An Introduction to Viruses: General Characteristics
Chapter 6 provides an overview of the general characteristics of viruses, which are obligate intracellular parasites containing either DNA or RNA. Viruses depend on the synthetic machinery of the host cell
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PowerPoint presentation about 'An Introduction to Viruses: General Characteristics'. This presentation describes the topic on Chapter 6 provides an overview of the general characteristics of viruses, which are obligate intracellular parasites containing either DNA or RNA. Viruses depend on the synthetic machinery of the host cell. The key topics included in this slideshow are . Download this presentation absolutely free.
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Slide1Chapter 6 Chapter 6 An Introduction to Viruses An Introduction to Viruses
Slide2General characteristics General characteristics Viruses = obligate, intracellular parasites that contain either DNA or RNA; depend on synthetic machinery of host cell for replication of the infectious particle called the virion Viruses = obligate, intracellular parasites that contain either DNA or RNA; depend on synthetic machinery of host cell for replication of the infectious particle called the virion Size - submicroscopic; most are beyond limit of resolution of light microscope (exception = poxviruses; can be seen under light microscope with proper illumination); size varies from 25 m to 250 m (approximately 0.1 diameter of E. coli cell) Size - submicroscopic; most are beyond limit of resolution of light microscope (exception = poxviruses; can be seen under light microscope with proper illumination); size varies from 25 m to 250 m (approximately 0.1 diameter of E. coli cell)
Slide3Structural components Structural components Nucleic acid core = genome - may be either DNA or RNA but never both; may be either single stranded (ss) or double stranded (ds); used in classification (ssDNA, ssRNA, dsDNA, or dsRNA); may be either linear or circular; found in all mature virus particles (virions) Nucleic acid core = genome - may be either DNA or RNA but never both; may be either single stranded (ss) or double stranded (ds); used in classification (ssDNA, ssRNA, dsDNA, or dsRNA); may be either linear or circular; found in all mature virus particles (virions)
Slide4Structural components Structural components Protein coat = capsid - shell surrounding and protecting nucleic acid core; composed of identical subunits called capsomeres; subunits undergo self-assembly into geometric shapes around nucleic acid core; number of capsomeres is unique for each virus; contain receptors that combine with complementary sites on host cell surface for attachment; found in all mature viruses Protein coat = capsid - shell surrounding and protecting nucleic acid core; composed of identical subunits called capsomeres; subunits undergo self-assembly into geometric shapes around nucleic acid core; number of capsomeres is unique for each virus; contain receptors that combine with complementary sites on host cell surface for attachment; found in all mature viruses
Slide5Structural components Structural components Envelope - lipid and protein membrane surrounding virion; origin = host cell membrane; lipid of envelope make viruses susceptible to lipid solvents and are more heat labile than non-enveloped viruses; found only in viruses that exit cell through cell membrane or bud through nuclear membrane during maturation; viruses without envelope are called naked Envelope - lipid and protein membrane surrounding virion; origin = host cell membrane; lipid of envelope make viruses susceptible to lipid solvents and are more heat labile than non-enveloped viruses; found only in viruses that exit cell through cell membrane or bud through nuclear membrane during maturation; viruses without envelope are called naked Glycoprotein spikes - surface projections coded for by viruses; manufactured in host cell and become incorporated into host cell membrane prior to virion maturation; often act as receptors for host cells; may agglutinate rbc's Glycoprotein spikes - surface projections coded for by viruses; manufactured in host cell and become incorporated into host cell membrane prior to virion maturation; often act as receptors for host cells; may agglutinate rbc's
Slide6ShapeShape Protein shell may assume either icosahedral (20 equilateral triangular faces = cubic symmetry) or helical (rod-shaped particles composed of repeating protein subunits surrounding core in a helical fashion = helical symmetry) shape; some viruses (e.g. poxviruses) have a complex shape Protein shell may assume either icosahedral (20 equilateral triangular faces = cubic symmetry) or helical (rod-shaped particles composed of repeating protein subunits surrounding core in a helical fashion = helical symmetry) shape; some viruses (e.g. poxviruses) have a complex shape
