Viruses, viroids and prions

Something that is not on the map

The five kingdoms are for cellular organisms. Some infectious agents are not cells at all: viruses, viroids and prions. They are not included in Whittaker's five kingdoms.

The discovery of the virus

• 1892: D. J. Ivanowsky studied the mosaic disease of tobacco. The sap of a sick plant, passed through a filter that stops bacteria, still made healthy plants sick. So the cause was smaller than a bacterium.
• 1898: M. W. Beijerinck showed that the extract of infected plants could infect healthy plants. He called the cause Contagium vivum fluidum (a contagious living fluid).
• 1935: W. M. Stanley showed that viruses could be crystallised, like a chemical. The crystals were made mostly of protein.

You should find that pores of $0.5\ \mu\text{m}$ stop the bacterium but let the viruses through, as in Ivanowsky's work. Even Mycoplasma slips through pores this wide.

What a virus is

• The word virus is Latin for poison.
• A virus is inert outside a living cell. It does not eat, grow or make energy by itself. Inside a host cell it takes over the cell and makes copies of itself, so it is an obligate parasite.
• It has a nucleic acid inside a protein coat, the capsid. The capsid is made of small units called capsomeres, and it protects the nucleic acid.
• The nucleic acid is either DNA or RNA, never both. The nucleic acid is the infectious part.
• Plant viruses usually have single-stranded RNA. Animal viruses have single-stranded or double-stranded RNA, or double-stranded DNA.
• Bacteriophages are viruses that infect bacteria. They usually have double-stranded DNA, with a head, a tail and tail fibres.

Viruses cause mumps, smallpox, herpes, influenza and AIDS in humans. In plants, the signs are mosaic patterns, rolling and curling of leaves, yellowing, clearing of veins and dwarfing.

Viroids

In 1971, T. O. Diener found an infectious agent smaller than a virus. It caused the potato spindle tuber disease. It was free RNA, with no protein coat, and the RNA was of low molecular weight.

Prions

A prion is an abnormally folded protein that is infectious. It has no nucleic acid. Prions cause bovine spongiform encephalopathy (BSE), or mad cow disease, in cattle, and a similar disease in humans, Creutzfeldt-Jakob disease (CJD).

Try it in Python

This program keeps a table of the genetic material of six viruses, and counts them.

Practise with new numbers

Common mistakes

• Putting viruses in Monera. Viruses are not cells, and they are in none of the five kingdoms.
• Giving a virus both DNA and RNA. It has one or the other.
• Calling a prion a small virus. A prion has no nucleic acid; it is a misfolded protein.
• Mixing up the scientists. Ivanowsky: the filter. Beijerinck: the contagious fluid. Stanley: the crystals.

Remember these

• Viruses, viroids and prions are not in the five kingdoms.
• Ivanowsky (1892), Beijerinck (1898) and Stanley (1935) studied the tobacco mosaic virus.
• A virus has DNA or RNA, never both, in a protein capsid. It is an obligate parasite.
• A viroid is free RNA. A prion is an abnormally folded protein.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Who showed that viruses can be crystallised?

2. Which agent has no nucleic acid?

3. What is a bacteriophage?

4. What did Ivanowsky's filter show about the cause of tobacco mosaic?