Structure and diversity of viruses

The big idea: A virus is not a living cell — it is a tiny non-cellular (acellular) infectious particle.

It has no cytoplasm, no organelles, no ribosomes and no metabolism of its own. Outside a host it just sits there, completely inert.

At its simplest a virus is only two things: a piece of genetic material (the genome) wrapped in a protein coat (the capsid).

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.

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.

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.

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.

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.

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.

How this is tested: HL questions on viruses usually ask you to describe the basic structure of a virus, to compare a virus with a cell, or to 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.

✓ 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 is to call a virus a 'cell' or to say its genome is 'DNA' only — remember a viral genome can be DNA or RNA.