Chapter 11 and 12 - Nucleus and Control of Gene Expresion

Double stranded regions eif4g links the 5 capped end

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Unformatted text preview: gions eIF4G links the 5 capped end to the polyadenylated 3 end once 43S binds 5 end of mRNA, the 43S complex scans the mRNA until it reaches a recognizable sequence usually AUG once AUG is recognized, eIF2-GTP is hydrolyzed and eIF2-GDP (and other eIFs) are released and the large ribosomal subunit (60S) joins the complex The Ribosome o o o o o o o they are molecular machines during translation a ribosome undergoes a cycle of mechanical changes that are driven with energy released by GTP hydrolysis ribosomal RNAs play a role in selecting tRNAs, ensuring accurate translation, binding protein factors, polymerizing amino acids 1990s major advances in crystallization of ribosomes and reports of Xray crystallographic structure of prokaryotic structure Each ribosome has 3 sites for association with tRNA: A site (aminoacyl), P site (peptidyl) and E site (exit) tRNAs move from A to P to E site in successive steps in the elongation cycle tRNAs bind within sites and span the gap between the small and large ribosomal subunits anticodon ends contact with small subunit, amino acid ends contact with large subunit 3 Translation Elongation o series of repeated steps as amino acids are polymerized into peptide chain STEP 1: aminoacyl tRNA selection o o o initiator tRNA is in P site, ribosome is available for entry of second aa-tRNA into A site before the 2nd aatRNA can bind to exposed mRNA in A site: it must combine with protein elongation factor called EFTu (eEF1alpha in eukaryotes) only aa-tRHNA-Tu-GTP whose anticodon is complementary to mRNA in the A site will trigger conformational change once aa-tRNA-Tu-GTP bound to mRNA, codon GTP is hydrolyzed and Tu-GDP is released STEP 2: Peptide bond formation o o o o o at the end of step 1, 2 amino acids are next to each other peptide bond forms between these 2 amino acids tRNA in A site now has attached dipeptide (nascent polypeptide chain goes from P site to A site) tRNA in P site is deacylated process catalyzed by peptidyl transferase which is part of the large subunit STEP 3: Translocation o o o o o tRNA of P site now has no linked amino acid ribosome moves ratchet-like to the next codon dipeptidyl-tRNA moves from A site to P site deacylated tRNA moves from P site to E site translocation is driven by conformational change in an elongation factor EF-G in bacteria and eEF2 in eukaryotes STEP 4: Releasing tRNA o o o deacylated tRNA leaves the ribosome, emptying the E site new aminoacyl-tRNA enters the A site after hydrolysis of GTP, EF-G-GDP leaves the ribosome Elongation o o o o for each cycle at least 2 molecules of GTP are hydrolyzed ribosome moves 3 nucleotides at a time (one codon) along the mRNA reading frame is fixed at the time the ribosomes bind to the intiation codon some of the most destructive mutations are frameshift mutations ribosome starts reading at the wrong nucleotide and reads wrong codons Termination o o termination occurs at one of the 3 stop codons: UAA, UAG and UGA no tRNAs exist with complementary anticodons termination requires release factors, which recognize the stop codons 4 o o o o o o o o there are 2 groups of release factors, class I RFs and class II RFs Class II RFs are GTP binding and not well understood Bacteria have 2 class I RFs (RF1- UAA and UAG and RF2- UAA and UGA) Eukaryotes have 1 class I RF eRF1 Class I RFs enter the A site of the ribosome The ester bond linking the nascent polypeptide to the tRNA is hydrolyzed Complete polypeptide is then released at this...
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This document was uploaded on 04/10/2014.

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