Explain how the use of ribosomal RNA (rRNA) sequences led to the proposal of the three-domain system of classification, and outline one limitation of this system.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
rRNA sequences are highly conserved across all life, so they can be compared even between very distant organisms.
By aligning 16S/18S rRNA sequences, scientists found that Archaea are as different from Bacteria as they are from Eukaryotes. This evidence led to the three‑domain system: Bacteria, Archaea and Eukarya.
The system places Archaea and Eukarya together because they share features such as similar RNA polymerases and histones, indicating a closer relationship than either has with Bacteria.
A limitation is that the system relies mainly on a single molecule (rRNA). Phylogenies based on other genes can give different relationships, and horizontal gene transfer can blur the true evolutionary history.
By aligning 16S/18S rRNA sequences, scientists found that Archaea are as different from Bacteria as they are from Eukaryotes. This evidence led to the three‑domain system: Bacteria, Archaea and Eukarya.
The system places Archaea and Eukarya together because they share features such as similar RNA polymerases and histones, indicating a closer relationship than either has with Bacteria.
A limitation is that the system relies mainly on a single molecule (rRNA). Phylogenies based on other genes can give different relationships, and horizontal gene transfer can blur the true evolutionary history.
Examiner tips
- Use the exact phrase ‘highly conserved’ and mention 16S/18S rRNA; state the three domains and the key shared features; give a concise limitation that cites single‑gene reliance or HGT.
- Keep the answer to four clear points, matching the mark scheme structure.
Common mistakes
- Confusing 18S rRNA with 16S rRNA or omitting the conservation point; not mentioning the shared RNA polymerase/histone evidence; giving a limitation unrelated to rRNA or not mentioning horizontal gene transfer.
Mark scheme (4 marks)
- rRNA sequences are highly conserved across all living organisms, making them suitable for comparing very distantly related organisms
- Comparison of rRNA sequences (particularly 16S/18S rRNA) revealed that Archaea are as distinct from Bacteria as they are from Eukaryota, supporting three separate domains: Bacteria, Archaea, and Eukarya
- The three-domain system places Archaea and Eukarya as more closely related to each other than either is to Bacteria / supported by shared features such as similar RNA polymerases and histones
- One limitation: the three-domain system is based primarily on a single gene/molecule (rRNA), and phylogenies based on other genes can give conflicting relationships / horizontal gene transfer can obscure true evolutionary history
Key terms in this question
ribosomal RNA (rRNA) · three-domain system
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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