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The Viruses

The Viruses. January 14 th , 2010. Virus Basics. Viruses are nucleic acid and protein structures Very small; typically between 20-200 nm No cellular structures No ribosomes No metabolic pathways (Glycolosis, Kreb’s cycle, electron transport chain, etc.) Few or no enzymes. Virus Basics.

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The Viruses

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  1. The Viruses January 14th, 2010

  2. Virus Basics • Viruses are nucleic acid and protein structures • Very small; typically between 20-200 nm • No cellular structures • No ribosomes • No metabolic pathways (Glycolosis, Kreb’s cycle, electron transport chain, etc.) • Few or no enzymes

  3. Virus Basics • Viruses carry out NO growth or metabolism on their own • They are dependent on living cells for their replication • Can exist in the environment, but do not replicate • To replicate, they must come in contact with a host organism

  4. Virus Basics • Viruses replicate by infecting a host cell and hijacking the host cell’s replication machinery to produce more viruses • Host cell DNA replication • Host cell RNA transcription • Host cell RNA translation • Host cell protein and membrane building capacity

  5. Virus Basics • They are generally host-specific • Infect only certain cell types • Influenza and lung tissue • Norovirus and intestines • Hepatitis B and liver cells

  6. Structure

  7. Virus structure • Viruses are mostly nucleic acid and protein • Protein shell • May be surrounded by a lipid envelope • Nucleic acid inside

  8. Virus structure

  9. The viral capsid • Made of protein subunits • Repeating patterns • Symmetrical structure • Can have proteins, lipids, and sugars on the surface • These outer structures interact with host cells

  10. Example: poliovirus

  11. Virus envelopes • Viruses can be enveloped or non-enveloped • Enveloped • Lipid bilayer surrounds the capsid • Similar to a cell membrane • Non-enveloped • Protein shell only

  12. Internal structures • Capsid forms a shell around the nucleic acid • Some viruses carry their own enzymes inside the capsid

  13. http://biology.kenyon.edu/slonc/gene-web/Lentiviral/Lentivi2.htmlhttp://biology.kenyon.edu/slonc/gene-web/Lentiviral/Lentivi2.html

  14. Nucleic Acids • Typically a single nucleic acid • Can be DNA or RNA • Single or double stranded • Size: 32-kbp • Encode proteins the virus needs to take over a cell and reproduce itself • Replication proteins • Structural proteins

  15. Example: the poliovirus genome http://www.jci.org/articles/view/22139/figure/1

  16. Viral Life Cycle

  17. Virus attachment • Viruses attach to cells via a receptor • Molecule on the surface of a cell that the virus can recognize and attach to • Like a lock and key • These cell receptors often serve a useful function for the host cell; viruses have simply evolved to exploit them

  18. Virus entry • Once attached, virus goes through the cell membrane into the cytoplasm • Entry happens through different mechanisms • Penetration • Membrane fusion

  19. Virus attachment

  20. Production of new viruses • Take over the cell’s replication machinery • Stop the cell’s own nucleic acid replication, transcription, and translation processes • All the cell’s energy goes into creating more viruses

  21. Production of viral protein

  22. Viral replication: DNA viruses • Virus DNA serves as the template • Host cell enzymes replicate more pieces of viral DNA • Host cell enzymes transcribe viral DNA into mRNA • Host cell ribosomes translate viral mRNA into proteins

  23. Viral replication: RNA viruses • Viral RNA serves as the template • Translated directly into proteins by the host cell ribosomes • However, the viral RNA still needs to be replicated for making more viruses • Human and animal cells do not have enzymes for replicating RNA • These enzymes are encoded on the viral genome • The host cell ribosomes produce these enzymes, allowing the virus to replicate its own nucleic acid

  24. Replication

  25. Virus assembly • Nucleic acids associate with capsid proteins • Proteins assemble into capsid structures • Complete virus particles assemble in the cytoplasm of the cell

  26. Virus assembly

  27. Release from the cell • Viruses can pass through the cell membrane • May rupture the cell and escape • Enveloped viruses: may “bud” out, taking part of the cell membrane to form their envelope • Infect adjacent cells • Release into the environment • Release into host cell body fluids that can spread infection

  28. Release

  29. Viruses in the environment • Can remain viable in a variety of environments • Water • Air • Soil • Food • Can remain infectious for long periods until they encounter a host cell

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