Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.
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.
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
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