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Living or non-living? Characteristics of Viruses • Non-cellular structure, no organelles • Outer proteins coat called a capsid • Nucleic acid (RNA or DNA) – only contain genes for replication of new viruses • Obligate intracellular parasites – dont carry out any metabolic reactions on their own – require the cell machinery of host cells • 50 times smaller than the average bacterial cell Living versus non-living Living • Contain genetic material • Contain structures made of protein • Have the ability to replicate Non-living • Non-cellular • Have no cellular organelles • Contain only one type of nucleic acid • Cannot reproduce by themselves Shapes of Viruses Rodshaped Round Complex Various viruses Replication • Attachment – host cell surface proteins act as receptors • Entry – Virus penetrates and viral nucleic acid enters – Capsid may be left outside • Replication – Viral DNA is replicated and instructs host organelles to produce viral protein • Assembly – Many copies of whole virus particles are put together • Release – The cell bursts and the new viruses escape Economic Importance • Advantages – Used to transfer genes from one organism to another in genetic engineering – Used to control pests • e.g. insect virus against European pine sawfly • Disadvantages – Crop diseases – Human diseases – Animal livestock diseases Diseases caused by viruses: Human: smallpox, chicken pox, shingles, warts, verrucas (direct physical contact with contaminated surfaces), hepatitis B.common cold, influenza, mumps, measles (all spread by droplet infection), polio (droplets/faeces),German measles, rabies (from a bite of an infected dog), Hepatitis A, Ebola, AIDS. • Smallpox Virus • Chicken Pox Mumps • Plant: tobacco mosaic virus (gives tobacco plant a spotted, mosaic appearance), potato mosaic virus and sugar beet virus • Most plant viruses are transmitted by insects (vectors) e.g. greenfly, which suck sap. • Animal: cowpox, foot and mouth (in cattle and sheep), rabies (affects nervous system of dogs mainly), distemper (dogs, fatal for puppies unless immunised), Newcastle disease in poultry and swine fever in pigs. Medical Importance • Advantages – vaccine manufacture (e.g. Measles, mumps and rubella - MMR) – gene therapy (using viruses to replace abnormal gene with normal one in diseased tissues) • Disadvantages – cause many infectious diseases • Measles, mumps, rubella, influenza, common cold, rabies – some cancers are caused by viral infections Human Diseases caused by viruses • • • • • • • • • AIDS Cold-sores Measles, mumps and rubella (“MMR”) Influenza and the common cold Chicken pox and shingles Viral meningitis SARS Haemorrhagic fever and Ebola Genital warts and genital herpes AIDS - Acquired Immune Deficiency Syndrome • HIV (human immunodeficiency virus) is a retrovirus. • HIV virus attacks helper T-lymphocytes upon which we are dependent for our immune system (and so can’t produce antibodies to pathogens or to organisms that don’t normally cause medical problems i.e. opportunistic infections e.g. pneumonia). Transmission - by body fluids e.g. blood, semen, saliva. It can also be passed from mother to child across placenta or in breast milk. High-risk groups = Receivers of blood transfusions, intravenous drug users (via contaminated needles), haemophiliacs receiving blood products and those with multiple sexual partners of unknown status. • Diagnosis • Blood tests of HIV positive people will reveal the presence of antibodies. Some HIV+ will get rid of the virus and so are immune but for others the virus is dormant and may turn into full-blown AIDS at any stage. Control and prevention: • At present no cure and no vaccine due to rapid mutation of virus. Methods of prevention: • Avoid unknown or multiple sexual partners. • Do not share needles, toothbrushes, razors etc.. • Only use screened blood products. • Use condoms if partner’s status unknown. • Avoid contact (use gloves) with blood/body fluids/wounds. • Don’t donate blood, semen or body organs if have AIDS or tested positive for the antibodies. Statistics: • Approx. 150 million people worldwide suffer from AIDS. Number growing by 20%, every year, especially in developing countries. Treatment of viral disease • Antibiotics do not work against viruses – they are only effective against bacteria • Vaccines – injections of antibodies are often used as vaccine but also as a treatment • There has only been limited success with antiviral drugs – Acyclovir (“Zovirax”) against Herpes simplex i.e. treatment of coldsores. – AZT against HIV in treatment of HIV/AIDS.