Chapter 13

1. Chapter 13 Characterizing and Classifying Viruses, Viroids, and Prions

2. Characteristics of Viruses Virus Minuscule, acellular infectious agent having either DNA or RNA Causes many infections of humans, animals, plants, and bacteria Causes most of the diseases that plague the industrialized world MDufilho .

3. Characteristics of Viruses Cannot carry out any metabolic pathway Neither grow nor respond to the environment Cannot reproduce independently Recruit the cell’s metabolic pathways to increase their numbers No cytoplasmic membrane, cytosol, organelles (with one exception) Have extracellular and intracellular state MDufilho

4. Characteristics of Viruses Extracellular State Called virion Protein coat (capsid) surrounding nucleic acid Nucleic acid and capsid also called nucleocapsid Some have phospholipid envelope Outermost layer provides protection and recognition sites for host cells Intracellular State Capsid removed Virus exists as nucleic acid MDufilho

5. Figure 13.1 Virions-overview MDufilho

6. Characteristics of Viruses Genetic Material of Viruses Show more variety in nature of their genomes than do cells Primary way scientists categorize and classify viruses May be DNA or RNA, but never both dsDNA, ssDNA, dsRNA, ssRNA Linear and segmented or single and circular Much smaller than genomes of cells MDufilho

7. Viralgenome Partial genomeof E. coli Figure 13.2 The relative sizes of genomes MDufilho

8. Characteristics of Viruses Hosts of Viruses Most viruses infect only particular host’s cells Affinity of viral surface proteins for proteins on host cell May be so specific they infect only particular kind of cell in a particular host Generalists – infect many kinds of cells in many different hosts MDufilho

9. Figure 13.3 Hosts of viral infections-overview MDufilho

10. 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) MDufilho

11. Characteristics of 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 Three basic shapes - helical,polyhedral,complex MDufilho

12. Figure 13.5 The shapes of virions-overview MDufilho

13. Figure 13.6 Bacteriophage T4-overview MDufilho

14. Characteristics of 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 glycoproteins (spikes) Envelope’s proteins and glycoproteins often play role in host recognition MDufilho

15. Figure 13.7 Enveloped virion-overview MDufilho

16. Table 13.2 Families of Human Viruses MDufilho

17. Viral Replication Lysogeny Modified replication cycle Infected host cells grow and reproduce normally for generations before they lyse Temperate phages Prophages – inactive phages Lysogenic conversion results when phages carry genes that alter phenotype of a bacterium MDufilho

18. Viral Replication Replication of Animal Viruses Same basic replication pathway as bacteriophages Differences result from Presence of envelope around some viruses Eukaryotic nature of animal cells Lack of cell wall in animal cells MDufilho

19. Viral Replication Replication of Animal Viruses Attachment of animal viruses Chemical attraction Animal viruses do not have tails or tail fibers Have glycoprotein spikes or other attachment molecules that mediate attachment Uncoating Direct penetration Membrane fusion MDufilho

20. Viral Replication Replication of Animal Viruses Synthesis of animal viruses Requires different strategy depending on its nucleic acid DNA viruses often enter the nucleus RNA viruses often replicate in the cytoplasm Must consider How mRNA is synthesized What serves as template for nucleic acid replication MDufilho

21. Figure 13.13 Synthesis of proteins and genomes in animal RNA viruses-overview MDufilho

22. Viral Replication Replication of Animal Viruses Assembly and release of animal viruses Most DNA viruses assemble in nucleus Most RNA viruses develop solely in cytoplasm Number of viruses produced depends on type of virus and size and initial health of host cell Enveloped viruses cause persistent infections Naked viruses are released by exocytosis or lysis MDufilho

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

24. Viral Replication Replication of Animal Viruses Latency of animal viruses When animal viruses remain dormant in host cells May be prolonged for years with no viral activity Some latent viruses do not become incorporated into host chromosome Incorporation of provirus into host DNA is permanent MDufilho

25. The Role of Viruses in Cancer Animal’s genes dictate that some cells can no longer divide or are prevented from unlimited division Genes for cell division “turned off” or genes inhibiting division “turned on” Neoplasia Uncontrolled cell division in multicellular animal Mass of neoplastic cells is tumor Benign vs. malignant tumors Metastasis Cancers MDufilho

26. Normal state: DNA Gene for repressor Protooncogene Represses mRNA Repressor Result: No cancer Figure 13.16 The oncogene theory of the induction of cancer in humans First “hit”: Virus inserts promoter DNA Oncogene Gene for repressor Represses mRNA Repressor Result: Still no cancer Second “hit”: Virus inserts into represssor gene DNA Oncogene No repressorprotein becausegene is segmented mRNA MDufilho Result: Cancer Causes cell division Protein

27. The Role of Viruses in Cancer Environmental factors that contribute to the activation of oncogenes Ultraviolet light Radiation Carcinogens Viruses MDufilho © 2012 Pearson Education Inc.

