The big idea: A virus is not a living cell — it is a tiny non-cellular (acellular) infectious particle.
No cell machinery
- It has no cytoplasm, no organelles, no ribosomes and no metabolism of its own.
- Outside a host it just sits there, completely inert.
Only two parts: At its simplest a virus is only two things: a piece of genetic material (the genome) wrapped in a protein coat (the capsid).
Two contrasting viruses: a bacteriophage (polyhedral protein capsid 'head' + tail) and an enveloped virus (capsid wrapped in a lipid envelope studded with glycoprotein spikes). Both are just a nucleic acid core inside a protein coat — non-cellular.
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- Virus
- A non-cellular infectious particle: a nucleic acid genome enclosed in a protein capsid (sometimes with a lipid envelope). It can only replicate inside a host cell.
- Non-cellular (acellular)
- Not made of cells; lacking cytoplasm, organelles, ribosomes and metabolism.
- Genome
- A virus's genetic material — either DNA or RNA, single- or double-stranded — carrying the genes to make new virus particles.
- Capsid
- The protein coat surrounding the genome, built from many repeating protein sub-units called capsomeres.
Why we don't simply call viruses 'alive': Living things respire, grow and reproduce by themselves. A virus does none of these on its own — it has no machinery to do so.
It only ever copies itself by hijacking a host cell's ribosomes and enzymes. So a virus sits on the border between living and non-living.
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Read each part as structure → function: every feature of a virus exists to get its genome into a host cell and protect it on the way.
Start with the two parts every virus has, then the extras some viruses add.
| Part | What it is | What it does |
|---|---|---|
| Genome (nucleic acid core) | DNA or RNA — single- or double-stranded | Carries the genes that instruct the host cell to build new virus particles |
| Capsid | A protein coat built from repeating sub-units called capsomeres | Protects the genome and helps the virus attach to and enter a host cell |
| Envelope (only in some viruses) | A lipid membrane stolen from the host cell as the virus leaves | Surrounds the capsid; helps the virus fuse with and hide from the next host |
| Glycoprotein spikes (envelope only) | Protein 'keys' sticking out of the envelope | Bind to specific receptors on the host cell so the virus can attach and enter |
Capsid → entry; envelope + spikes → targeting: The capsid does two jobs: it shields the fragile genome and gives the virus a shape that can dock onto a host cell.
Envelope and spikes
- Some viruses (e.g. influenza, HIV, coronaviruses) also wrap themselves in a lipid envelope taken from the host's own membrane as they bud out.
- Sticking out of that envelope are glycoprotein spikes.
- A spike works like a key that fits a specific lock — it binds a particular receptor on the host cell.
- This is why a given virus can usually only infect certain cells or species.
Despite very different shapes, sizes and genomes, both share the same minimal plan — genome + capsid (± envelope).
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Three ways viruses differ enormously
- Size — from tiny ones around 20 nm across up to giant viruses bigger than some bacteria.
- Capsid shape — helical (a spiral rod), polyhedral / icosahedral (a many-sided 'ball'), or complex (like a bacteriophage with a polyhedral head and a tail).
- Genome type — DNA or RNA, and single-stranded (ss) or double-stranded (ds). This single feature is the main reason viruses are sorted into so many different groups.
A worked structure → function example: Take an enveloped influenza virus. Its RNA genome holds the genes. The capsid protects that RNA. The lipid envelope (host-derived) hides it from the immune system, and the glycoprotein spikes lock onto receptors on cells lining your airway — so the virus attaches there and not, say, in your liver.
When the spikes change: Change the spikes (as flu does each year) and the key changes shape — which is why last year's immunity may no longer fit.
One rule that applies to ALL viruses: However big, however shaped, whatever genome — every virus is an obligate intracellular parasite.
It cannot replicate on its own. It must get inside a living host cell and use the host's ribosomes, enzymes and raw materials to build copies of itself. 'Obligate' means it has no choice — there is no free-living option.
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The big idea: Viruses are very varied, but they share only a few features. Learn the five on this list: every one can earn a mark.
The points to remember
- Small, fixed size: most viruses are 20 to 300 nm across, and every particle of one virus is the same size.
- Nucleic acid as genetic material: DNA or RNA, never both. Not every virus has DNA.
- A capsid made of protein encloses the nucleic acid.
- No cytoplasm: there is nothing inside but the nucleic acid (and in some, a few proteins).
- Few or no enzymes: a virus cannot make ATP or proteins, so it must use a host cell's.
Remember it as: Small, a nucleic acid, a protein coat, and nothing else.
Real example: norovirus: Norovirus, which causes stomach upsets, is about 30 nm across: a protein capsid around one strand of RNA. It has no cytoplasm and carries no enzymes of its own.
Name the feature exactly when you write it down:
Say which molecule: 'Genetic material' alone does not earn the mark: write 'nucleic acid', 'DNA or RNA'. And never write that all viruses contain DNA: many contain RNA instead.
How this is tested: HL questions on viruses usually ask you to do one of three things.
The usual tasks
- Describe the basic structure of a virus
- Compare a virus with a cell
- Outline the diversity of viruses (shape and genome type)
A common reasoning twist: Explain why a particular structure (e.g. spikes) matters, or why viruses are not classed as fully living. Always anchor your answer to the same parts: genome + capsid (± envelope + spikes) and the phrase obligate intracellular parasite.
Exam-style question — structure and diversity of viruses
Outline the basic structure of a virus, and explain two ways in which viruses show diversity. [4]
How to score all four marks
- State the universal plan. Every virus has a genome — a nucleic acid (DNA or RNA) — enclosed in a protein capsid made of capsomere sub-units. (Mark 1)
- Add the optional parts. Some viruses also have a lipid envelope (from the host membrane) carrying glycoprotein spikes used to attach to host cells. (Mark 2)
- Diversity 1 — capsid shape. Capsids vary: helical, polyhedral/icosahedral, or complex (e.g. a bacteriophage with head + tail). (Mark 3)
- Diversity 2 — genome type. The genome can be DNA or RNA and single- or double-stranded — the main basis for grouping viruses. (Mark 4)
Final answer
A virus is a nucleic acid genome (DNA or RNA) inside a protein capsid, sometimes with a lipid envelope and glycoprotein spikes. Viruses differ in capsid shape (helical / icosahedral / complex) and in genome type (DNA vs RNA, single- vs double-stranded).
Why this scores full marks: It names the two universal parts (genome + capsid), adds the optional envelope/spikes, and gives two genuinely different axes of diversity — shape and genome — rather than two examples of the same thing.
A frequent way to lose a mark: Calling a virus a 'cell', or saying its genome is 'DNA' only — remember a viral genome can be DNA or RNA.
| Feature | A living cell | A virus |
|---|---|---|
| Cytoplasm / organelles | Yes | No |
| Ribosomes | Yes — makes its own proteins | No — must borrow the host's |
| Own metabolism (respiration etc.) | Yes | No |
| Can reproduce on its own | Yes | No — only inside a host cell |
| Genetic material | Always DNA | DNA OR RNA |
Measles virus multiplies inside cells of the human airway. Describe one reason that viruses such as measles are not considered to be living.
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