The big idea: Translation starts when the small subunit binds the 5' end of mRNA, finds the start codon and the initiator tRNA, and then the large subunit joins.
The points to remember
- The small subunit binds to the 5' end of the mRNA (at the cap).
- It moves along the mRNA, 5' to 3', to the start codon AUG.
- The initiator tRNA, anticodon UAC, carrying methionine, pairs with AUG.
- The large subunit then binds; the initiator tRNA is in its P site.
- A second tRNA arrives at the next codon, in the A site, and elongation begins.
Remember it as: Small finds start, Met joins, large closes, then the second tRNA.
Real example: bacteria. Bacterial initiator tRNAs carry a modified methionine, formyl-methionine. Our immune cells can recognise it as a sign of a bacterial infection.
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Three sites in elongation: The large subunit has three binding sites for tRNA. Each tRNA passes through them in the order A, P, E.
| Site | What happens there |
|---|---|
| A (arrival) | A tRNA carrying its amino acid binds to the next codon |
| P (polypeptide) | The tRNA holding the growing polypeptide; the chain is joined to the A-site amino acid |
| E (exit) | The tRNA, now without an amino acid, leaves the ribosome |
Say the amino acid: A site: the tRNA arrives with its amino acid. E site: the tRNA leaves without one. Answers that leave out the amino acid miss the point.
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How it is asked: The steps of initiation after the small subunit binds, the roles of the A and E sites, or a 1A item on what happens if one site is blocked.
IB-style questionDescribe[3 marks]
Describe the initiation of translation.
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