HardBiologyHigh School (11–12)Diversity of Living Organisms › Viruses, Viroids and PrionsConceptualBoard Exam
Crystals of tobacco mosaic virus are kept in a sealed tube for 50 years…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Crystals of tobacco mosaic virus are kept in a sealed tube for years. When a little is rubbed on a tobacco leaf, the leaf soon shows the mosaic disease. Is a virus a living thing? Give one reason for each side.
Need a reminder? The key ideaShow
Viruses, viroids and prions are not in the five kingdoms. A virus is an obligate parasite: DNA or RNA, never both, inside a protein capsid. Ivanowsky (1892) found that the cause of tobacco mosaic passed through a bacterial filter; Beijerinck (1898) called it Contagium vivum fluidum; Stanley (1935) crystallised the virus. Diener (1971) found the viroid, free RNA with no coat. A prion is an abnormally folded protein (BSE, CJD).
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Not living. Outside a cell a virus is inert. It can be crystallised like a chemical, and it has no metabolism of its own.
Step 2. Living. Inside a host cell it takes over the cell and makes copies of itself, with its own genes.
Step 3. Where it stands. So a virus lies on the border of life. This is one reason it is not in the five kingdoms.
Answer: non-living outside a cell (it can be crystallised); living-like inside a host cell (it reproduces).
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