Intro to Virology Flashcards
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)
- Genome, as a nucleoprotein complex
- Enzymes
- Auxiliary proteins that aid in disassembly after entry
- Structural proteins
- Attachment proteins
- 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)
- Attachment
- Penetration
- Uncoating
- Synthesis of components
- Assembly of viral components
- Exit/maturation
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
+mRNA
The replication pathway a virus uses is reflected in, and depends upon, the ______ it encodes
enzymes
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
budding
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
