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Viruses, Viroids, and Prions. Characteristics of Viruses. Obligate intracellular parasites Contain either DNA or RNA, but not both Consist of a protein coat surrounding the nucleic acid core Multiply only within a living cell Display specific host range

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Viruses, Viroids, and Prions


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    1. Viruses, Viroids, and Prions

    2. Characteristics of Viruses • Obligate intracellular parasites • Contain either DNA or RNA, but not both • Consist of a protein coat surrounding the nucleic acid core • Multiply only within a living cell • Display specific host range • Lack enzymes for protein synthesis and ATP generation

    3. Are viruses living organisms? • They are not free living • Cannot “live” outside a live host • Basically a protein coat around a nucleic acid core • Viruses do multiply inside their host cell, is this alive?

    4. Viral Structure • Genetic material of a virus is either DNA or RNA, but never both • Protein coat is called CAPSID; surrounds the nucleic acid core • Capsid is composed of sub-units called CAPSOMERES • Some viruses may have a lipid envelope around the outside • Envelope may or may not be covered with SPIKES

    5. Viral Structure(Nonenveloped virus)

    6. Enveloped Virus

    7. Hemagglutinin spikes 500 per virus Allows virus to recognize and attach to body cells Antibodies made to these spikes Neuraminidase spikes 100 per virus Help virus separate from infected host cell Spikes

    8. Growing Viruses • Need a living host • Could NOT grow viruses on nutrient agar because the agar is not a living media • What to use? • Embryonated eggs • Cell cultures • Living animals

    9. Cytopathetic Effect (CPE)

    10. Complex Virus(Bacteriophage)

    11. How are viruses categorized? • Type of nucleic acid • How they replicate • Morphology

    12. Bacterial Viruses • Bacteriophage - from Greek phagein “to eat” • Viruses that ONLY infect most types of bacteria • Phages may be the most common entities in the biosphere • Two cycles of viral multiplication • LYTIC CYCLE • LYSOGENIC CYCLE

    13. Lytic cycle • Five step cycle that will result in the death of the infected bacterial cell • ONLY the viral DNA enters the cell • Protein coat of the virus remains outside the cell • This discovery was important on showing that DNA is the genetic material • VIRULENT BACTERIOPHAGE

    14. Lysogenic Cycle • Virus attaches and injects DNA • BUT the viral DNA incorporates into the bacterial chromosome and NO new virus is made • Integrated viral DNA is called a PROPHAGE • Viral DNA can remain “dormant” for many cell divisions • However, the lysogenic cycle can be induced to become lytic and kill the host cell • TEMPERATE BACTERIOPHAGE

    15. Lysogenic Cycle

    16. Phage Therapy • Phages ONLY infect bacteria • Could phages then be used to treat bacterial infections in humans? • Answer - YES • Phage therapy has been used with success in Russia sense the 1940’s • Current research is aimed at using phages to treat antibiotic resistant bacteria • In the US, the FDA has allowed phage spraying on certain food and meats to combat Listeria contamination.

    17. Bacterial Attachment Penetration Biosynthesis Maturation Release Animal Attachment Penetration Uncoating Biosynthesis Maturation Release Comparison of bacterial virus and animal virus multiplication

    18. Animal virus penetration of host cell

    19. DNA virus multiplication

    20. Herpesvirus • DNA viruses that infect most animals • HSV-1 causes cold sores in humans • HSV-2 causes genital herpes in humans • HHV-3 causes chickenpox in humans • All herpes viruses exhibit a property called LATENCY • The viral DNA integrates into your DNA and stays with you forever causing periodic out break of disease • NO CURE for latent diseases

    21. Retroviruses(RNA Viruses) • Retro means backward • HIV virus that causes AIDS is a retrovirus • Backward refers to the ability of retroviruses to this: RNA → DNA • The enzyme REVERSE TRANSCRIPTASE makes the above reaction possible • Now the viral DNA moves into the host chromosome and never leaves • Viral DNA directs the synthesis of new virus particles that infect more cells

    22. Retroviruses(continued) • The integrated viral DNA is protected from the immune system • The “infection” remains inside your cells permanently • So HIV infections can only be treated NOT cured because the viral DNA is now a part of YOU!

    23. Retrovirus multiplication

    24. Viral budding and the envelope

    25. Viroids • Infectious RNA • No protein coat • Naked pieces of RNA that cause disease • GOOD NEWS – viroids cause disease only in plants, at least so far!

    26. Prions • Infectious proteins! • Naked protein molecules that cause disease and death in humans • Two well known prion diseases are SCRAPIE and MAD COW DISEASE • Human prion disease is variant Creutzfledt-Jakob disease • Disease causes fatal brain encephalitis