3. Viruses Extremely small (can only be viewed using an electron microscope) Non-cellular pathogen Core of genetic material – DNA or RNA Surrounded by wall of protective protein  Capsid

6. Viruses – Living or Non - Living Viruses have none of the characteristics of living things. They only respond when they make contact with a suitable host All types of cells are attacked by viruses – most viruses are specific to one cell type. Do not have ribosomes, mitochondria or other cytoplasmic organelles  cannot carry out metabolisms on their own Must invade host cell to carry out rxn’s

7. LivingNon-Living Possess genetic material DNA/RNA Possess protein coat Can replicate (inside a living cell) Non cellular Cannot reproduce by themselves Do not possess ribosomes, mitochondria etc Only have one type of nucleic acid

8. A Bacteriophage is a virus that infects bacteria. - Most complex and best studied viruses

9. Virus Replication * As viruses are not cells the term replication is used instead of reproduction  Attachment – virus attaches to a suitable host cell  Entry – the whole virus or its nucleic acid enters the host cell

10. New virus DNA made using bacterial organelles  Synthesis – the host nucleic acid is inactivated. The viral nucleic acid uses the host’s organelles to produce new viral nucleic acids and protein  Assembly – Viral nucleic acid and proteins come together forming new viruses

11. Bacterium bursts Many viruses released  Release – host cell bursts to release between 100 and 100,000 new viruses  Bursting of host cell is called lysis

12. Retroviruses Contain RNA instead of DNA Contain an enzyme that converts virus RNA to DNA  DNA then makes new copies of virus RNA and new viruses inside the host cell. Example: HIV is a retrovirus

13. Some viruses do not destroy host DNA. Instead the viral DNA joins with the host DNA and remains inactive When the host DNA is copied, the viral DNA is also copied and passed on to the daughter cells. New host cells may behave normally or may produce new chemicals due to the presence of the virus Example : Diphtheria, scarlet fever and botulism

14. Economic importance of Viruses Crop damage e.g. mosaic disease of potatoes, tomatoes and tobacco Livestock diseases e.g. foot and mouth, rabies Work days lost due to viral infections e.g. flu, colds

15. Medical Importance of Viruses Causes disease e.g. cold, cold sores, flu, measels, rubella, mumps, chickenpox, warts, hepatitis, viral meningitis, HIV(AIDS) Some cancers are caused by viruses Vaccines – immunisation against viral diseases

16. Beneficial uses of Viruses Act as vectors in genetic engineering  used to transfer genes from one organism to another. Biological control : against pest rabbit populations

17. Control and Immunity Controlled by the body’s general defence system and specific defence system Immunity to certain viruses can be induced artificially by vaccination Antibiotics kill bacteria but have no effect on viruses Some antiviral drugs are available – they interfere with the virus without affecting the host. E.g. Acyclovir helps treat herpes (cold sores)