Cambridge O Level Biology · Syllabus 5090 · Classification
Virus
What is Virus?
A virus consists of genetic material surrounded by a protein coat. It has no cellular structure and is not placed in any of the five kingdoms. A virus can produce new virus particles only inside a living host cell, whose machinery it uses to make them.
This definition is part of the Classification chapter in Cambridge O Level Biology.
Virus in context
Classification is the placing of living organisms into groups on the basis of the features they share, so that organisms with many features in common sit in the same group. Cambridge O Level Biology (5090) Chapter 2 covers four things: the definition of a species and the binomial naming system, dichotomous keys, the five kingdoms (animal, plant, fungus, prokaryote and protoctist), and the observable features that separate the vertebrate groups, the arthropod groups, ferns from flowering plants and monocotyledons from dicotyledons. Viruses sit apart from all five kingdoms because they have no cellular structure.
Common mistakes with Virus
- 6. Putting viruses in a kingdom Fix Viruses are not placed in any of the five kingdoms. State their features — genetic material in a protein coat — and that they replicate only inside living cells.
- "Viruses are in the prokaryote kingdom because they are tiny and have no nucleus." Repair Viruses are not placed in any kingdom. A prokaryote is a complete cell with cytoplasm and a cell wall; a virus is genetic material in a protein coat with no cell structure at all, and it can replicate only inside a living cell.
- "The virus would not grow on the agar because there was no food." Repair Nutrient agar contains plenty of nutrients. What it lacks is living cells, and a virus needs the machinery of a living host cell to produce new virus particles.
Questions students ask about Virus
Why are viruses not placed in any of the five kingdoms?
A virus is genetic material inside a protein coat, with no cellular structure at all — no cytoplasm, no cell wall and no nucleus. Because it is not a cell it cannot belong to any kingdom, and it cannot replicate on its own: it can make new virus particles only inside a living host cell, using that cell's machinery. That is also why a virus will not grow on nutrient agar, which contains nutrients but no living cells.