Chapter 11 and 12 - Nucleus and Control of Gene Expresion

Structure 2d cloverleaf structure has unusual bases

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Unformatted text preview: ture 2D cloverleaf structure has unusual bases disrupt the Hbonds and may be recognition sites for various proteins All mature tRNAs have the triplet CCA at their 3 end encoded in the tDNA (many prokaryotes) and added enzymatically, without DNA in eukaryotes 3D structure 2 double helixes, L shape The common shape of all tRNAs reflect their similar roles however each tRNA is unique They have anticodons located in the middle loop of the clover leaf, 7 nucleotides where the middle 3 are anticodon and vary amino acid binds to opposite end of the anticodon 61 different codons, therefore there should be 61 different tRNAs but because the 3rd nucleotide in the triplet is highly variable there are less than 61 tRNAs Crick proposed that the same tRNA may be abkle to recognise more than one codon wobble hypothesis It is important that each tRNA is connected to the correct amino acid cognate Amino acids covalently linked to cognate tRNA organisms typically have 20 enzyme (aminoacyl-tRNA synthetase aaRS), one for each amino acid aaRS recognises both ends of tRNA the acceptor stem (for amino acid linkage) and the anticodon aaRS is involved in a 2 step reaction 1) step 1 requires ATP forms adenylated amino acid bound to aaRS 2) step 2 is thermodynamically favourable which means it doesn t require energy transfers amino acid to 3 end of cognate tRNA If wrong amino acid is added, proofreading mechanism of the enzyme is activated Translation o o o Complex synthetic activity Requires: various tRNAs with attached amino acids, ribosomes, mRNA, numerous other proteins, cations and GTP Divided into: initiation, elongation and termination Initiation o o o o Initiation translation begins at the initiation codon AUG which then puts the ribosome in the proper reading frame mRNA does not bind to intact ribosome it first binds to the small subunit the small ribosomal subunit identifies the correct AUG codon in bacteria, Shine-Dalgarno sequence (5 to 10 nucleotides before initiation sequence) is complementary to 3 end to 16S ribosomal RNA leads to attachment of 30S ribosomal subunit to AUG initiation codon STEP 1 2 o o o several steps of initiation require initiatin factors IFs in bacteria and eIFs in eukaryotes IF1 and IF3 are needed to aid in the association of the ribosomal subunit with the mRNA at the AUG initiation codon IF2 is a GTP-binding protein required for attachment of first aminoacyl-tRNA STEP 2 o o o AUG is not just the initiation codon, but also codes for methionine always the first amino acid (in bacteria, Nformylmethionine) often removed enzymatically after the protein is made The formylmethionyl-tRNA becomes associated with the mRNA and the 30S ribosomal subunit by binding to IF2GTP IF1 and IF3 are released STEP 3 o o 50S subunit joins the complex, GTP is hydrolyzed and I F2-GDP is released First tRNA enters at site P and the rest enter at the A site Initiation In Eukaryotes o o o o o o o o o o Requires at least 12 initiation factors Several of the eIFs bind to the 40S subunit which prepares the subunit for binding to the mRNA Initiator aa-tRNA enters P site of the subunit in association with eIF2-GTP Small ribosomal unit together with associate eIFs and tRNA form the preinitiation complex (43S) The preinitiation complex finds the 5 end of the mRNA (methlguanosine cap) it is recruited to mRNA with the help of eIFs already bound to the mRNA eIF4E binds to the 5 cap of the mRNA eIF4A moves along the 5 end of the mRNA, removing double stranded re...
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This document was uploaded on 04/10/2014.

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