# Explain why viruses are not assigned to any of the three domains of life, and discuss the implications of this for their classification within a cladistic framework.

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

## Mark scheme (4 marks)

1. Viruses are not considered living organisms / do not possess cellular structure
2. The three domains (Bacteria, Archaea, Eukarya) are defined by cellular organisation / ribosomal RNA sequence comparisons, which viruses lack
3. Cladistics requires shared derived characters (synapomorphies) inherited from a common ancestor; viruses cannot be meaningfully rooted in the same tree of life
4. Viral classification uses alternative criteria (capsid structure, genome type, replication strategy) rather than phylogenetic ancestry / cladistics

## Key terms

- [domain](https://www.gradenine.co.uk/glossary/domain)

## Related

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