Ch_8_Translation - TRANSLATION OF mRNA mRNA's read from 5...

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TRANSLATION OF mRNA mRNA’s read from 5’ to 3’ direction polypeptide chains synthesized from amino to carboxy terminus. o Starts with methionine Translation carried out on ribosomes (in cytosol) o tRNAs serve as adapters b/t mRNA template and amino acids to become protein tRNA’s 70-80 nucleotides long Cloverleaf structure resulting from complementary base pairing Compact L-shapes for tRNAs to fit onto ribosomes CCA sequence at 3’ terminus Template recognized by anticodon loop Aminoacyl tRNA synthetases- “charger proteins”-enzymes that mediate attachment of amino acids to specific tRNA’s o Each enzyme recognizes a single amino acid and the correct tRNA(s) to attach. o Reaction 2 steps: amino acid activated by rxn with ATP to form aminoacyl AMP synthetase intermediate. Activated AA joined to 3’ terminus of tRNA, AMP released. Both steps catalyzed by aminoacyl tRNA synthetases o Very selective in recognizing tRNA’s and AA. o Codon-anticodon base pairing less stringent than standard A-U/G-C pairing b/c of redundancy 61 codons (3 stop codons:UAA, UAG, UGA) code for amino acids Most amino acids specified by more than one codon. o Some tRNA’s able to recognize more than one codon. Wobble- leniency in 3 rd position (5’ on the anticodon on tRNA) Ribosomes Site of protein synthesis 80S for eukaryotes. Subunits: 40S, 60S created separately o 40S composed of 18S RNA and 30 proteins o 60S composed of 28S, 5.8S. 5S rRNAs and 45 proteins. Ribosomes can be formed by self-assembly of RNA and protein parts o rRNAs required for in-vitro assembly of functional ribosomes Fundamental rxn of protein synthesis catalyzed by ribosomal RNA. (rRNA) Proteins play structural role Translation Starts at specific intiation sites
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5’ and 3’ ends have noncoding regions (5’/3’ untranslated regions-UTR) Polycistronic mRNAs code for multiple polypeptides Monocistronic mRNAs code for a singe polypeptide. Translation initiates with methionine- AUG o Alternative start codons also used- GUG (still encodes for methionine though) In eukaryotes: o 40S Ribosome binds to 7-methylguanosine cap at 5’ terminus- Cap associated proteins o then scans downstream of the cap until reaching AUG intiation codon. Scanning requires ATP Translation divided into 3 stages: 1. Initiation o Small subunit--tRNA-- Large subunit o Binding of intiator methionyl tRNA and mRNA to small ribosomal subunit. o Large ribosomal subunit joins complex, forming functional ribosome. In eukaryotes, 12 proteins needed for intiation. o eIF-1A and 3 bind to 40S ribosomal subunit o eIF-2(complexed to GTP) brings initiator methionyl tRNA to ribosome and eIF-1.
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This note was uploaded on 03/19/2008 for the course MMG 409 taught by Professor Arvidson during the Spring '08 term at Michigan State University.

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Ch_8_Translation - TRANSLATION OF mRNA mRNA's read from 5...

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