Explain how the structure of prokaryotic ribosomes differs from that of eukaryotic ribosomes, and outline the significance of this difference for antibiotic action.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Prokaryotic ribosomes are 70S, made of 50S and 30S subunits, whereas eukaryotic ribosomes are 80S, made of 60S and 40S subunits.
Because the subunit sizes differ, the ribosomal proteins and rRNA sequences are also different between prokaryotes and eukaryotes.
Antibiotics such as streptomycin or erythromycin bind specifically to the 70S ribosomal subunits, blocking bacterial protein synthesis.
Since eukaryotic (80S) ribosomes have a different structure, these antibiotics do not bind to them, so they are not toxic to host cells and selective toxicity is achieved.
Because the subunit sizes differ, the ribosomal proteins and rRNA sequences are also different between prokaryotes and eukaryotes.
Antibiotics such as streptomycin or erythromycin bind specifically to the 70S ribosomal subunits, blocking bacterial protein synthesis.
Since eukaryotic (80S) ribosomes have a different structure, these antibiotics do not bind to them, so they are not toxic to host cells and selective toxicity is achieved.
Examiner tips
- Use the 70S/80S subunit detail to show the structural difference. Show that antibiotics target 70S, not 80S, to explain selective toxicity. Keep answer concise and use exam‑specific terminology.
Common mistakes
- Confusing 70S with 80S or vice versa. Failing to mention the subunit composition (50S/30S vs 60S/40S). Claiming antibiotics affect eukaryotic ribosomes without justification.
Mark scheme (4 marks)
- Prokaryotic ribosomes are 70S (composed of 50S and 30S subunits), whereas eukaryotic ribosomes are 80S (composed of 60S and 40S subunits).
- The structural difference means the ribosomal proteins and rRNA sequences differ between prokaryotes and eukaryotes.
- Antibiotics such as streptomycin or erythromycin bind specifically to 70S ribosomal subunits, inhibiting bacterial protein synthesis.
- Because eukaryotic (80S) ribosomes are structurally different, these antibiotics do not bind to them, so human/host cells are not harmed / selective toxicity is achieved.
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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