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Chapter 13 Viruses , Viroids , and Prions. Not all pathogens are cellular!. Many infections of humans, animals, plants, and even bacteria are caused by acellular ( noncellular ) particles Acellular infectious particles include Viruses Viroids prions. Characteristics of Viruses. Virus

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Chapter 13 Viruses , Viroids , and Prions


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    1. Chapter 13 Viruses, Viroids, and Prions

    2. Not all pathogens are cellular! • Many infections of humans, animals, plants, and even bacteria are caused by acellular (noncellular) particles • Acellular infectious particles include • Viruses • Viroids • prions

    3. Characteristics of Viruses Virus Much smaller than bacteria(usually measured in nanometers) Acellularinfectious agent having either DNA or RNA They do not have both! Cause many infections in humans, animals, plants, and bacteria Cause most of the diseases that affect the industrialized world Examples: common cold, influenza, herpes, SARS, Polio, HIV

    4. E. coli (bacterium)(1000 nm  3000 nm) Figure 13.4 Sizes of selected virions Red blood cell(10,000 nm in diameter) Bacterialribosomes(25 nm) Smallpox virus(200 nm  300 nm) Poliovirus(30 nm) Bacteriophage T4(50 nm  225 nm) Bacteriophage MS2(24 nm) Tobacco mosaic virus(15 nm  300 nm)

    5. Characteristics of Viruses Cannot carry out any metabolic pathway outside of a cell Neither grow nor respond to the environment Cannot reproduce independently Recruit the cell’s (animal, plant, or bacterial cells) metabolic pathways to reproduce No cytoplasmic membrane, cytosol (liquid portion of cytoplasm), organelles Have extracellular and intracellular state

    6. Characteristics of Viruses Extracellular State (Naked or Enveloped) Protein coat (capsid) surrounding nucleic acid (naked) Some have a phospholipid envelope which surrounds the capsid (enveloped) Outermost layer provides protection and recognitionsites for host cells Intracellular State Capsid removed Virus exists inside the cell as nucleic acid (DNA or RNA)

    7. Figure 13.1 Virions-overview

    8. Differentiating Viruses We can differentiate viruses from one another based on their Genetic Material Show more variety in genomes than cells The genetic material a virus contains is the primary way scientists categorize and classify viruses

    9. Differentiating Viruses • We can differentiate viruses from one another based on their Genetic Material • Viral genome may be DNA or RNA, but never both • dsDNA, ssDNA, dsRNA, ssRNA • ds= double stranded, ss= single stranded • Linear and segmented or single and circular • Influenza virus genome has 8 linear segments of ssRNA • Much smaller than genomes of cells • Cells always have double stranded DNA • ssDNA and dsRNA are almost nonexistent in cells

    10. Differentiating Viruses Hosts of Viruses Most viruses infect only particular host’s cells Species specific Dog viruses don’t infect humans May be so specific they infect only particular kind of cell in a particular host HIV attacks helper T lymphocytes in humans but does not infect muscle or bone cells Generalists – infect many kinds of cells in many different hosts Rabies

    11. Tobacco mosaic virus infected leaf on left Figure 13.3 Hosts of viral infections-overview Bacteria (blue/gray) under attack from a bacteriophage (pink) Human WBC cytoplasmic membrane with HIV particles (blue) attached

    12. Differentiating Viruses • Host specificity • Due to viral surface proteins which have a precise affinity (attraction) for complementary proteins on the host cell membranes

    13. Differentiating Viruses • Hosts of Viruses • All types of cells can be infected by a virus • Archaeal, bacterial, plant, protozoan, fungal, animal • Most research focuses on animal and bacterial viruses • A virus that infects a bacterium is called a bacteriophage or phage

    14. Naked vs. Enveloped

    15. Differentiating Viruses Capsid Morphology Capsids Provide protection for viral nucleic acid Means of attachment to host’s cells Composed of proteinaceous subunits called capsomeres Capsomere made of single or multiple types of proteins

    16. Differentiating Viruses Viral Shapes Three basic shapes Helical (capsomeres bond in a spiral fashion) Polyhedral (close to spherical, geodesic dome) Complex (many different shapes, bullet shaped rabies virus)

    17. Figure 13.5 The shapes of virions-overview

    18. Figure 13.6 Bacteriophage T4-overview

    19. Differentiating Viruses The Viral Envelope Acquired from host cell during viral replication or release Envelope is portion of membrane system of host Composed of phospholipid bilayer and proteins Some proteins are virally coded proteins (spikes) Envelope proteins play a role in host recognition

