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Viral structure & properties
2. What is a virus 3. Viruses Virus is defined as a nucleoproteincomplex which infects cells and uses their metabolic processes to replicate Smallest known infective agents Metabolically inert - no metabolic activity outside host cell; must enter host cell to replicate 4. What is Life? Can grow, i.e. increase in size. Can reproduce. Metabolism: can acquire and utilize energy. 5. Kingdoms of Life AnemaliaPlantae ProtistaFungi Bacteria Archaea 6. Discovery of Viruses Beijerinck (1897) coinedthe Latin name virus meaning poisonHe studied filtered plant juices & found they caused healthy plants to becomesick copyright cmassengale6 7. UNCONVENTIONAL AGENTS Virion: viral particle VIROIDS RNA only viroids infect plants Hepatitis D virus (HDV) is a viroid7 8. PRIONS protein only do not contain any nucleic acid Cause animal diseases e.g.spongiform encephalopathy Cause human diseases e.g. Creutzfeldt-Jakob disease. 9. Why are some viruses harmful? Virus invades cellVirus forces cell to make copies of virusEventually the cell explodes, releasing all of the new virusesWhen your cells make viruses instead of operating normally, YOU get sick 10. Who do viruses infect? Viruses infect Bacteria These viruses are called bacteriophages Viruses infect Plants One example is the Tobacco Mosaic Virus Viruses infect AnimalsOne example is Influenza virus Viruses infect humans 11. Bacteriophage 12. How small are the viruses 13. The smallest viruses (parvoviruses)are 20 nm (nm=10-9 m) The largest (poxviruses) are 300nm 14. Viral Structure Nucleic acid: either RNA or DNA never both. Capsid: Protein shell surround thenucleic acid. Envelope: lipid membrane acquired from the host cell. Not all viruses have it. 15. Nucleic Acid: -DNA or RNA - RNA can be ve or +ve sense. - single - or double - stranded - linear or circular - segmented or not - encodes proteins 16. proteins: a)capsid: made of capsomeres b) enzymes: - differ from host cell enzymes. - targets of antiviral therapy 17. Envelope: layer made of lipid and protein Surrounds the capsid Not found in all viruses If destroyed the virus become noninfectious. Some enveloped viruses have a matrix Some have protrusions made of glycoprotein and called spikes. 18. Virus particles exhibit 3 types of capsid symmetry: helical : most helical viruses possessan outer envelope icosahedral or cubic: may or may not possess an outer envelope Complex: does not conform to either of above (e.g. Pox viruses) 19. Helical Capsid Helical capsid is rod-like structure with the RNA in the center of the helix. A helix is made by stacking repeating units in a spiral.RNAprotein coat 20. ICOSAHEDRAL SYMMETRY20 faces 12 vertices 23 21. COMPLEX SYMMETRY cross sectionPOXVIRUS FAMILY 25 22. Capsid symmetry IcosahedralHelical Naked capsidEnvelopedMatrix Lipid Glycoprotein 23. Enveloped viruses Envelope is sensitive to Drying, heat, detergents and acid Consequences Must stay wet during transmission Transmission in large droplets andsecretions Cannot survive in the gastrointestinal tract May not need to kill cells in order to spread 24. Naked Viruses Capsid is resistant to Drying, heat, detergents and acids. Consequences Can survive in the gastrointestinal tract Survive well on environmental surfaces Spread easily via fomites Must kill host cells to release maturevirus particles 25. Differences between bacteria and viruses 26. Bacteria Viruses Growth on artificial mediaOftenNoDivision by binary fissionYesNoContain DNA and RNAYesNoprotein synthesis machineryYesNoSensitive to antibioticsYesNo
