Chapter 23: Viruses Leaving Certificate Biology Higher Level

Viruses Viruses are obligate parasites – they can only reproduce inside a living host orViruses 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 non- living –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 Protein coat Piece of DNA or RNA

Viral Replication 1.Attachment: virus attaches to proteins on the target cell’s surface 2.Entry: either the whole virus enters the cell by pushing into the cell or the virus injects its DNA/RNA into the host cell 3.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 4.Assembly of new viruses: the newly synthesised DNA/RNA and viral proteins assemble themselves into new viruses 5.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