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Viruses

Viruses. General Structure of Viruses. -> Size range – most <0.2 μ m; requires electron microscope -> Virion – fully formed virus able to establish an infection. Viruses. Form and Function of Viruses. Viruses. General Structure of Viruses. Capsids

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Viruses

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  1. Viruses

  2. General Structure of Viruses -> Size range – • most <0.2 μm; requires electron microscope -> Virion – fully formed virus able to establish an infection Viruses

  3. Form and Function of Viruses Viruses

  4. General Structure of Viruses Capsids • All viruses have capsids - protein coats that enclose and • protect their nucleic acid. • Each capsid is constructed from identical subunits called • capsomers made of protein. • The capsid together with the nucleic acid are • nucleoscapsid. • Some viruses have an external covering called envelope; • those lacking an envelope are naked Viruses

  5. General Structure of Viruses Capsids - Envelope Viruses

  6. General Structure of Viruses Helical Capsid Viruses -> continuous helix of capsomers forming a cylindrical nucleocapsid Influenza virus

  7. General Structure of Viruses IcosahedralCapsid Viruses • -> 20-sided with 12 corners • vary in the number of capsomers • Each capsomer may be made of 1 or several proteins. • - Some are enveloped. Herpes simplex virus Adenovirus

  8. General Structure of Viruses Viruses Viral envelope • -> mostly animal viruses • -> acquired when the virus leaves the host cell • -> exposed proteins on the outside of the envelope, called • spikes, essential for attachment of the virus to the host cell

  9. Function of Capsid/Envelope Viruses -> Protects the nucleic acid when the virion is outside the host cell -> Helps to bind the virion to a cell surface and assists the penetration of the viral DNA or RNA into a suitable host cell

  10. Viral Morphology Viruses

  11. General Structure of Viruses Complex viruses: atypical viruses • -> Poxviruses lack a typical capsid and are covered by a • dense layer of lipoproteins. • -> Some bacteriophages have a polyhedral nucleocapsid • along with a helical tail and attachment fibers. Viruses

  12. General Structure of Viruses Nucleic Acids Viral genome – either DNA or RNA but never both -> Carries genes necessary to invade host cell and redirect cell’s activity to make new viruses -> Number of genes is low - varies for each type of virus – few to hundreds Viruses

  13. General Structure of Viruses DNA viruses • -> usually double stranded (ds) but may be single • stranded (ss) • -> circular or linear RNA viruses • -> usually single stranded, may be double stranded, • may be segmented into separate RNA pieces • -> ssRNA genomes ready for immediate translation are • positive-sense RNA. • -> ssRNA genomes that must be converted into proper • form are negative-sense RNA. Viruses

  14. General Structure of Viruses Viruses Pre-formed enzymes may be present. • -> polymerases – DNA or RNA • -> replicases – copy DNA • -> reverse transcriptase –synthesis of DNA from RNA • (AIDS virus)

  15. How Viruses are classified -> Main criteria presently used are structure, chemical composition, and genetic makeup. -> No taxa above Family (no kingdom, phylum, etc.) -> Currently recognized: 3 orders, 63 families, and 263 genera of viruses -> Family name ends in -viridae, i.e.Herpesviridae -> Genus name ends in -virus, Simplexvirus -> Herpes simplex virus I (HSV-I) Viruses

  16. How Viruses are classified Viruses

  17. How Viruses are classified Viruses

  18. Modes of Viral Multiplication General phases in animal virus multiplication cycle: • -> Adsorption - binding of virus to • specific molecule on host cell • -> Penetration - genome enters • host cell • -> Uncoating – the viral nucleic acid • is released from the capsid • -> Synthesis – viral components are • produced • -> Assembly– new viral particles • are constructed • -> Release – assembled viruses are • released by budding (exocytosis) • or cell lysis Viruses

  19. Adsorption and Host Range -> Virus coincidentally collides with a susceptible host cell and adsorbs specifically to receptor sites on the cell membrane -> Spectrum of cells a virus can infect – host range • -> hepatitis B – human liver cells • -> poliovirus – primate intestinal and nerve cells • -> rabies – various cells of many mammals Viruses

  20. Penetration Flexible cell membrane is penetrated by the whole virus or its nucleic acid by: • -> endocytosis – entire virus is engulfed and enclosed in a vacuole or vesicle • -> fusion – envelope merges directly with membrane resulting in nucleocapsid’s • entry into cytoplasm Viruses Endocytosis Fusion

  21. Replication and Protein Production Viruses -> Varies depending on whether the virus is a DNA or RNA virus -> DNA viruses generally are replicated and assembled in the nucleus. -> RNA viruses generally are replicated and assembled in the cytoplasm. -> Positive-sense RNA contain the message for translation. • -> Negative-sense RNA must be converted into positive-sense • message.

