct amino acids are the key to reading the genetic code the critical

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