Slide7Classification of Viruses Classification of Viruses Criteria for classification Criteria for classification Nature of disease (e.g. neurotropic viruses) Nature of disease (e.g. neurotropic viruses) Organ system most frequently involved (e.g. hepatitis viruses) Organ system most frequently involved (e.g. hepatitis viruses) Physiochemical characteristics Physiochemical characteristics Type of nucleic acid (ssDNA, ssRNA, dsDNA, dsRNA) Type of nucleic acid (ssDNA, ssRNA, dsDNA, dsRNA) Presence or absence of envelope - ether sensitive are enveloped while ether resistant = naked Presence or absence of envelope - ether sensitive are enveloped while ether resistant = naked Size (small = picornaviruses to large = poxviruses) Size (small = picornaviruses to large = poxviruses) Vertebrate viruses classified into 13 families of RNA- containing viruses and 7 families of DNA viruses Vertebrate viruses classified into 13 families of RNA- containing viruses and 7 families of DNA viruses
Slide8Replication Cycle Replication Cycle
Slide9Attachment & Adsorption Attachment & Adsorption cell surface phenomenon; adsorption involves specific interaction between sites on viruses and receptors on cell membrane of host cell via ionic or electrostatic interactions (e.g. HIV attaches via gp120 of virus to CD4 receptor on T cells); tropism = susceptibility of only certain cells to particular viral agent - may be related to presence or absence of receptors on host cell cell surface phenomenon; adsorption involves specific interaction between sites on viruses and receptors on cell membrane of host cell via ionic or electrostatic interactions (e.g. HIV attaches via gp120 of virus to CD4 receptor on T cells); tropism = susceptibility of only certain cells to particular viral agent - may be related to presence or absence of receptors on host cell
Slide10PenetrationPenetration usually occurs by phagocytosis (naked viruses) or fusion (enveloped viruses) usually occurs by phagocytosis (naked viruses) or fusion (enveloped viruses)
Slide11Uncoating & Biosynthesis Uncoating & Biosynthesis Uncoating - involves removal of protein coat and release of viral nucleic acid; may involve host enzymes (proteolytic) and/or viral coded enzymes brought in with virus or synthesized in new host Uncoating - involves removal of protein coat and release of viral nucleic acid; may involve host enzymes (proteolytic) and/or viral coded enzymes brought in with virus or synthesized in new host Biosynthesis Biosynthesis Involves synthesis of viral nucleic acid, viral coat proteins and virus-specific enzymes - each is synthesized separately Involves synthesis of viral nucleic acid, viral coat proteins and virus-specific enzymes - each is synthesized separately Site of viral synthesis varies depending on type of NA; generalizations include: Site of viral synthesis varies depending on type of NA; generalizations include: Most DNA viruses synthesize DNA in nucleus of host cell & proteins (structural & enzymatic) in cytoplasm Most DNA viruses synthesize DNA in nucleus of host cell & proteins (structural & enzymatic) in cytoplasm Most RNA viruses synthesize all viral components in cytoplasm Most RNA viruses synthesize all viral components in cytoplasm No infectious virus particles are detected during this period = eclipse period No infectious virus particles are detected during this period = eclipse period
Slide12Maturation (Assembly of virions) Maturation (Assembly of virions) Maturation (Assembly of virions) - involves combining of synthesized NA and protein; occurs in nucleus with most DNA viruses & cytoplasm with most RNA viruses; most capsids self-assemble around genome; defective (incomplete) viruses occur when NA is not coated by protein (empty coats or non-viral NA placed in coat) Maturation (Assembly of virions) - involves combining of synthesized NA and protein; occurs in nucleus with most DNA viruses & cytoplasm with most RNA viruses; most capsids self-assemble around genome; defective (incomplete) viruses occur when NA is not coated by protein (empty coats or non-viral NA placed in coat)
Slide13ReleaseRelease Host cell disintegration (lysis) - occurs when cell fills with virus particles; causes death of cell and release of naked viruses Host cell disintegration (lysis) - occurs when cell fills with virus particles; causes death of cell and release of naked viruses Slow release through cell membrane as part of budding process - results in release of enveloped viruses; cell may not die for a long time & can continue to shed virus particles; some proteins synthesized under viral control become incorporated into CM of host and become incorporated into virion as virus buds through membrane (glycoprotein spikes); important in cellular immunity (proteins are antigenic and host recognizes these proteins as foreign mounting immune response against cells with these proteins in their membranes) Slow release through cell membrane as part of budding process - results in release of enveloped viruses; cell may not die for a long time & can continue to shed virus particles; some proteins synthesized under viral control become incorporated into CM of host and become incorporated into virion as virus buds through membrane (glycoprotein spikes); important in cellular immunity (proteins are antigenic and host recognizes these proteins as foreign mounting immune response against cells with these proteins in their membranes)
