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D1.2.17 (HL)—Translation initiation

Translation initiation and ribosomal tRNA sites organize start-codon recognition, amino acid entry, peptide transfer and tRNA exit during protein synthesis.

Syllabus
First assessment 2025
Objective
D1.2.17
Level
HL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1
Typical marks1–3

Common command terms

  • Outline

Scoring notes

Common mistake
Putting the initiator tRNA into the E site or A site instead of linking initiation to the start codon and P site.

Recent exam appearances

May 2025Paper1A ["HL"] · TZ311[ 1 ]D1.2.17 (HL)—Translation initiation
November 2023Paper2 ["HL"] · TZ22(c)[ 3 ]D1.2.17 (HL)—Translation initiation
November 2022Paper1 ["HL"] · TZ028[ 1 ]D1.2.17 (HL)—Translation initiation
November 2019Paper1 ["HL"] · TZ027[ 1 ]D1.2.17 (HL)—Translation initiation
May 2017Paper2 ["HL"] · TZ26(b)[ 4 ]D1.2.17 (HL)—Translation initiation

Coverage 2017–2025 · Updated 16 Jul 2026

Translation Initiation Places the Start Codon in the P Site

HL only
1

The small ribosomal subunit binds the mRNA and positions the start region for scanning or recognition.

A translation initiation complex has AUG paired with an initiator methionine tRNA in the ribosomal P site, with A and E sites labelled.
2

The initiator tRNA carrying methionine pairs with AUG. This tRNA is placed directly in the P site, establishing the reading frame.

3

The large subunit joins to complete the ribosome. The A site is now open for the tRNA matching the next codon.

AUG inside an already established reading frame can code for methionine without serving as a new start. Initiation depends on context and assembly, not the triplet alone.

A, P and E Sites Track Each tRNA's State

HL only
Ribosomal site Typical tRNA state during elongation
A (aminoacyl) incoming charged tRNA paired with the next codon
P (peptidyl) tRNA carrying the growing polypeptide before peptide transfer
E (exit) uncharged tRNA leaving after translocation

Before peptide transfer: growing chain in P, new amino acid in A.

After peptide transfer and translocation: the chain-bearing tRNA occupies P, the empty tRNA moves to E, and A opens for the next charged tRNA.

The site letters describe functional positions in the ribosome, not permanent labels on a tRNA. A single tRNA can move A → P → E during one cycle.

Control Protein Output at Several Checkpoints

HL only
Checkpoint What can change Consequence for protein output
transcription initiation promoter and transcription-factor activity amount of pre-mRNA made
RNA processing cap, tail and exon combination mRNA stability, export and encoded sequence
translation initiation successful AUG recognition and ribosome assembly whether and where translation begins
post-translational processing folding, cleavage, modification and assembly protein activity or location
proteasomal degradation ubiquitin tagging and breakdown rate protein lifetime and abundance

Across these checkpoints, sequence direction stays consistent: RNA is made 5′→3′, the ribosome reads mRNA 5′→3′ and the polypeptide grows from N-terminus to C-terminus.

Gene expression is therefore not a single on/off event. A cell controls which message is made, how it is processed, whether it is translated, how the product is activated and how long the protein remains.

Translation initiation

HL only

[ 4 ]

Outline the roles of the different binding sites for tRNA on ribosomes during translation.

Concept essentials

  • The small ribosomal subunit binds mRNA early in initiation.
  • The start codon AUG establishes the reading frame for translation.
  • The A site receives incoming aminoacyl-tRNA.
  • The P site holds the growing polypeptide, and the E site releases empty tRNA.