# 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.

> IB DP Biology Higher Level (2023 syllabus) — D1.2 Protein synthesis · Explain · 4 marks

## Mark scheme (4 marks)

1. A charged (aminoacyl) tRNA with a complementary anticodon enters the A (aminoacyl) site of the ribosome
2. 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)
3. The tRNA at the P site (now uncharged/deacylated) moves to the E (exit) site and is released
4. 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

- [elongation](https://www.gradenine.co.uk/glossary/elongation)
- [translocation](https://www.gradenine.co.uk/glossary/translocation)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-the-molecular-events-that-occur-37b64391) · Published by Druglandscape Ltd.