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Lecture 12

BIO240H Lecture 12.doc

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University of Toronto St. George
Jennifer Harris

Lecture 12 TranslationSlide 1Slide 2 Firstly remember that tRNA brings AAs to the ribosomes The tRNA is the one that recognizes the mRNA through anticodon to codon pairing If you think about this the tRNA already has an AA attached You have to make sure that you have the right AA attached to the right tRNA otherwise youre in trouble What it means is that you need to have 20 different enzymes 20 different tRNA synthaseseach of the 20 tRNAs binds to a particular AA Diagram tRNA that binds to a tryptophanthis particular tRNA synthase only binds to Trp as well it binds to any transfer RNA that has an anticodon that recognizes the codon for tryptophan Recall for the degeneracy inherent in DNA there is a subset of tRNA that will have the correct anticodon In the case of tryptophan there is only one but for others there may be subsetsreplacements This particular enzyme is binding to the AA and is attaching the AA to the tRNA and its happening in the cytoplasm before we get to the ribosome The enzyme attaches the AA onto the 3 end of the tRNA remember tRNA is just RNA When its like this there is a special nomenclature when it has AA attached it is called amino acyl transfer RNA when its got the combo The reason why is amino acyl side group and tRNA As we will see the transfer RNA is going to the ribosome and base pair with the correct codons and you will have the appropriate AA being added There are two steps to maintain fidelitynotion that the anticodon must correspond to the correct AA The synthase is going to make sure that its only binding to the right AA tryptophan and any tRNA with the right anticodon Another part of fidelity is that in the mRNA there must be good base pairingso tRNA anticodon must correctly pair with codon of mRNA This ensures that you get the correct AA sequence of the protein Ex With Arg there are 6 different mRNA codons therefore there are 6 different tRNAs that can fit in there One synthase will recognize Arg plus the subset of tRNAs it can recognize all 6 tRNAs with different anticodons tRNAs dont really react with each other since theres only 3 nucleotides there isnt that big a patch for combination so the dangers of tRNAs binding together due to complementary base pairing is very smallSlide 3 What happens with tRNA synthase it binds to tRNA adds AA onto the 3 end of the tRNA If it adds on the incorrect AA then what happens is the AA will flip into the editing site If AA were the correct one then it would not be able to fit into the editing site Normally the correct AA gets added on but if something goes wrong then this AA can pop into the editing site of the enzyme and gets chopped off If AA is added on correctly it cannot fit into the editing site therefore you dont get editingthis is only like proofreadinglike a check This is much like exonuclease activity of DNA polymerase very similar The chopping off of the amino acid is called hydrolytic editingit chops off either the AA or nucleotideSlide 4 The ribosome is the action place where protein synthesis happens They are found in both eucaryotic cells and prokaryotic cells There is a lot of RNA in ribosomes We see prokaryotic ribosomes on the left these are found in bacteria and on the right is eucaryotic ribosome There are two subunits to each ribosome one large and one small subunit They have similar kinds of shapes they both have RNAs and lots of proteins that assemble with the RNAs Ribosomes are made of combinations listed in the slide there is an extra RNA in the large subunit in eucaryotic cellsYou dont need to memorize all the small details just know that there is a lot of RNA in protein in the ribosomes
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