Lecture 18 - Lecture18 fig7.

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Lecture 18 fig 7.3 tRNA is charged to form aminoacyl-tRNA by forming an ester link between the: 2’ or 3’OH group of the adenylic acid at the end of the acceptor arm COOH group of the amino acid the tRNA is the adapter that is going to be the conduit between the codon triplet in the mRNA and the amino acid that gets covalent attached in a peptide bond. tRNA synthetases have the responsibility to recognizing the exact structure of each tRNA, all tRNAs have a clover leaf; most of the bases are invariant, some have an extra arm. They range in size from 73 to 94 nucleotides in these invariant arms that the syn- thetases are looking for. What is being recognized is the anticodon bases and a few bases around the anti- codon, and the exact angle that the tRNA folds up to look like an L. Have to realize that the system had to make all the tRNAs look similar enough so they fit into the ribosome. So all tRNAs fit in either the A or P site of the ribosome. Yet the syn- thetases have to discriminate one tRNA from another to know which amino acid to covalently attach the correct tRNA when you have multiple tRNA for the same amino acid in a cell, called cognate tRNAs, the same synthetase charges all of the these tRNAs. So the tRNAs must look similar in a way, not just based off of their anticodons but based of the tertiary structure so that the synthetase can distinguish between the tRNAs to know which amino acid should be charged. Fig 7.5 The sequence of the anticodon is solely responsible for the specificity of the aminoacyl-tRNA Experiment: shows that the ribosome has a large and small subunit, within each sub- unit comprised of at least one ribosomal RNA and some have three different rRNA. Usually 30 to 40 different proteins comprise each subunit. function of the small subunit is to mediate the anticodon/codon base pairing. This is where the arm of the tRNA is sticking into the small subunit and the opening in the binding site of the tRNAs allow it to site over the mRNA so that anticodon triple is available for h-bonding to the codons. peptide bond formation occurs in the large subunit, the tRNA is tall enough that it spans both subunits. When you have a complete ribosome, the A and P sites are considered full or complete meaning that they span both the large and small sub-
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unit, and the top of the tRNA (the acceptor arm) is going to stick up where the pep- tide bond is going to form in the large subunit One the tRNA enters and pairs with the codon it has no way of knowing if the cor- rect amino acid is on that tRNA, what ever amino acid is there will get joined in a peptide bond based on the codon/anticodon match.
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Lecture 18 - Lecture18 fig7.

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