Translation initiation and ribosomal tRNA sites organize start-codon recognition, amino acid entry, peptide transfer and tRNA exit during protein synthesis.
Coverage 2017–2025 · Updated 16 Jul 2026
Translation initiation assembles the ribosome at the mRNA start codon and establishes the reading frame; A, P and E sites then organize tRNA movement during elongation.
The small ribosomal subunit attaches near the mRNA 5′ terminal and moves to the start codon. An initiator tRNA pairs with that codon; the large subunit attaches, and the next matching tRNA can enter.
| Ribosome site | Role during elongation |
|---|---|
| A (aminoacyl) | Accepts the incoming charged tRNA |
| P (peptidyl) | Holds the tRNA carrying the growing polypeptide |
| E (exit) | Holds/releases the uncharged tRNA before it leaves |
After peptide-bond formation and translocation, tRNAs shift through the sites: the growing-chain tRNA moves to P, the empty tRNA moves to E, and A becomes available for the next tRNA.
The start codon fixes the triplet reading frame. A/P/E describe tRNA positions, not three separate ribosomes.
This objective is assessed through structured response, commonly using Outline.
Outline
Build the answer around this relationship: The small ribosomal subunit binds mRNA early in initiation.
Putting the initiator tRNA into the E site or A site instead of linking initiation to the start codon and P site.
Representative question
Outline the roles of the different binding sites for tRNA on ribosomes during translation.
RNA polymerase reads template DNA 3' to 5' and synthesizes RNA 5' to 3'; ribosomes translate mRNA codons in the 5' to 3' direction. Promoters mark transcription start regions and orientation; transcription factors help RNA polymerase bind and initiate in eukaryotes. Non-coding DNA does not code for polypeptide amino acid sequences and includes introns, regulatory sequences, telomeres, rRNA genes, and tRNA genes. Eukaryotic pre-mRNA is modified before export and translation by adding a 5' cap and poly-A tail and removing introns by splicing. Alternative splicing joins different exon combinations from one pre-mRNA, so one gene can produce multiple protein variants in different cells or stages. Translation initiation assembles ribosomal subunits at the start codon AUG; initiator tRNA enters the P site and A, P, and E sites organize tRNA movement. Newly made polypeptides may be folded, cleaved, or chemically modified; preproinsulin processing to active insulin is a key example. Proteasomes degrade tagged, damaged, or unneeded proteins; amino acid recycling supports new protein synthesis and proteome quality control.
