Translation initiation positions the ribosome on the start codon so the very first amino acid is read correctlyInitiation is the first stage of translation, in which the ribosome assembles on the mRNA at the correct starting point.The players are the same as in the rest of translation: the mRNA template, the two ribosomal subunits, and the initiator tRNA carrying methionine.Key Players in TranslationThe small subunit binds the mRNA, scans to the start codon, and the initiator tRNA reads AUG in the P siteThe small ribosomal subunit binds the 5′ end of the mRNA.In eukaryotes the small subunit recognises the 5′ cap that marks this end of the mRNA.The small subunit then scans along the mRNA in the 5′ to 3′ direction until it reaches the start codon AUG.The initiator tRNA carries methionine and has the anticodon UAC, which base-pairs with AUG.The initiator tRNA settles into the P site of the small subunit, the only tRNA able to bind there directly.NoteIn prokaryotes the small subunit binds instead to the Shine-Dalgarno sequence just upstream of the start codon.In eukaryotes the start codon is usually the first AUG the scanning subunit meets.The large subunit joins to complete the ribosome, leaving the A site empty and ready for elongationOnce the initiator tRNA is in place, the large ribosomal subunit joins the small subunit to form the complete ribosome.The initiator tRNA stays in the P site while the A site is left empty, ready to accept the next tRNA when elongation begins.Translation initiation Structure of the ribosomeHintAccurate initiation is what fixes the reading frame, so getting the ribosome onto the right AUG keeps every downstream codon correct.Theory of KnowledgeHow does the near-universal use of AUG as the start codon reflect the shared evolutionary history of life?Could that shared code make it easier to engineer proteins across species in synthetic biology?Active recallWhich ribosomal subunit binds the mRNA first during initiation?In which direction does the small subunit scan the mRNA to find the start codon?What is the anticodon of the initiator tRNA, and which amino acid does it carry?Which site does the initiator tRNA occupy when the complete ribosome forms?What is the state of the A site at the end of initiation?
