1. Viruses

2. Contents Definition Shapes Basic structure of a virus
Viruses are obligate parasires
How viruses infect a cell
Viral Reproduction / Effect of viruses on cell
Medical importance of viruses
Economic importance of viruses
Beneficial viruses
Conditions caused by viruses

3. Definition
virus: (plural = viruses) are non-cellular micro-organisms, made up of a protein coat and one type of nucleic acid (DNA or RNA) - obligate parasites = can only multiply inside a living cell - cause disease.
Are they living or non-living?
Examples of diseases caused by viruses are influenza (flu), HIV (causes AIDS), chicken pox, measles, mumps, rubella.

4. Shapes
Viruses are found in a number of different shapes e.g. bacteriophage, etc.
Note: names of various shapes not needed for L.C.

5. Basic Structure of a Virus
Consists of a nucleic acid (DNA or RNA) and a protein coat.
Some may have a membrane coat outside of this, made from the membrane of the cell that was infected; with virus proteins inserted into it e.g. influenza.

6. Viruses are obligate parasites
Can only replicate within other living cells.
Can only replicate within a limited no. of organisms.
Some only replicate within a specific type of cell of the host.

7. How viruses infect a cell
To infect a cell, the surface proteins of the virus only bind with certain proteins on the surface of the cell.
Viruses are specific.

8. Viral Reproduction / Effect of Viruses on Cells (1/2)
When a virus enters a cell one of two things may happen: -
1. Viral DNA takes over the host cell – makes the host cell produce copies of the viral protein and DNA (or RNA) from materials within the cell.
These are then assembled into new viruses and released by the bursting (lysing) of the host cell.

9. Viral Reproduction / Effect of Viruses on Cells (2/2)
Viral DNA may be inserted into the host DNA.
Becomes integrated with host DNA and is passed on from generation to generation when cell replication takes place.
These are lysogenic cells – and the new DNA has no apparent effect.
Generations lateer – environmental conditions may change (e.g. radiation, chemicals, etc.) and activate the virus to make more copies of itself and lysing the host cell.

10. The possible effects of a virus on a cell

11. Medical importance of viruses
They cause many diseases.
Antibodies cannot cure viral diseases.
Vaccinations available for some viral diseases e.g. chickenpox, measles, etc.
No protection against other viral diseases e.g. AIDS.
Some new medicines interfere with viral replication e.g. medication against cold sores.

12. Economic importance of viruses
They damage crops e.g. tobacco mosaic virus.
Sick people effect the economy – out of work – need to be cared for.
In parts of Africa 10% of the population is infected with HIV – devastates their societies.

13. Beneficial viruses Very hard to find.
Bacteriophages used to destroy certain harmful bacteria. Some kill disease-causing bacteria / fungi e.g. Lambda bacteriophage attack E. coli.
Used in medical research e.g. in genetic engineering and the production of DNA profiles e.g. retroviruses produce the enzyme reverse transcriptase which is needed for cutting the DNA molecules in these processes.
Some viruses live in rhubarb and reduce size of plant.

14. Conditions caused by viruses
not examinable
for information only

15. Influenza

16. Other conditions caused by viruses
Foot & mouth disease
Distemper
Myxomatosis
Rabies (hydrophobia)
Common cold
Measles
Mumps
Rubella = German measles
Smallpox
Chickenpox
Poliomyelitis = Polio
Viral pneumonia
Genital herpes
AIDS

17. END