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Viruses

Viruses. Objectives. The components of a virus The differences between lytic and lysogenic cycles. What a virus is NOT. A bacterium An independently-living organism; a virus needs a host in order to survive and Antibiotics do not “kill” it.

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Viruses

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  1. Viruses

  2. Objectives • The components of a virus • The differences between lytic and lysogenic cycles

  3. What a virus is NOT • A bacterium • An independently-living organism; a virus needs a host in order to survive and • Antibiotics do not “kill” it http://www.scientificamerican.com/article.cfm?id=experts-where-did-viruses-come-fr

  4. Structure of a Virus This figure is not necessarily correct – why?

  5. Viral genomes • Single or double-stranded DNA, or single- or double-stranded RNA, but not both DNA and RNA. • Usually a single linear or circular nucleic acid molecule http://www.ncbi.nlm.nih.gov/genomes/GenomesHome.cgi?taxid=10239

  6. Capsid • The protein shell, built from a large number of often identical protein subunits • Help determine the shape of the virus – helical, polyhedreal, or even more complex shapes like those of bacteriophages

  7. Virus Sizes and Shapes

  8. Viral Envelope • Derived from membranes of the host cell, but also can include viral proteins and glycoproteins • May cloak the capsids of viruses found in animal. HIV

  9. What are bacteriophages? • Viruses that infect bacteria • Unique viral structure • Have been used in research for decades

  10. Viruses only reproduce in host cells • Obligate intracellular parasites – need a host • Have no metabolic enzymes or machinery to reproduce on their own – must hijack host cell • Viruses are specific to their host – host and viral receptors must match in order for infection to occur http://www.youtube.com/watch?v=41aqxcxsX2w Virus Infection Video

  11. Two life cycles – lytic and lysogenic

  12. Lytic Cycle of bacteriophages: • Bacteriophage attaches to host cell and injects its viral DNA into the cell http://www.youtube.com/watch?v=41aqxcxsX2w Video of T4 phage infecting a cell

  13. What happens next? • Transcription and translation of viral genes produces an enzymethat targets host DNA and chops it up. Why might the virus do this? • Capsid proteins are then made and assembled into viral particles • These viral particles build up until the cell bursts, releasing more infectious virions into the environment

  14. Lysogenic Cycle • Instead of a lytic path, the viral DNA may be incorporated into the host genome. The viral DNA is now called a prophage. • These genes are then repressed • Reproduction of the host cell, reproduces the viral DNA along with the bacterial DNA. • At this point, the thelytic cycle may begin

  15. End Result: RSV virus leaving cells SARS virus leaving cells Budding HIV

  16. RNA viruses • What do RNA viruses (retroviruses) need to do in order to replicate?

  17. Nature Medicine5, 740 - 742 (1999) doi:10.1038/10462 HIV-1 entry inhibitors: Evading the issue

  18. More Info on HIV http://www.stanford.edu/group/virus/retro/2005gongishmail/HIV.html

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