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Microbiology Pretest 1. Are viruses alive? 2. Vaccines are effective against: (a) viral diseases (b) bacterial diseases (c) both of these 3. Antibiotics are effective against: (a) viral diseases (b) bacterial diseases (c) both of these 4. Name a disease caused by a prion. 5. Viruses contain: (a) DNA (b) RNA (c) either DNA or RNA (d) Both DNA and RNA 6. Approximately what percent of bacteria are harmful to humans? (a) 1% (b) 25% (c) 50% (d) 75% 7. Name 2 useful functions of bacteria. 8. Name a viral disease. VIRUSES “A virus is a piece of bad news wrapped in a protein.” -Sir Peter Medawar Nobel Laureate Why are viruses considered non-living? They lack the ability to reproduce on their own. They do not transform their own energy, or demonstrate homeostasis. The lack cell membranes, cytoplasm and cell organelles. They are dormant unless in contact with a host cell. I. Early Studies • Tobacco mosaic virus which causes discoloration of tobacco leaves, was the first identified virus • TMV can form crystals. • The ability to form crystals is a property of chemicals, not cells. Viral Vaccine History • In 1796, Edward Jenner experimented with cowpox and smallpox • noticed that milkmaids got cowpox, but never acquired smallpox • injected a young boy with cowpox, then smallpox • boy survived; first successful vaccine for smallpox II. Composition and shape A. Protein coat known as capsid. B. Inside the coat, the virus contains either DNA or RNA but never both. C. Many have an envelope surrounding the capsid. D. Envelope contains proteins derived from host, which help the virus gain entry to cells. Shapes include: • Polyhedral (adenovirus) • Helical (TMV and rabies) • enveloped (often appears spherical – example is the influenza virus) • filovirus (long and threadlike – example is Ebola) Viruses can infect plants, bacteria and animals Bacteriophages These are viruses that infect bacteria. • The bacteriophage injects its genetic material into a bacterium. The protein coat remains outside the cell. capsid DNA tail sheath tail fiber Viruses of eukaryotes often fuse with cell membrane III. Replication inside living cells http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap11.html The Lytic Cycle Causes death of the host cell (lysis) 1. 2. 3. 4. 5. Attachment or Docking Entry Biosynthesis of the viral components Assembly Release of completed virus from host cell In 24 minutes, 1000 viruses can be produced Lysogenic cycle • Virus’s nucleic acid is inserted into the host cell’s chromosome and is known as a prophage. • Prophage is replicated along with host cell’s chromosome • At any time, prophage can be activated and enter the lytic cycle. Lytic or Lysogenic? • Common cold lytic • HIV lysogenic • Herpes lysogenic • Influenza lytic Two Main Groups of Viruses A. DNA Viruses • herpes virus, poxviruses B. RNA Viruses • cause diseases such as the common cold, rabies, measles Retrovirus: a type of RNA virus • Retroviruses have a core of RNA. • Use enzyme called reverse transcriptase to form DNA. • Viral DNA is inserted into the host DNA. • HIV is an example of a retrovirus. • Other retroviruses can cause some cancers. Emerging Viruses • An existing virus can spread from one type of host to another. Humans can get viruses from other species. • An existing virus can spread from a small population to become more widespread. Hantavirus can spread from a small mouse population to the human population. • Other emerging viruses are Ebola, HIV, SARS, and bird flu. PRIONS and MAD COW DISEASE What are prions? • discovered in 1981 • "proteinaceous infectious particle" • an abnormal (misfolded) protein, with no nucleic acid • present in the brain and neurons • pathogenic variants of proteins • agents that cause a group of fatal and degenerative brain diseases Why are prions so deadly? • No immune response • Long incubation period • Mutated proteins multiply in the brain • When a normal protein (PrPc) comes in contact with an infectious prion (PrPsc), it’s shape is changed and it is converted into an infectious prion. Prion Diseases • affect the brain, disrupting or destroying neurons in large numbers (all are lethal) • Creutzfeldt Jakob Disease (CJD) in humans • Bovine Spongiform Encephalopathy (BSE) or ”mad cow disease" How could prions infect me? • dairy products are “safe” • muscle tissue is “safe” • you can only get mad cow disease if you ingest the abnormal prion • these are present in nervous tissue (spinal cord, brain) Viroids • discovered in 1971 • smaller than viruses • single circular strand of nucleic acid (RNA) with no protein coat • serious metabolic disruption and stunting in plants 1 nanometer (nm) = one billionth of a meter 100 nm eukaryotics cells 10,000-100,000 nm viruses 50-200 nm prokaryotics cells 200-10,000 nm viroids 5-150 nm prion 2-10 nm