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MEDICAL VIROLOGY Assist Prof.Dr.Baydaa Hameed

MEDICAL VIROLOGY Assist Prof.Dr.Baydaa Hameed. VIRUSES CAN BE USEFUL. VACCINE DEVELOPMENT GENE THERAPY TOOLS TO INVESTIGATE HOST CELLS. CLASSIFICATION. NUCLEIC ACID CAPSID PRESENCE OF ENVELOPE REPLICATION STRATEGY. CLASSIFICATION NUCLEIC ACID. RNA or DNA segmented or non-segmented

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MEDICAL VIROLOGY Assist Prof.Dr.Baydaa Hameed

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  1. MEDICAL VIROLOGYAssist Prof.Dr.Baydaa Hameed

  2. VIRUSES CAN BE USEFUL • VACCINE DEVELOPMENT • GENE THERAPY • TOOLS TO INVESTIGATE HOST CELLS

  3. CLASSIFICATION • NUCLEIC ACID • CAPSID • PRESENCE OF ENVELOPE • REPLICATION STRATEGY

  4. CLASSIFICATIONNUCLEIC ACID • RNA or DNA • segmented or non-segmented • linear or circular • single-stranded or double-stranded • if single-stranded RNA • is genome mRNA (+) sense or complementary to mRNA (-) sense

  5. CLASSIFICATION CAPSID • symmetry • icosahedral, helical, complex • number of capsomers if icosahedral • enveloped or non-enveloped

  6. :Classification and nomenclature of viruses • viral genome properties: • Type of nucleic acid (may be DNA or RNA). • Molecular weight of nucleic acid. • Number of strands (whether single strand or double strand)of nucleic acid. • Nucleic acid form(may be linear or circular ). • Number and size of nucleic acid segments. • Manner of replication.

  7. Capsid properties including : • Shape and size of viral particle. • Number of capsomeres in capsid. • Type of symmetry of nucleoprotein. • Presence or absence of envelope. • Presence or absence of spikes.

  8. Biological properties including: • Susceptibility to physical and chemical agents , especially ether and detergents , and stability to pH and temperature. • Host specificity (natural host range). • Mode of viral transmission. • Organ specificity(tissue tropism) and pathogenicity .

  9. The nomenclature of viruses • is not as in other organisms, but classified into groupings which called families, the family names have the suffix-viridae . Each family , subdivided into genera. The genus names carry the suffix-virus.

  10. The nomenclature of viruses • Names of viruses are derived from: • The name of disease caused by virus(eg:Influenza virus, Hepatitis virus). • The locality where the virus was first isolated (such as ; West Nile virus). • The name of scientists responsible for isolating virus(such as; Epstein-Barr virus). • Unique epidemiological characteristics of virus (such as; Arboviruses, these are arthropod-borne viruses).

  11. DNA VIRUSES DOUBLE STRANDED SINGLE STRANDED COMPLEX NON-ENVELOPED ENVELOPED PARVOVIRIDAE POXVIRIDAE ENVELOPED NON-ENVELOPED HERPESVIRIDAE HEPADNAVIRIDAE CIRCULAR LINEAR All families shown are icosahedral except for poxviruses PAPILLOMAVIRIDAE POLYOMAVIRIDAE (formerly grouped together as the PAPOVAVIRIDAE) ADENOVIRIDAE Modified from Volk et al., Essentials of Medical Microbiology, 4th Ed. 1991

  12. RNA VIRUSES SINGLE STRANDED SINGLE STRANDED DOUBLE STRANDED positive sense negative sense ENVELOPED NONENVELOPED ENVELOPED NONENVELOPED HELICAL ICOSAHEDRAL HELICAL ICOSAHEDRAL ICOSAHEDRAL ORTHOMYXOVIRIDAE FLAVIVIRIDAE CORONAVIRIDAE PICORNAVIRIDAE REOVIRIDAE PARAMYXOVIRIDAE CALICIVIRIDAE ASTROVIRIDAE TOGAVIRIDAE RHABDOVIRIDAE RETROVIRIDAE FILOVIRIDAE BUNYAVIRIDAE ARENAVIRIDAE Modified from Volk et al., Essentials of Medical Microbiology, 4th Ed. 1991

  13. BASIC STEPS IN VIRAL LIFE CYCLE • ADSORPTION • PENETRATION • UNCOATING AND ECLIPSE • SYNTHESIS OF VIRAL NUCLEIC ACID AND PROTEIN • ASSEMBLY (maturation) • RELEASE

