eukaryotic 80s ribosome and tRNA

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  • Eukaryotic 80S ribosome and transfer RNA
    • Prokaryotic ribosome
      • 70S - made of 50S + 30S
      • cotranscriptional translation
    • Eukaryotic ribosome
      • 80S - made of 60S + 40S
      • transcription and translation separated physically and temporally
    • ribosomes made of 65% rRNA and 35% ribosomal protein
      • small ribosomal subunit
        • decoding centre required for reading the mRNA genetic code
      • large subunit
        • contains peptidyltransferase centre where AA are added onto the nascent polypeptide
    • multiple ribosomes loaded onto an actively translating mRNA = polysome
    • tRNA
      • acts as molecular adaptor that associates and mRNA codon to a specific AA
      • more than one tRNA associates with a particular AA
      • fold into a series of 4 base-paired stems which adopt an L shape (although they are drawn with a cloverleaf structure)
      • the anticodon loop is a single-stranded area which contains the anticodon that hybridises to a corresponding 3 base codon on the mRNA
      • the 5' and 3' ends of the tRNA base pair together to form the acceptor stem where the AA is added
        • the 3' end terminates in the residues CCA which becomes charged when the relevant AA when it is transferred by aminoacyl tRNA synthetase
      • added complexity in tRNA structure arises from base modifications
        • this changes specific bases to generate inosine, pseudouridine, dihydrouridine and ribothymidine
        • if a modified base is in the anticodon, the base pairing specificity canchange, resulting in a phenomenon called wobble
          • this is where the first 2 anticodon bases follow standard base pairing, but the 3rd base affords a large degree of flexibility
            • e.g. a U on mRNA can be recognised by A, G or I
            • the consequence is ththe consequenece is that a given codon can be reocgnised by multiple tRNAs, which ensures the process is more efficient
            • mutations affecting the 3rd base do not normally have a phenotypic effect because they do not alter the AA sequence


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