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Viruses The Smallest Organisms?

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What are viruses? Very small particles Too small to see with a light microscope Can be seen with an electron microscope Invade living cells Experts think that it is probable that all living cells are subject to infection by one or more viruses

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Discovery Scientists suspected the existence of viruses by the end of the 1800s. Question: How could scientists find something that they could not see?

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Discovery The first virus that scientists discovered was the tobacco mosaic virus (TMV). TMV is a virus that infects a variety of plants including tobacco, tomatoes and peppers. Photo of a tobacco leaf with symptoms of tobacco mosaic virus Public Domain (from USDA Forest Service) http://commons.wikimedia.org/wiki/File:Tobacco_mosaic_vir us_symptoms_tobacco.jpg http://commons.wikimedia.org/wiki/File:Tobacco_mosaic_vir us_symptoms_tobacco.jpg

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Discovery 1883 Showed tobacco mosaic disease was infectious if you took plant juice from a diseased plant and placed it on healthy plants the plants developed the disease. 1889 Filtered the juice to remove all particles large enough to see with a light microscope and the juice still caused the disease.

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Discovery Next experiment proved that the disease could reproduce in the plant this is a characteristic of living things Unlike other living things the germ could not be grown outside the host plant

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Discovery 1935 Used chemical techniques to isolate the germ from the juice. Ended up with a crystalline substance that did not grow, breathe, eat, reproduce or perform any other life function but caused the disease in plants Called this germ a virus Are viruses living?

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Tobacco Mosaic Virus Public Domain http://en.wikipedia.org/wiki/File: TMV.jpg http://en.wikipedia.org/wiki/File: TMV.jpg

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So What Is a Virus? Three things to remember: 1. Tiny particles that can invade living cells 2. Do not perform any of the life function of cells 3. Can reproduce only within the host cell

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Host vs. Parasite Host = living thing that provides a home and/or food for a parasite Parasite = an organism that survives by living on or in another organism and harms the host in the process

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Host vs. Parasite ALL VIRUSES ARE PARASITES Each virus can infect only a few specific kinds of cells, both specific species and specific cells within the host organism.

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What do viruses look like? Diagram of a Flu Virus Public Domain http://en.wikipedia.org/wiki/File:Flu_und_legende_color_c.jpg http://en.wikipedia.org/wiki/File:Flu_und_legende_color_c.jpg Swine Flu virus electron micrograph Public Domain: CDC http://commons.wikimedia.org/wiki/File:B00528- Swine-flu.pnghttp://commons.wikimedia.org/wiki/File:B00528- Swine-flu.png

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What do viruses look like? Electron micrograph of the poliovirus Public Domain: CDC http://en.wikipedia.org/wiki/File:Polio_EM_PHIL_1875_lores.PNG http://en.wikipedia.org/wiki/File:Polio_EM_PHIL_1875_lores.PNG Computer model of Adenovirus Public Domain: National Cancer Institute http://commons.wikimedia.org/wiki/File:Adenovirus.jpg http://commons.wikimedia.org/wiki/File:Adenovirus.jpg

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What do viruses look like? Vesiculovirus, a member of the Rhabdoviridae similar in morphology to the rabies virus. Public Domain: This image is a work of the Centers for Disease Control and Prevention, part of the United States Department of Health and Human Services http://en.wikipedia.org/wiki/File:Vesicular_stomatitis_virus_(VSV )_EM_18_lores.jpg http://en.wikipedia.org/wiki/File:Vesicular_stomatitis_virus_(VSV )_EM_18_lores.jpg HIV-1. Transmission electron micrograph Public Domain: CDC http://commons.wikimedia.org/wiki/File:HIV- 1_Transmission_electron_micrograph_AIDS02bbb_lores.jpg http://commons.wikimedia.org/wiki/File:HIV- 1_Transmission_electron_micrograph_AIDS02bbb_lores.jpg

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What do viruses look like? Ebola, Transmission Electron Micrograph Public Domain: CDC http://en.wikipedia.org/wiki/File:Ebola_virus_em.jpghttp://en.wikipedia.org/wiki/File:Ebola_virus_em.jpg Virus particles of G. indiensis polydnavirus that infects a parasitoid wasp. The virions are formed by 5 to 10 nucleocapsids enclosed by only one viral envelope. Public Domain: USDA http://commons.wikimedia.org/wiki/File:GiBV-em.jpghttp://commons.wikimedia.org/wiki/File:GiBV-em.jpg

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Structure of Viruses Two Basic Parts 1. A core of hereditary material DNA or RNA Controls the reproduction of the virus 2. An outer protein coat Protects the virus Allows the virus to identify and attach to the host cell

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Bacteriophages Viruses that infect bacteria Photo Credit: Dr. Jan Mast, CODA http://commons.wikimedia.org/wiki /File:Bacteriofaag.gifCODA http://commons.wikimedia.org/wiki /File:Bacteriofaag.gif

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Reproduction of Viruses Three basic steps 1. The virus gets its hereditary material (DNA or RNA) into the host cell 2. The host cell makes new viruses 3. The new virus particles leave the host cell and infect new host cells

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Reproduction of Viruses Points to note: The entire virus may enter the cell or it might inject the hereditary material leaving the protein coat outside the cell. When the new virus particles leave the cell they may kill the cell by bursting it or they may not kill the cell in which case the cell may go on producing more and more virus particles.

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Viruses and Humans Viruses cause diseases including: Cold sores Warts Colds AIDS Measles Mumps Hepatitis Chicken pox Small pox Influenza

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Viruses and Humans Vaccines weakened or killed disease- causing organisms used to stimulate an immune response (antibodies) allowing the body to react immediately to exposure to the disease-causing organism and prevent it from causing disease Most, but not all, vaccines are against viruses. Includes mumps, measles, polio, hepatitis, chicken pox, and rubella.

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Viruses and Humans Uses for viruses Control other pests for example rabbits in Australia. Remember that most viruses are very specific as to what they can infect. As possible transmitters of replacement genes to cure genetic diseases such as cystic fibrosis.

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The End