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BIOL-L 211: Translation
Translation |
production of a protein from the info in an mRNA
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Transfer RNA (tRNA)
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read the mRNA by base-pairing anticodon with codon; translate mRNA into protein; carry specific aa at the 3' end
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Aminoacyl-tRNA synthetase
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Attach aas to tRNAs
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Ribosome
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carry out translation; 1/3 protein and 2/3 RNA; move 5'-3' along mRNA
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Small subunit of ribosome
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Deciphers mRNA
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Large subunit of ribosome
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Mediates chemical bond formations
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Translation factors
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Often called GTPases; associate with ribosomes and help with translation
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4 stages of translation
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Initiation, elongation, termination, ribosome recycling
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tRNA structure
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4 regions of dsRNA with 3 stem-loops
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Acceptor stem
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5' and 3' ends of tRNA base-pair; conserved 3' CCA tail that binds the aa
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Anticodon loop
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3 nucleotides that base pair with the 3 nucleotides of the codon in mRNA
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Triplet code
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Each codon specifies a single aa (sense) or no aa (stop/ nonsense); simplest code that can specify all 20 aas; some aas encoded by more than one codon
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Isoacceptors
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Different tRNAs that carry the same aa
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Reading the 3 mRNA positions
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First 2 are read by strict Watson-Crick base pairing with positions 2 and 3 of the anticodon; 3rd interacts with position 1 of the anticodon via wobble pairing
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Genetic code
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Almost the same in all organisms (AUG-methionine; UAA, UAG, and UGA-stop); mutations that change the aa encoded usually result in a similar aa taking its place |
Aminoacylation process
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2 steps requiring ATP that attach aas to tRNAa; fewer than 1 error per 104 aminoacylation events |
Identity elements
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sequence and structural features by which aminoacyl-tRNA synthetases identify tRNAs |
Exit tunnel
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part of large subunit where growing polypeptide emerges; often a target for antibiotics
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Interface between subunits
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important for movement of tRNAs and mRnA
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3 tRNA binding sites in ribosome
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aminoacyl (A) site, peptidyl (P) site, exit (E) site
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Initiation |
AUG identified by initiation factors (IFs), ribosome, and special initiator methionine tRNA; methionine-loaded tRNA is bound to P site
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Elongation |
Elongation factor Tu (eEF1A) loads next charged tRNA into A site; peptide bond formation catalyzed between aa in P and aa in A; EFG (EF2) then promotes translocation
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Translocation
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Movement of mRNA-tRNA through ribosome; peptidyl-tRNA in A site moves to P site, new codon moves to A site, tRNA in E site leaves ribosome
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Termination |
ribosome reaches a stop codon (UAG, UAA, or UGA); stop codon recognized by release factors; promotes release of polypeptide from ribosome
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Class I release factors that recognize stop codons
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In bacteria, RF1 for UAA and UGA, RF2 for UAA and UGA
In eukaryotes, eRF1 for all 3 |
Ribosome recycling
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Large and small subunits dissociate and release remaining tRNA and mRNA; recycling factor RRF and EFG help dissociation in bacteria
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3 steps of translation initiation
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small subunit identifies start codon in mRNA; methionyl-tRNA is loaded into P site and base pairs with start codon; large subunit joins complex
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Initiator tRNA in eukaryotes vs. bacteria
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decodes AUG initiator codon; tRNA1Met in eukaryotes and tRNAfMet in bacteria, f denoting a formyl group
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Polycistronic
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Bacterial mRNAs have several open reading frames, each with its own start and stop codon
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Shine-Dalgarno sequence
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Polypurine tract 6-8 bases upstream of initiator AUG in bacteria; pairs with polypyrimidine region in 3' end of 16S rRNA (anti-Shine-Dalgarno sequence); guides initiator AUG to P site
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Shine-Dalgarno Consensus
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AGGAGGU; sequence deviations from consensus control strength of translation
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Differences between eukaryotic initiation and bacterial initiation |
No eukaryotic equivalents to Shine-Dalgarno sequence; small subunit does not bind directly to mRNA; more initiation factors needed; monocistronic mRNAs; initiation can occur at 2nd and 3rd AUG in addition to 1st |
3 steps of elongation |
Decoding, peptide bond formation, translocation |
Decoding |
aminoacyl-tRNA with anti-codon complementary to mRNA codon is chosen by the ribosome
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Peptide bond formation
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transfer of polypeptide chain to the aminioacyl-tRNA in the A site
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Ribosome recycling factor (RRF)
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In bacteria, promotes ribosome disassembly
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Nonsense suppression
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stop codons are misread and termination fails to occur
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Frameshifting
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mRNA shifts so that peptide synthesis proceeds in a different reading frame
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Antibiotics |
Small molecules that kill or disrupt organism growth; most effective target bacterial or fungal process but don't disrupt the same process in mammals
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Antibiotics and translation
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Small differences between bacterial and eukaryotic translation allow good antibiotics to target the ribosome and other translation proteins
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