C39: Viruses Mr. E Murphy

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

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

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

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

Virus Replication

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

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

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