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Flashcards in Intro to Virology Deck (30):

Virus Characteristics

  • Obgligate Intracellular Parasites
    • Rely on metabolic machinery of host cells
  • Virion size and structure can vary rapidly - virions can be "naked" or "enveloped" with membrane
  • Genome can be either RNA or DNA
  • Intracellular phase - replication, transcription, synthesis of proteins and assembly of virions take place within infected cell
  • Extracellular phase - particles must pass from cell to cell, or throughout body, or between individuals


Viruses range in diameter from ___ nM to ___ nM

20; 300


The best way to see viruses is through:

Electron Microscopy


What does polythetic mean in relation to viruses

Any given virus group is described using a collection of individual properties - important for diagnosis, identification of new viruses, clarification of life cycle, drug design


Properties used for classification

Particle type

Tissue tropism (skin, respiratory tract)

Disease etiology

Serology (Cross-reacting epitopes)

Genome type (RNA vs DNA)


Virion structure: Capsids

  • Capsids can be made from one or few proteins that have repeating protein-protein contacts
  • Capsids can form naked viruses or be surrounded by a membrane for enveloped viruses
  • Capsid/envelope are the packaging, protection, and delivery vehicle during transmission
  • Exposed proteins on capsid and membrane are targets of neutralizing antibodies


Why is it important to distinguish Naked vs. Enveloped viruses

Major difference for virus structure, entry mechanism, environmental susceptibility...


Baltimore Classification

All viruses have to make RNA - classification scheme

  • Based on groups
    • Group 2 - single stranded DNA used to make double stranded DNA
    • Group 1 - double stranded DNA used to make +mRNA
    • Group 3 - double stranded RNA used to make +mRNA
    • Group 4 - +RNA(can be directly translated to protein) and -RNA used to make +mRNA (not efficient)
    • Group 5 - retrovirus (-RNA) to +mRNA
    • Group 6 - +RNA to single stranded (-)DNA to double stranded DNA


Are there more RNA viruses or DNA viruses?

RNA viruses


Define Terms

  • Virion:
  • Virus:
  • MOI:
  • CPE:
  • Viremia:

  • Virion: the viral particle
  • Virus: an infectious particle
  • MOI: multiplicity of infection
  • CPE: cytopathic effect
  • Viremia: spread of virus throughout the body via the bloodstream



Quantification of infectious virus

  • Plaque Assay - titration of the number of infectious progeny (unit = plaque forming unit, "pfu")
  • Focus forming Assay - for viruses promoting cell growth rather than death
  • Single strep growth curve - provides quantitation of "burst size"


Tissue Culture models of infection:

  • Cytopathic effect
  • Cytolytic effect
  • Transforming
  • Induction/production/release of diagnostic enzymes
  • Expression of diagnostic antigens


What do virions contain

Naked Capsid virus = Nucleocapsid = DNA or RNA + structural proteins +/- enzymes and nucleic acid proteins

Enveloped virus = nucleocapsid + glycoproteins and membrane


Components of virion particle? (6)

  1. Genome, as a nucleoprotein complex
  2. Enzymes
  3. Auxiliary proteins that aid in disassembly after entry
  4. Structural proteins
  5. Attachment proteins
  6. Fusion proteins


Issues in understanding virus

  • Route of transmission
  • Cell/tissue tropism
  • Binding to and entering cells
  • Replication/ macromolecular synthesis
  • Morphogenesis (assembly)
  • Release from cells (egress) and host (transmission)


How is viral replication unique?

Viruses replicate by xeroxing - From one you get many new genomes


Basic steps in viral life cycle (6)

  1. Attachment
  2. Penetration
  3. Uncoating
  4. Synthesis of components
  5. Assembly of viral components
  6. Exit/maturation

A image thumb

Virion attachment

  • For both naked and enveloped viruses, a viral surface protein recognizes a receptor on the target cell
  • Binding of virus surface protein to cell surface receptor: protein or carbohydrate
  • Receptors may be ubiquitous or cell type specific
  • Virus recognition of its receptor is important for determining host range and cell tropism


Virion entry

  • Naked viruses enter the cell via endocytosis and are surrounded by endosomal membrane
  • Some enveloped viruses use their glycoprotein to induce fusion of virus and cell membranes
  • Fusion releases capsid into the cytoplasm, leaving tracs of viral protein in the plasma membrane
  • Some enveloepd viruses are taken up by endocytosis, and fusion of their membrane with the membrane of an acidic endosomal compartment releases the capsid into the cytoplasm


All viruses must synthesize _______ to make proteins



The replication pathway a virus uses is reflected in, and depends upon, the ______ it encodes



Assembly/Morphogenesis - interplay between viral components and cell trafficking pathways

  • Proteins must be sorted to the correct nuclear or cytoplasmic location
  • Capsid proteins must assemble
  • The viral genome must be selectively encapsidated
  • Viral membrane proteins must be synthesiszed, modifies, and trafficked to the correct membrane


Encapsidation into progeny virions

Once viral replication proteins are made, the viral genome is replicated - different viruses employ differen strategies and compartments for their replication - replicated genomes are assemblesd into progeny virions which are released from the cell


Many naked viruses rely on ____ _____ for release

cell lysis


Most enveloped viruses are released by ______ at the plasma membrane



Productive infection:

Latent infection:

Persistent infection:

Abortive infection:

Productive infection: Generally leads to cytopathic effect

Latent infection: No production of infectious virus particles, but genetic information of virus remains

Persistent infection: Chronic virus production

Abortive infection: Virus life cycle incomplete and virus lost


Virus effects on host cell

  • Acute cytopathic effects
    • Membrane fusion of adjacent cells to form giant multi-nucleate cells called synctia
    • Shut off of host metabolism
    • Apoptosis
    • Necrosis
  • Cell transformation
  • No apparent effect


What factors influence the species/organ/tissue/for disease? How are viruses spread between contacts?

  • Tropism
    • receptor
    • tissue specific
    • temperature or pH or activating proteases
  • Transmission
    • Respiratory
    • fecal/oral
    • direct contact
    • indirect contact


Mechanisms of pathogenesis

Virus destruction of infected cels

Viral modification of infected cell function

Immune and inflammatory responses to virus infection

Combination of several factors


Host Defenses

Innate response

Adaptive Response: humoral and cell mediated

Memory response