Chapter 16 Translation How does information from DNA produce cellular proteins Learning objectives By the end of this class you should be able to Understand how the cell uses 40 tRNAs to recognize 61 different codon Describe how translation is initiated elongated and terminated Comprehend how the translation machinery identifies were to start and stop translation Describe the differences between a tRNA and an aminoacyl tRNA Central Dogma How does an mRNA triplet specify an amino acid mRNA ribosome amino acids ATP GTP Adaptor molecule No Protein Protein Transfer RNA Enzymes called aminoacyl tRNA synthetases catalyze the addition of amino acids onto tRNAs called charging a tRNA 20 amino For each of the acids there is a different aminoacyl tRNA synthetase and one or more tRNAs The combination of a tRNA covalently linked to an amino acid is then called an aminoacyl tRNA How many tRNAs are there Last couple of classes we learned that the 20 most common amino acids are coded for by how many different mRNA codons So how many different tRNAs would you expect there to be Wobble Hypothesis The contradiction of there being 61 codons but only tRNAs was a big problem Several scientists including Crick noticed that many of the codons for the same amino acid share the same first two codon Wobble Hypothesis Scientist tested tRNA that corresponded to codons with the same first two nucleotides Found that one tRNA can bind more than one codon Anticodon GUU can bind CAA or CAG Certain bases in the third position of tRNA anticodons can bind to bases in a manner that does not match complementary pairing CAG CAA Transfer RNA The addition of amino acids onto tRNAs called a tRNA For each of the 20 amino acids there is a different tRNA synthetase and one or more tRNAs The combination of a tRNA covalently linked to an amino acid is then called an tRNA Structure and Function of Ribosomes Initiating Translation 1 mRNA binds to small ribosomal subunit 2 Initiator tRNA binds to start codon carrying f Met The first codon is always 3 Large subunit binds completing the complex Initiating Translation Deciphering the genetic code While the first codon is always this codon can be used in the rest of the protein sequence AUG AAG AGG AUG GGG Elongation Extending the Polypeptide When both the P site and A site are occupied by tRNAs they are in the enzyme s active site bond formation occurs Elongation Extending the Polypeptide 1 aminoacyl tRNA arrives an binds in A site 2 formation 3 Translocation repeated down the length of mRNA Terminating Translation Terminating Translation When translocation opens the site and exposes one of the stop codons a protein called release factor fills the A site This breaks the bond linking the tRNA in the P site to the polypeptide chain Remember an mRNA is being translated by multiple ribosomes Watch a video that will describe transcription and translation http media pearsoncmg com bc bc 0media bio biof lix bioflix htm c8eproteinsynthesis
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