  14. ADSORPTION

  15. ADSORPTION • TEMPERATURE INDEPENDENT • REQUIRES VIRAL ATTACHMENT PROTEIN • CELLULAR RECEPTORS

  16. PENETRATION - ENVELOPED VIRUSES FUSION WITH PLASMA MEMBRANE ENTRY VIA ENDOSOMES

  17. PENETRATION herpesviruses, paramyxoviruses, HIV

  18. PENETRATION - ENVELOPED VIRUSES FUSION WITH PLASMA MEMBRANE ENTRY VIA ENDOSOMES, FUSION WITH ACIDIC ENDOSOME MEMBRANE

  19. H+

  20. PENETRATION - ENVELOPED VIRUSES from Schaechter et al, Mechanisms of Microbial Disease, 3rd ed, 1998

  21. VIRUS UPTAKE VIA ENDOSOMES • CALLED • VIROPEXIS / ENDOCYTOSIS / PINOCYTOSIS

  22. PENETRATIONNON-ENVELOPED VIRUSES entry directly across plasma membrane:

  23. H+

  24. UNCOATING • NEED TO MAKE GENOME AVAILABLE • ONCE UNCOATING OCCURS, ENTER ECLIPSE PHASE • ECLIPSE PHASE LASTS UNTIL FIRST NEW VIRUS PARTICLE FORMED

  25. SYNTHESIS OF VIRAL NUCLEIC ACID AND PROTEIN • MANY STRATEGIES • NUCLEIC ACID MAY BE MADE IN NUCLEUS OR CYTOPLASM • PROTEIN SYNTHESIS IS ALWAYS IN THE CYTOPLASM

  26. ASSEMBLY AND MATURATION • NUCLEUS • CYTOPLASM • AT MEMBRANE

  27. smallpox virus cytoplasmic assembly and maturation F. A. Murphy, School of Veterinary Medicine, University of California, Davis. http://www.vetnet.ucdavis.edu/fam_graphics/download.html

  28. RELEASE • LYSIS • BUDDING THROUGH PLASMA MEMBRANE • NOT EVERY RELEASED VIRION IS INFECTIOUS

  29. HIV budding and maturation Hsiung, GD et al., Diagnostic Virology 1994 p204 (D. Medina)

  30. Viral Replication • When a virus infects a cell, nucleic acid must be uncoated and gain access to metabolic machinery of cell. • Virus life cycle is characterized by: • attachment • penetration, with entry of nucleic acid into cell • early expression of virus genes (either directly by translation, if virus contains "+" RNA, or indirectly after transcription and then translation) • replication of virus nucleic acid • synthesis of new virion components • packaging and assembly of new virions • exit from cell

  31. Attachment • specific binding of a virion protein (the anti-receptor) to a constituent of the cell surface (the receptor) • e.g. hemagglutinin of influenza virus • some complex viruses (HSV) may have more than one species of anti-receptor molecule • Penetration • energy-dependent step • occurs almost instantaneously after attachment

  32. After the virus attaches to the host cell, it can enter the cell by several mechanisms: • Transfer of the entire viral particle across the cell membrane by endocytosis • Transfer of only the viral genome through the cell membrane • Fusion of the viral envelope with the host cell membrane

  33. Uncoating • at same time as penetration or shortly after • separation of viral nucleic acid (n.a.) from outer structural components • Released as: • free n.a. (picornaviruses) • as nucleocapsid (reoviruses) = may need acidic pH in endosome • viruses only infectious agent for which dissolution of infecting agent obligatory step in replicative pathway • Expression of viral genome and synthesis of viral components

  34. After the viral nucleic acid is released inside the host cell: • The transcription and translation processes of the host cell are redirected for the production of viral proteins and nucleic acids • The different types of nucleic acid genomes are expressed and replicated in several ways: • DNA genomes undergo replication-using processes similar to cellular replication • RNA genomes may be +ssRNA; Can be read directly as an mRNA or reverse transcribed by reverse transcriptase into DNA • RNA genomes may also be -ssRNA; The RNA must first be used as a template to form +mRNAs

  35. Assembly and Release • Components of capsid synthesis directed by late genes • Assembly of enveloped viruses needs interaction with plasma membrane which has been modified • Final stage of infection • Enveloped viruses released gradually by budding or exocytosis • Naked viruses accumulate in cytoplasm and released during lysis

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