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Lesson: Chapter 13 — The Living World

13.1. The three domains 5 of 5

Where viruses sit

Viruses have no place in the three domain classification. The reason is that they have never been definitely established as living things at all.

Living or not?

Viruses, first observed in 1892 by the Russian scientist D. J. Ivanovski, have not been definitely identified as living things. They have the characteristics of living things and of non-living things alike.

The one characteristic of living things a virus does have is multiplying inside a host cell.

Figure to be added

Figure 13.6 — the electron microscope structures of various viruses: the AIDS virus, a bacteriophage and the bird flu virus.

So they sit outside the table

A classification is for living things. Since whether a virus is one at all is unsettled, no domain can be given to it. That is not a gap in the classification so much as a peculiarity of the virus.

You have met this already

The structure of a virus, the living and non-living characteristics it has, and the diseases it causes were all covered in chapter 8, "Viruses". What is added here is one thing only — that they take no place in the classification.