1. C39: Viruses
Mr. E Murphy

3. Objectives What is a virus? Structure of a virus Virus replication
Medical and economic importance of viruses

4. What is a virus?
Viruses are composed of a core of nucleic acid surrounded by a protein coat
Outside living cells, they are considered non-living chemicals
Inside a living cell, they replicate using the host cell
Viruses have features of both living and non-living material
Smallpox virus

5. What is a virus?
Viruses can only multiply inside living cells, therefore they are then obligate parasites
Viruses cause diseases in humans, animals and plants
Different kinds of viruses have different shapes
They can only be seen with an electron microscope

6. Structure of a virus
A virus is an infectious agent that consists of nucleic acid (DNA or RNA) enclosed in a protein coat
They are not made of cells, therefore don't have the cell machinery for their own metabolism
Can only grow in living tissue
Cannot be grown on agar
Antibiotics do not work against them, as there is no cell machinery for the antibiotic to damage

7. Virus Replication

8. Virus Replication
Most of our knowledge on viruses has emerged from work carried out on bacteriophages
Viruses attach to host cell
Either the whole virus or its nucleic acid enters the cells cytoplasm
Viral nucleic acid takes over the bacteria’s own DNA, making the cell replicate the viral nucleic acids and proteins
Nucleic acids and proteins are put together to make new viruses
New viruses use enzymes to burst out of the host cell

9. Medical and economic importance of viruses
Disadvantages
Human Diseases
Viruses cause many diseases such as measles, the common cold and AIDS
Plant diseases
Plant viruses cause mosaic disease in plants such as tobacco, potato and corn
Animal disease
Foot and mouth, SARS and rabies

10. Medical and economic importance of viruses
Advantages
Genetic engineering
Control of infections