  22. Release Assembled viruses leave host cell in one of two ways: • -> budding – exocytosis; • nucleocapsid binds to membrane • which pinches off and sheds the • viruses gradually; cell is not • immediately destroyed • -> lysis – nonenveloped and • complex viruses released when • cell dies and ruptures Number of viruses released is variable • -> 3,000-4,000 released by poxvirus • -> >100,000 released by poliovirus Budding Viruses Budding of HIV from T-cells

  23. Damage to Host Cells Cytopathic effects - virus-induced damage to cells: • 1. Changes in size & shape • 2. Cytoplasmic inclusion bodies • 3. Nuclear inclusion bodies • 4. Cells fuse to form multinucleated cells. • 5. Cell lysis • 6. Alter DNA • 7. Transform cells into cancerous cells Viruses

  24. Damage to Host Cells Cytopathic effects Viruses Herpes simplex virus ->multinucleated giant cells (skin cells) Cytosolic inclusion bodies

  25. Damage to Host Cells Viruses

  26. Persistent Infections -> Cells can maintain a carrier relationship Viruses -> Persistent infections - cell harbors the virus and is not immediately lysed -> Can last weeks or host’s lifetime; several can periodically reactivate – chronic latent state • - measles virus – may remain hidden in brain cells for many years • - herpes simplex virus – cold sores and genital herpes • - herpes zoster virus – chickenpox and shingles

  27. Persistent Infections -> Cells can maintain a carrier relationship Viruses -> Some animal viruses enter host cell and permanently alter its genetic material resulting in cancer – transformationof the cell. -> Transformed cells have increased rate of growth, alterations in chromosomes, and capacity to divide for indefinite time periods resulting in tumors. -> Mammalian viruses capable of initiating tumors are called oncoviruses. • - Papillomavirus – cervical cancer • - Epstein-Barr virus – Burkitt’s lymphoma

  28. Bacteriophages ->Bacteriophages – bacterial viruses (phages) -> Most widely studied are those that infect Escherichia coli – complex structure, DNA -> Multiplication goes through similar stages as animal viruses. -> Only the nucleic acid enters the cytoplasm - uncoating is not necessary. -> Release is a result of cell lysis induced by viral enzymes and accumulation of viruses - lytic cycle. Viruses

  29. Bacteriophages - Replication 1. Adsorption – binding of virus to specific molecule on host cell 2. Penetration –genome enters host cell 3. Replication – viral components produced 4. Assembly - viral components assembled 5. Maturation – completion of viral formation 6. Release – viruses leave cell to infect other cells Viruses

  30. Bacteriophages – Life Cycles Viruses

  31. Bacteriophages Lysogeny: The Silent Virus Infection Viruses -> Not all phages complete the lytic cycle. -> Some DNA phages, called temperate phages, undergo adsorption and penetration but don’t replicate. -> The viral genome inserts into bacterial genome and becomes an inactive prophage - the cell is not lysed. -> Prophage is retained and copied during normal cell division resulting in the transfer of temperate phage genome to all host cell progeny – lysogeny. ->Induction can occur resulting in activation of lysogenic prophage followed by viral replication and cell lysis.

  32. Bacteriophages Lysogeny: The Silent Virus Infection Viruses -> Lysogeny results in the spread of the virus without killing the host cell. -> Phage genes in the bacterial chromosome can cause the production of toxins or enzymes that cause pathology– lysogenic conversion. • - Corynebacterium diphtheriae • - Vibrio cholerae • - Clostridium botulinum

  33. Bacteriophages Viruses

  34. Detection and Treatment of Animal Viral Infections Viruses -> More difficult than other agents -> Consider overall clinical picture -> Take appropriate sample • - Infect cell culture- look for characteristic cytopathic effects • - Screen for parts of the virus • Screen for immune response to virus (antibodies) -> Antiviral drugs can cause serious side effects

  35. Prions Prions - misfolded proteins, contain no nucleic acid • -> cause transmissible spongiform encephalopathies - neurodegenerative diseases (fibrillation of proteins) • -> common in animals: - scrapie in sheep & goats • - bovine spongiform encephalopathies (BSE), • mad cow disease • - humans – Creutzfeldt-Jakob Syndrome (CJS) !!! Extremely resistant to usual sterilization techniques !!! Viruses

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