Explain why viruses are not classified within any of the recognised kingdoms or domains of life, and outline one piece of evidence that supports their exclusion from living classification systems.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Viruses are acellular – they have no cell structure, which is a basic requirement for all recognised kingdoms or domains.
They cannot carry out metabolism on their own; metabolic activity only occurs inside a host cell.
They cannot self‑replicate; they need host ribosomes and enzymes to copy their genome and assemble new virions.
Evidence: viruses lack ribosomes (or any organelles) and therefore cannot translate proteins independently, a feature shared by all cellular life.
They cannot carry out metabolism on their own; metabolic activity only occurs inside a host cell.
They cannot self‑replicate; they need host ribosomes and enzymes to copy their genome and assemble new virions.
Evidence: viruses lack ribosomes (or any organelles) and therefore cannot translate proteins independently, a feature shared by all cellular life.
Examiner tips
- Use the four criteria in separate sentences; keep each point brief and to the point.
- Mention the lack of ribosomes as the specific evidence; this is what the mark scheme expects.
- Avoid extra explanation or examples that are not asked for.
Mark scheme (4 marks)
- Viruses are acellular / do not possess a cell, which is a fundamental criterion shared by all recognised living organisms.
- Viruses cannot carry out metabolism independently / have no metabolic activity outside a host cell, so they do not meet the metabolic criterion for life.
- Viruses cannot self-replicate / reproduce independently; they require a host cell's ribosomes and enzymes to replicate their genetic material and assemble new particles.
- Named evidence: viruses lack ribosomes (or any organelles), which are present in all cellular life, supporting their exclusion; OR viruses show no 16S/18S rRNA sequences that are used in universal phylogenetic trees to place all cellular organisms.
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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