Lecture 13

Lecture 13 - LS3 MOLECULAR BIOLOGY LECTURE 12 Overview...

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Unformatted text preview: LS3 MOLECULAR BIOLOGY LECTURE 12 Overview TRANSLATION I: GENETIC CODE AND TRANSFER RNA (tRNA) Dr. Randolph Wall Reading: Watson et al., 411-423, 461-477 Three Possible Open Reading Frames (ORF) of Triplet Codons in mRNA Translation in both prokaryotes and eukaryotes initiates with an AUG which encodes methionine. This is the START of translation, though additional signals are also required. ORF = Open Reading Frame 1. Codons in mRNA are read 5 --> 3 2. Codons are non-overlapping and there are no gaps between codons in mRNA 3. mRNA is translated in a fixed reading frame set by the initiation or start codon (e.g., AUG) Three Rules Govern the Genetic Code RBS, Ribosome Binding site 3-dimensional tRNA structures differ significantly in only 2 places: -the anticodon loop -the 3 acceptor stem C C A Charging of tRNA Catalyzed by tRNA synthetases - amino acids attached to 3-CCA-0H end present on all tRNAs Two-step reaction requiring ATP 1. ATP adenylylation -> activation of amino acid 2. transfer of amino acid to CCA-0H of tRNA One tRNA synthetase for each amino acid (20 synthetases) Each synthetase charges all tRNAs that carry the same amino acid (called isoaccepting tRNAs, have different anticodons) Accuracy of protein synthesis is absolutely dependent upon tRNA synthetase recognition of specific amino acid and its charging of correct tRNA - no proof reading is carried out by ribosome tRNA structures recognized by synthetases: acceptor stem (major determinant), discriminator base (4th base from 3 end of CCA acceptor stem), anticodon loop Wobble pairing allows a single tRNA to read more than one codon Pairing Combinations with the Wobble Concept 5 Base in tRNA Anticodon G C A U I 3 Base in mRNA Codon U or C G U A or G A, U, or C LS3 MOLECULAR BIOLOGY LECTURE 13 May 13, 2008 TRANSLATION II: Ribosomes and Protein Synthesis Dr. Randolph Wall Reading: Watson et al. 423-453, 458-459 Overview of the Processes in Translation: The ribosome cycle (M. Nirenberg, 1961) mRNA E. coli cell free extract labeled amino acids ATP, GTP labeled proteins associated with large particles - Ribosomes! In vitro Translation System Ribosomes: Protein Synthesis Machines Cell free extract was fractionated using rate zonal centrifugation. Prokaryotic ribosomes made up of two subunits: 50S and 30S Prokaryotic and Eukaryotic Ribosomes Functions of ribosomal subunits: Small subunit mediates interaction between mRNA codons and tRNA anticodons Large subunit catalyzes peptide bond formation binding sites for G-proteins that assist in initiation, elongation and termination...
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This note was uploaded on 07/02/2008 for the course LS 3 taught by Professor Lin during the Spring '06 term at UCLA.

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Lecture 13 - LS3 MOLECULAR BIOLOGY LECTURE 12 Overview...

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