Topic 1.3 · Supplement
Viruses
Viruses sit outside every kingdom covered on the previous pages, for a simple reason: they are not made of cells. For this chapter, what you need is narrower and more precise than a full account of virus biology.
The required structural features of a virus, for classification purposes in this topic, are limited to two: genetic material, and a protein coat surrounding the genetic material. That pairing is the essential structural pattern to recognise — genetic material + protein coat— and it’s worth being able to state it in exactly that form, since it’s the detail an unfamiliar-diagram question on viruses is usually checking for.
Because viruses aren’t built from cells, none of the cell-based features used to separate the five kingdoms— cell walls, chloroplasts, nuclei — apply to them at all. That’s exactly why viruses are treated as a structure of their own here rather than squeezed into one of the five kingdoms. More detailed virus biology, including how they infect cells, is not required at this point in the course.