# 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.

> IB DP Biology Higher Level (2023 syllabus) — A3.1 Diversity of organisms · Explain · 4 marks

## Mark scheme (4 marks)

1. rRNA sequences are highly conserved across all living organisms, making them suitable for comparing very distantly related organisms
2. 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
3. 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
4. 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

- [ribosomal RNA (rRNA)](https://www.gradenine.co.uk/glossary/ribosomal-rna-rrna)
- [three-domain system](https://www.gradenine.co.uk/glossary/three-domain-system)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-use-of-ribosomal-2b3b87d9) · Published by Druglandscape Ltd.