Slide14Effects of Viruses on Host Cells Effects of Viruses on Host Cells Effects of viruses on host cells = cytopathic effects (CPE) – morphologic alterations that occur in virus-infected cells; often visible in light microscope Effects of viruses on host cells = cytopathic effects (CPE) – morphologic alterations that occur in virus-infected cells; often visible in light microscope Inclusions = intracellular structures; may be cytoplasmic or nuclear; represent aggregates of viral nucleic acid or protein, mature virions or degenerative cell reaction products; serve as markers of certain viral infections (e.g. Negri bodies = intracytoplasmic inclusions of rabies infection; Owl's eyes = cytomegaloviruses) Inclusions = intracellular structures; may be cytoplasmic or nuclear; represent aggregates of viral nucleic acid or protein, mature virions or degenerative cell reaction products; serve as markers of certain viral infections (e.g. Negri bodies = intracytoplasmic inclusions of rabies infection; Owl's eyes = cytomegaloviruses)
Slide15Effects of Viruses on Host Cells Effects of Viruses on Host Cells Syncytia (multinucleated giant cells) - represent fusion of virus-infected cells with contiguous uninfected cells; form giant cells with many nuclei; caused by virus- induced changes in CM; allows cell to cell spread of virus without coming in contact with humoral immunity (e.g. respiratory syncytial viruses) Syncytia (multinucleated giant cells) - represent fusion of virus-infected cells with contiguous uninfected cells; form giant cells with many nuclei; caused by virus- induced changes in CM; allows cell to cell spread of virus without coming in contact with humoral immunity (e.g. respiratory syncytial viruses) Cell necrosis or death - rate of destruction depends on degree of shut-down of host cellular functions and rate of synthesis with accumulation of toxic viral structural proteins; cytolytic effects are visible microscopically and when destruction is extensive, effects can be seen with naked eye forming areas referred to as plaques in tissue cultures; in vivo, destruction of large numbers of cells results in impairment of tissue or organ structure & function Cell necrosis or death - rate of destruction depends on degree of shut-down of host cellular functions and rate of synthesis with accumulation of toxic viral structural proteins; cytolytic effects are visible microscopically and when destruction is extensive, effects can be seen with naked eye forming areas referred to as plaques in tissue cultures; in vivo, destruction of large numbers of cells results in impairment of tissue or organ structure & function
Slide16Host Cell Response Host Cell Response Non-immunological = interferon production Non-immunological = interferon production Immunological Immunological Humoral - inhibits adsorption of virus to host cell surface (blocks receptors on virus) Humoral - inhibits adsorption of virus to host cell surface (blocks receptors on virus) Cell mediated - acts against viral-specific proteins incorporated into host cell membrane; involves destruction of infected cell by sensitized T cells Cell mediated - acts against viral-specific proteins incorporated into host cell membrane; involves destruction of infected cell by sensitized T cells
Slide17Laboratory Techniques with Viruses Laboratory Techniques with Viruses Viral isolation - require present of living cells Viral isolation - require present of living cells Laboratory animals - suckling animals usually more susceptible than adult animals (e.g. suckling mice or guinea pigs) Laboratory animals - suckling animals usually more susceptible than adult animals (e.g. suckling mice or guinea pigs) Embryonated eggs - chick or duck eggs used most often; viruses grow in embryo or on membranes Embryonated eggs - chick or duck eggs used most often; viruses grow in embryo or on membranes
Slide18Laboratory Techniques with Viruses Laboratory Techniques with Viruses Tissue culture Tissue culture Primary cell lines - prepared directly from tissue excised from humans or animals (e.g. primary monkey kidney = PMK cells) Primary cell lines - prepared directly from tissue excised from humans or animals (e.g. primary monkey kidney = PMK cells) Continuous cell lines - cells that have been passaged indefinitely; are usually derived from malignant or premalignant cells (e.g. HeLa from human cervical carcinoma and HEp-2 from human laryngeal carcinoma Continuous cell lines - cells that have been passaged indefinitely; are usually derived from malignant or premalignant cells (e.g. HeLa from human cervical carcinoma and HEp-2 from human laryngeal carcinoma
Slide19Laboratory Techniques with Viruses Laboratory Techniques with Viruses Serological methods Serological methods Patient's serum - show rising titer of antibodies to specific viruses Patient's serum - show rising titer of antibodies to specific viruses Tissue culture - used to identify viruses that have been grown in the laboratory; have specific antisera for viruses; most often show neutralization of effect by antisera as indication of identity of virus Tissue culture - used to identify viruses that have been grown in the laboratory; have specific antisera for viruses; most often show neutralization of effect by antisera as indication of identity of virus