# Explain the evidence from ribosomal RNA (rRNA) sequence comparisons that led to the proposal of the three-domain system of classification.

> IB DP Biology Standard Level (2023 syllabus) — A3.2 Classification and cladistics · Explain · 4 marks

> Carl Woese proposed a radical revision of the classification of life in the 1970s based on comparisons of ribosomal RNA sequences. This led to the current three-domain system: Bacteria, Archaea, and Eukarya.

## Mark scheme (4 marks)

1. rRNA sequences are highly conserved across all life and can be compared between organisms to determine evolutionary relatedness
2. rRNA sequence comparisons showed that Archaea and Bacteria are as distantly related to each other as either is to Eukarya
3. This evidence contradicted the previous two-kingdom or five-kingdom systems which grouped all prokaryotes together
4. rRNA evidence also showed Archaea share some molecular features with Eukarya, supporting the three-domain arrangement where Archaea and Eukarya share a more recent common ancestor

## 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-the-evidence-from-ribosomal-rna-847058d8) · Published by Druglandscape Ltd.