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Chapter 23: Viruses Leaving Certificate Biology Higher Level Viruses • Viruses are obligate parasites – they can only reproduce inside a living host • Viruses are non-cellular: made of a piece of DNA or RNA surrounded by a protein coat • Syllabus: “Viruses: identify the problem of definition” – The problem of definition comes from the debate as to whether or not viruses are living or nonliving – Viruses do not show all the characteristics of living things: organisation; nutrition; excretion; response; reproduction Virus Shapes • Round/spherical-shaped • Rod-shaped • Complex-shaped Virus Structure • Viruses are composed of a single piece of either DNA or RNA surrounded by a protein coat Piece of DNA or RNA Protein coat Viral Replication 1. 2. 3. 4. 5. Attachment: virus attaches to proteins on the target cell’s surface Entry: either the whole virus enters the cell by pushing into the cell or the virus injects its DNA/RNA into the host cell Synthesis of viral DNA and viral proteins: the virus causes the cell’s own DNA to become inactive and uses the cells ribosomes and organelles to make new DNA/RNA and protein Assembly of new viruses: the newly synthesised DNA/RNA and viral proteins assemble themselves into new viruses Release of new viruses: the number of new viruses build up in the host cell until the host cell bursts and releases all the newly-formed viruses which go on to infect other cells Economic and Medical Importance of Viruses • Two harmful effects of viruses: – AIDS (acquired immunodeficiency syndrome) caused by the HIV (human immunodeficiency virus) – which is a retrovirus (contains RNA) – Human papilloma virus: causes cervical cancer by integrating its own DNA into the host cells DNA and remains dormant • Beneficial effect of viruses: – Used in genetic engineering to transfer genes especially between bacteria – It is possible that bacteriophages might be used to kill bacterial infections in the future