Introduction to Viruses

1. IntroductiontoViruses

2. Thestudentshould be ableto • Define the structure of viruses • Describe the importance of structure for transmission • Classify the viruses • List the steps in viral replication • List at least two virus attachment protein and receptor • List the ways of viral transmission • Describe the risk factors for viral infection

3. Viruses • Small size • Filtrable agents • Obligate intracellular parasites: Can not make energy or proteins independently of a host cell

4. Viral genome • RNA or DNA • Never both !!!!!! • A naked capsid or +envelope • Do not replicate by division=binary fission • Viral components are produced in the host cell and assembled.

5. Consequences of viral properties • Viruses are not living • Viruses must be infectious to endure in nature. • Viruses must be able to use host cell processes to produce their components (viral messenger RNA, protein and identical copies of the genome)

6. Consequences of viral properties • Viruses must encode any required processes not provided by the cell. • Viral components must self-assemble.

7. Knowledge of • the structural (size and morphology) • Genetic (type and structure of nucleic acid) Provides insight how the virus replicates, spreads and causes disease.

8. Viruses • Very small • Nanometers (nm) • Clinically important viruses range from 18nm (parvoviruses) to 300 nm (poxviruses).

9. Viruses • Range from small and simple (parvoviruses and picornaviruses) to • Large and complex viruses (pox viruses and herpesviruses)

11. Naming of viruses • Structure: size, morphology and nucleic acid (picornavirus (small RNA) togavirus (cloak) • Members of its family : papovavirus (papilloma, polyoma, vacuolating viruses) • The disease they cause:smallpox(poxviruses)

12. Naming of viruses • Tissue or organ tropism: adenovirus, enterovirus, reovirus(respiratory,enteric, orphan) • Place of isolation: Norwalk, Coxsackie and many toga,arena and bunyaviruses are named after African places where they were first isolated.

13. Means of classification • Biochemical characteristics: structure and mode of replication: current means • Host cell (host range): animal (human, mouse, bird), plant, bacteria • Means of transmission: arboviruses by insects • Disease: ensephalites, hepatitis viruses

14. Viruses • DNA viruses: 6 families • Poxviridae • Herpesviridae • Adenoviridae • Hepadnaviridae • Papovaviridae • Parvoviridae

15. RNA viruses • Paramyxoviridae • Orthomyxoviridae • Coronaviridae • Arenaviridae • Rhabdoviridae Picornaviridae • Filoviridae Togaviridae • Bunyaviridae Flaviviridae • Retroviridae Caliciviridae • Reoviridae Delta

16. Virus capsid • Helical (rod) • İcosahedral (spherical)

20. Naked viruses • DNA or RNA +structural proteins= Nucleocapsid

21. Enveloped viruses Nucleocapsid+envelope

26. DNA genome • Double stranded • Single stranded • Linear • Circular

27. DNA viruses • Enveloped: • Pox • Herpes • Hepadna • Naked: • Papova • Adeno • Parvo (ss)

28. DNA viruses • Enveloped: • Pox (ds-linear) • Herpes (ds,linear) • Hepadna(ds, circular, contains a single stranded region!!!!!) • Naked: • Papova (ds, circular) • Adeno (ds, linear) • Parvo (ss,linear)

29. RNA viruses • Mostly single stranded • Reoviruses ds • Segmented: orthomyxoviruses, reoviruses, arenaviruses

30. Naked viruses • Stable to environmental conditions. • Temperature, acid, proteases, detergents, drying

31. Naked viruses • Can be spread easily(on fomites, from hand to hand) • Can dry out but retain infectivity • Many of them are transmitted by fecal-oral route • Resistant to acid and bile of the enteric tract

32. Enveloped viruses • Environmentally labile • Must stay wet • Spreades in large droplets, secretions, respiratory droplets, organ transplants and blood transfusion

33. Steps in viral replication (I) • Recognition of the target cell • Attachment • Penetration • Uncoating • Macromolecular synthesis • Assembly of virus • Buddding of enveloped viruses • Release of virus

34. Steps in viral replication (II) • Macromolecular synthesis: -early mRNA and nonstructural protein synthesis -replication of the genome -latemRNA and structural protein synthesis -posttranslational modification of protein

35. Viral attachment proteins(VAP) • Rhinovirus VP1-VP2-VP3 complex • Adenovirus Fiber protein • Rotavirus VP7 • Rabies G protein • Influenza A Hemaglutinin • HIV gp120

36. Viral attachment proteins(VAP) • Epstein-Barr virus gp350 and gp220 • Measles HA

37. Viral receptors • Epstein-Barr virus: Target cell: B cell (C3d complement receptor) • HIV: Target cell: Helper T cell(CD4 molecule and chemokine coreceptor) • Rhinovirus: Target cell: Epitelial cell (ICAM-1)

38. Viral receptors • Rabies virus: Target cell: Neuron(Acatylcoline receptor) • Influenza A virus: Target cell: Epitelial cells(sialic acid) • B19 parvovirus: Target cell: Erythroid precursors ( Erythrocyte P antigen-globoside)

39. Host range • Viruses may only bind to receptors only on spesific cell types on certain species Human, mouse • Susceptible target cell defines the tissue tropism neurotropic, lymphotropic

44. Penetration • Viropexis (receptor-mediated endocytosis): naked viruses • Fusion enveloped viruses

45. Release • Budding (enveloped) • Lysis (naked)

49. Positive-strand RNA viral genomes • Picornaviruses • Caliciviruses • Coronaviruses • Flaviviruses • Togaviruses

50. Positive-strand RNA viral genomes • They act as mRNA • Bind to ribosomes • Direct protein synthesis • Naked RNA is sufficient to initiate infection