Explain the molecular events that occur at the ribosome during the elongation stage of translation, from the arrival of a charged tRNA at the A site to the translocation of the ribosome.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A charged tRNA with an anticodon complementary to the mRNA codon enters the A site. Peptidyl transferase, a ribozyme in the 23S rRNA, catalyses a peptide bond between the amino acid on the A‑site tRNA and the polypeptide chain on the P‑site tRNA, extending the chain. The P‑site tRNA, now deacylated, moves to the E site and is released. Translocation then shifts the ribosome one codon forward (5′→3′), moving the peptidyl‑tRNA from the A to the P site; this step requires GTP hydrolysis by EF‑G.
Examiner tips
- Use the exact terms: A site, P site, E site, peptidyl transferase, GTP hydrolysis, translocation
- Show the sequence of events in order
- Mention the ribozyme activity of rRNA
Common mistakes
- Confusing the direction of translocation (5′→3′)
- Omitting the role of GTP hydrolysis
- Mixing up which tRNA moves to which site
Mark scheme (4 marks)
- A charged (aminoacyl) tRNA with a complementary anticodon enters the A (aminoacyl) site of the ribosome
- A peptide bond is formed between the amino acid at the A site and the growing polypeptide chain held by the tRNA at the P site, catalysed by peptidyl transferase (rRNA with ribozyme activity)
- The tRNA at the P site (now uncharged/deacylated) moves to the E (exit) site and is released
- Translocation occurs — the ribosome moves one codon (3 nucleotides) along the mRNA in the 5′→3′ direction, requiring GTP hydrolysis, so the tRNA carrying the elongating polypeptide shifts from the A site to the P site
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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