28. The Role of Viruses in Cancer Viruses cause 20–25% of human cancers Some carry copies of oncogenes as part of their genomes Some promote oncogenes already present in host Some interfere with tumor repression Specific viruses are known to cause ~15% of human cancers Burkitt’s lymphoma Hodgkin’s disease Kaposi’s sarcoma Cervical cancer MDufilho

29. Are Viruses Alive? Some consider them complex pathogenic chemicals Others consider them the least complex living entities Use sophisticated methods to invade cells Have the ability to take control of their host cell Are able to replicate themselves MDufilho

30. 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 May appear linear due to H bonding MDufilho

31. Genome of bacteriophage T7 PSTV Figure 13.20 The RNA strand of the small potato spindle tuber viriod (PSTV) MDufilho

32. Figure 13.21 One effect of viroids on plants MDufilho

33. Other Parasitic Particles: Viroids and 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 MDufilho

34. Other Parasitic Particles: Viroids and Prions ANIMATION Prions: Overview MDufilho

35. Other Parasitic Particles: Viroids and Prions Characteristics of Prions Normally, nearby proteins and polysaccharides force PrP into cellular shape PrP mutations result in formation of prion Pr MDufilho

36. Other Parasitic Particles: Viroids and Prions ANIMATION Prions: Characteristics MDufilho

37. Other Parasitic Particles: Viroids and Prions Characteristics of Prions Prion diseases Fatal neurological degeneration, fibril deposits in brain, and loss of brain matter Large vacuoles form in brain Characteristic spongy appearance Spongiform encephalopathies Prions only destroyed by incineration or autoclaving in 1 N NaOH MDufilho

38. Other Parasitic Particles: Viroids and Prions ANIMATION Prions: Disease MDufilho

39. Viral Diseases of the Skin and Wounds Herpes Infections Signs and symptoms Slow spreading skin lesions Recurrence of lesions is common Pathogen and virulence factors Caused by human herpesviruses 1 and 2 Produce various proteins that act as virulence factors Pathogenesis Painful lesions caused by inflammation and cell death Cause fusion of cells to form syncytia MDufilho

40. Figure 19.11 Oral herpes lesions MDufilho

41. Viral STDs Genital Herpes Signs and symptoms Small blisters on or around the genitals or rectum Pathogen and virulence factors Human herpesvirus 2 causes most cases Virus can become latent in nerve cells Pathogenesis Herpesvirus kills epithelial cells at infection site Blisters may form at sites far from initial infection site Babies can become infected during birth MDufilho

42. Figure 24.12 Herpes lesions of the eyes and skin-overview MDufilho

43. Viral Diseases of the Skin and Wounds Herpes Infections Epidemiology Spread between mucous membranes of mouth and genitals Herpes infections in adults are not life threatening Diagnosis, treatment, and prevention Diagnosis made by presence of characteristic lesions Immunoassay reveals presence of viral antigens Chemotherapeutic drugs help control the disease but do not cure it MDufilho

44. Viral Diseases of the Skin and Wounds Warts Benign epithelial growths on the skin or mucous membranes Can form on many body surfaces Various papillomaviruses cause warts Most warts are harmless Transmitted via direct contact and fomites Diagnosed by observation Various techniques to remove warts New warts can develop due to latent viruses MDufilho

45. Figure 19.13 Various kinds of warts--lesions caused by papillomaviruses-overview MDufilho

46. Figure 24.13 Genital warts MDufilho

47. Viral STDs Genital Warts Signs and symptoms Warts on the genitalia and surrounding areas Large growths called condylomata acuminata may form Pathogen Caused by human papillomaviruses (HPV) HPV can cause various cancers MDufilho

48. Viral STDs Genital Warts Pathogenesis and epidemiology HPVs invade skin or mucous membranes during sex Most common STD in the U.S. Diagnosis, treatment, and prevention Diagnosis made by presence of warts Variety of methods available to remove warts Vaccine available against HPV strain associated with cervical cancer MDufilho

49. Viral Diseases of the Skin and Wounds Chickenpox and Shingles Signs and symptoms Chickenpox characterized by lesions on the back and trunk that spread across body Shingles lesions localized to skin along an infected nerve Pathogen Varicella-zoster virus (VZV) causes both diseases Pathogenesis Infected dermal cells cause rash characteristic of chickenpox Virus becomes latent in nerve ganglia Reactivated VZV causes shingles MDufilho

50. Viral Diseases of the Skin and Wounds Chickenpox and Shingles Epidemiology Chickenpox occurs mostly in children Disease is more severe in adults Risk of shingles increases with age Diagnosis, treatment, and prevention Diagnosis based on characteristic lesions Treatment based on relief of symptoms Vaccine available against chickenpox MDufilho