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VIRUSES

Edexcel
Do not confuse viruses with bacteria. Both are sometimes called germs, and poor hygiene will enable both to cause infections, but there are important differences from a biological point of view.

Viruses are much smaller (and simpler) than bacteria. They vary in size from 10 to 50 nm (nanometres).

1 nm =10-9 m, or 10-6 mm, or 10-3 Ám. They can only be seen using an electron microscope.

Viruses do not grow except inside cells of other (true) organisms, so they do not (on their own) need food, etc. In fact they do not carry out most of the commonly accepted characteristic processes which other living organisms perform.

Structure of a typical virus

virus particle

The shape may be almost spherical or a fairly simple 3D geometric figure, e.g. icosahedron, as shown in this diagram. Viruses do not actually move of their own accord - this one has simply been gratuitously and amateurishly animated so as to make it look more interesting on the WWW!
rotating icosahedral virus

Alternatively, it may have a rod or tube structure. Viruses which infect bacteria (bacteriophages, or 'phages for short) may have a more complex shape with a broad head and a tubular tail section, and fibres, looking like the lunar landing module!

Phage attacking bacterium

In the space below, list the differences between the structure of a virus and a typical (animal) cell.

There is no cytoplasm, cell membrane, true nucleus or chromosomes, cell organelles, e.g.mitochondria, ribosomes etc. Virus has a simple coat of protein only. Virus size about 1/1000 of animal cell.

List below some examples of (human) diseases caused by viruses.

cold, flu/influenza, AIDS/HIV, Hepatitis(B), polio(myelitis), etc,

In the space below, list the (7) characteristic processes of life and briefly note whether viruses seem to perform them at all.

1 movement not of their own accord

2 nutrition no - uses materials taken in by host's cells

3 respiration no need for energy when outside host's cells

4 growth no change in size - only growth in numbers inside host's cells

5 excretion no metabolism therefore no waste products

6 reproduction the main process - but only inside the host's cells

7 sensitivity not reacting to stimuli, but perhaps in targeting specific cells

Stages in the replication of a virus

A virus first needs to enter a host's cell, or at least "inject" its nucleic acid (DNA or RNA) into the cytoplasm. This may lay dormant, or start to be active straight away. The virus DNA (or RNA) then takes over the control of the cell's metabolism and starts making hundreds of copies of itself, as well as proteins to make up its coat and also the special proteins (e.g. enzymes) that it needs in the process. Meanwhile, the cell is not performing its usual tasks because the virus has "hi-jacked" the cell.

Eventually these components - protein and nucleic acid - are "assembled" into hundreds of new virus particles, inside the host cell. Usually these new virus particles can only escape by breaking open the host cell, destroying it in the process.

Use this information and your own knowledge to answer the following questions.

Why may there be an interval of several hours between exposure to an infection and the appearance of symptoms of the disease?

gap between injection of nucleic acid and production of viral particles

Why do the symptoms of a cold include inflammation of the nasal passages?

destruction of cells lining passages causes white cells to "mop up" dead cells and attack virus particle



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