Acellular Life

A virus is a piece of genetic material wrapped in protein. The genetic material is the genome, either DNA or RNA, and the protein coat around it is the capsid. The capsid is built from repeated protein subunits called capsomeres, which are proteins and nothing else. Some viruses add a layer outside that coat, the envelope, and the whole assembled particle in its complete, mature, infectious form is the virion, the phrase the papers also use as "complete virus". Four words, four levels of description, and almost every structure question in this chapter turns on picking the right one.

Outside a host cell a virus does nothing that a living thing does. It has no organelles and no metabolic machinery of its own, so it cannot generate energy or build its own macromolecules, and that is the reason a host cell is compulsory. One account goes further and calls viruses an exception to cell theory, alive only while inside the host cell and behaving as dead matter outside it. The line can also be drawn the other way round, with "cannot excrete" as the keyed answer against crystallise, mutate and infect bacteria, and Stanley's crystallisation of tobacco mosaic virus in 1935 is the reason crystallise belongs on the list at all. The word itself is Latin, derived from a term meaning poison or slimy liquid, and all viruses are parasites.

The discovery of that word is a story about filters, and it is examined. Sindh gives Charles Chamberland, an assistant of Louis Pasteur, as the man who built the porcelain filter in Paris in 1884, with a pore size of $0.1\ \mu\text{m}$, small enough to remove every bacterium of $0.2\ \mu\text{m}$ or more from the liquid drawn through it. Ivanowski used the filter on tobacco mosaic leaf extract in 1892 and found the filtrate still diseased the plant, Beijerinck called the agent a "contagious living fluid" in 1899, and Stanley crystallised it in 1935. The pore size of a porcelain filter is keyed as $100$ to $1000\ \text{nm}$, the same instrument in nanometres, and Stanley is named as the first to purify filterable agents. The envelope is where the host is written in: it is derived from the host cell membrane and not from viral capsid protein, new particles bud from the plasma membrane to acquire it, and it is the feature by which a virus recognises its specific host, because the surface glycoproteins on it bind host cell receptors.