    20. Figure 13.7 Enveloped virion-overview

    21. Viral Replication Dependent on hosts’ organelles and enzymes to produce new viral particles Lytic replication Replication cycle usually results in lysis and death of host cell Basic stages of lytic replication cycle Recognition and Attachment Entry Chromosome degraded Synthesis Assembly Release

    22. Figure 13.12 Three mechanisms of entry of animal viruses-overview

    23. Attachment Bacteriophagegenome Entry Tail sheath Outer membrane Peptidoglycan Cytoplasmicmembrane Figure 13.8 The lytic replication cycle in bacteriophages-overview Bacterialchromosome Entry Attachment Phage DNA Lytic replicationcycle of bacteriophage Bacterialchromosomedegraded Release Synthesis Phage proteins Assembly Assembly Sheath Base DNA Mature virion Tail Tail fibers Mature head Capsid

    24. Viral Replication Lysogeny Modified replication cycle Infected host cells grow and reproduce normally for generations before they lyse Inactive bacteriophage is called a prophage Induction occurs and the prophage is excised from the host chromosome Induction can occur through DNA damaging chemicals, UV light, X rays After induction the lytic cycle will occur

    25. Attachment Prophagein chromosome Entry Lambda phage Lyticcycle Figure 13.11 The lysogenic replication cycle in bacteriophages: phage lambda and E. coli Lysogeny Synthesis Release Replication ofchromosomeand virus;cell division Assembly Induction Further replications andcell divisions

    26. Transduction

    27. Viral Replication Replication of Animal Viruses Same basic replication pathway as bacteriophages Recognition and Attachment (capsid or envelope proteins recognize host cell receptors) Entry (fusion with cell membrane or endocytosis) Synthesis (DNA virus in the nucleus, RNA virus in the cytoplasm) Assembly Release (budding, exocytosis, lysis)

    28. Viral Replication Replication of Animal Viruses Envelopedviruses cause persistent infections Released from cell by budding Naked viruses are released by exocytosis or lysis

    29. Envelopedvirion Budding ofenveloped virus Figure 13.14 The process of budding in enveloped viruses Cytoplasmicmembraneof host Viral glycoproteins Viral capsid

    30. Viral Replication Replication of Animal Viruses Latency of animal viruses (chicken pox and herpes virus) Called latent viruses or proviruses When animal viruses remain dormant in host cells May be prolonged for years with no viral activity

    31. The Role of Viruses in Cancer Viruses cause 20–25% of human cancers Some viruses carry copies of oncogenes as part of their genomes Oncogenes are involved in cell division and are usually repressed (not activated) and no cancer results Some promote oncogenes already present in host Specific viruses are known to cause human cancers Kaposi’s sarcoma (HIV) Cervical cancer (HPV)

    32. Are Viruses Alive? Infectious agents with both living and non-living characteristics Living characteristics: Reproduce, but only in living host cells Can mutate Nonliving characteristics: Acellular: no cytoplasm or organelles Cannot carry out metabolism on their own Have DNA or RNA but not both

    33. Other Parasitic Particles: Viroids and Prions Characteristics of Viroids Extremely small, circular pieces of RNA that are infectious and pathogenic in plants Similar to RNA viruses, but lack capsid No known animal diseases are known to be caused by viroids

    34. Figure 13.21 One effect of viroids on plants

    35. Characteristics of Prions • Proteinaceous infectious agents • Are ONLY protein • Cause spongiform encephalopathies: • Mad cow • Scrapie • Kuru • Creutzfeld-Jakob syndrome • Resistant to proteases, UV light, heat, disinfectants • Prions only destroyed by incineration or autoclaving in NaOH

    36. Prions Characteristics of Prions Prion diseases Fatal neurological degeneration, and loss of brain matter Large vacuoles form in brain Characteristic spongy appearance

    37. Chronic wasting disease

    38. Prions • Characteristics of Prions • Proteinaceous infectious agents • Cellular PrP protein • Made by all mammals • Normal structure with -helices called cellular PrP • Prion PrP • Disease-causing form with -pleated sheets called prion PrP • Prion PrP changes shape of cellular PrP so it becomes prion PrP

    39. Figure 13.22 The two stable, three-dimensional forms of prion protein (PrP)-overview

    40. Shared characteristics of viruses, viroids, and prions • Simple compared to cells • Lack cell membranes • Composed of 1 or a few organic molecules • Lack most of the characteristics of life