Spring 09 lecture 30

Spring 09 lecture 30 - Translation Translational machinery...

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11 Translation Learning Objectives Review Explain how wobble base pairing increases the efficiency of protein synthesis Describe the process of tRNA charging Compare the RNA and protein composition of prokaryote and eukaryote ribosomes Translational machinery
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22 Translation Translation – processing of mRNA into a polypeptide/protein mediated by ribosomes RNA carries out instructions encoded by DNA mRNA - carries genetic information from DNA in the form of “codons” tRNA - translates mRNA code. Each amino acid has its own tRNA. rRNA - associates with proteins to form ribosomes which catalyze the assembly of protein chains
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33 Review 4 nucleotides If 1 nucleotide was used to code for one AA · 41=4 combination Possible combinations of three nucleotides used to code one AA 43=64 64 code words can be formed – more then enough for 20 essential AA This triplet code - CODON 61 codons specify individual AA, 3 are stop codons Many AA - more then one codon – synonymous codons Code is degenerate
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44 ORF (=CDS): Precise linear array of ribonucleotides in group of three in mRNA from start to stop codon It specifies precise linear sequence of amino acids and signals where protein synthesis starts and stops Overlapping information in some sequences - 2nd reading frame Start codon AUG - Methionine CUG - in some Bacterial mRNA (for first Methionine) CUG -in eukaryotes (for Methionine-very rarely) Stop codons UAA-Ochre UGA-Opal UAG-Amber
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55 1961- Gobind Khorana and Marshall Nirenberg Nobel Prize for genetic code in 1968 Experiments to break genetic code: Cell free extracts from bacteria, contains : tRNAs for all AA, ribosomes, energy source, AAs Added synthetic mRNA:
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66 Next- synthetic mRNA with alternating bases: ACACACACACAC Next- synthetic mRNA : AACAACAACAACAAC Stimulated synthesis of three kinds of polypeptide chains: either or or The only codon in common between two synthetic mRNAs was ACA, the only AA in common was Thr T ACA is codon for Thr and T CAC is codon for His…
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77 Second set of experiments: -20 cell free extracts with all 20 charged tRNAs -in each sample different AA was radioactively labeled -instead of synthetic mRNA - trinucleotides (64 combinations) were added: different trinucleotides added into each sample - trinucleotide is enough to attract the appropriate charged tRNA and bind it to ribosome -samples filtered through nitrocellulose filters
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88 UUU codes for Phe
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Spring 09 lecture 30 - Translation Translational machinery...

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