Explain the role of ribosomes in ensuring the accuracy of translation during protein synthesis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Ribosomes consist of a small and a large subunit that join at the start codon on the mRNA, aligning the mRNA for accurate reading.
The small subunit contains the decoding centre (A‑site) where the mRNA codon is read and the correct tRNA anticodon is selected, ensuring codon–anticodon fidelity.
The large subunit houses the peptidyl‑transferase centre (PTC) in the 23S/28S rRNA, a ribozyme that catalyses peptide‑bond formation between adjacent amino acids.
After each peptide bond, the ribosome translocates 5′→3′ by one codon, shifting the mRNA so that the next codon enters the A‑site, maintaining the correct reading frame and sequential translation.
The small subunit contains the decoding centre (A‑site) where the mRNA codon is read and the correct tRNA anticodon is selected, ensuring codon–anticodon fidelity.
The large subunit houses the peptidyl‑transferase centre (PTC) in the 23S/28S rRNA, a ribozyme that catalyses peptide‑bond formation between adjacent amino acids.
After each peptide bond, the ribosome translocates 5′→3′ by one codon, shifting the mRNA so that the next codon enters the A‑site, maintaining the correct reading frame and sequential translation.
Examiner tips
- Use the four key points in order: subunit assembly, decoding centre, PTC, translocation.
- Show the ribosome moves 5′→3′ and shifts by one codon each cycle.
- Mention the ribozyme activity of the PTC to highlight catalytic role.
Common mistakes
- Confusing the small subunit with the large one for peptide bond formation.
- Omitting the 5′→3′ direction of translocation or the one‑codon shift.
- Failing to state that the PTC is a ribozyme in the rRNA.
Mark scheme (4 marks)
- Ribosomes have two subunits (small and large) that come together at the start codon on the mRNA, positioning the mRNA correctly for reading.
- The small subunit contains the decoding centre / A site region where the mRNA codon is read, ensuring the correct codon–anticodon pairing occurs.
- The large subunit contains the peptidyl transferase centre (PTC), a ribozyme activity in the 23S/28S rRNA, which catalyses peptide bond formation between adjacent amino acids.
- The ribosome moves along the mRNA in the 5′ to 3′ direction (translocation), shifting by one codon at a time, so that each codon is read sequentially and in the correct reading frame.
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 →
More Protein synthesis questions
- Explain how the wobble hypothesis accounts for the fact that the number of tRNA …
- Explain the molecular events that occur at the ribosome during the elongation st…
- Explain how the sequence of nucleotides in a gene is used to determine the seque…
- Explain how the process of splicing of pre-mRNA contributes to proteome diversit…