Introduction Flashcards
(31 cards)
Virion
Extracellular virus particle
Inc nucleic acid and protein coat (and sometimes lipoprotein membrane envelope)
Size: 20-300 nm
Common features of viruses
Obligatory intracellular growth (virion = vehicle)
Replication - via synthesis in host vs binary fission
Contain one kind of nucleic acid (RNA or DNA)
Eclipse period
Virion undetectable within hours after initial infection of cell
- virion disintegrates -> releases nucleic acid (=”eclipse”) -> subsequently assembled into new virions
Baltimore classification
Via type of genome and type of replication ex: dsDNA (Adeno, Herpes, Pox) ssDNA (Parvo) \+ssRNA (Picorna, Toga) - "+" is mRNA -ssRNA (Orthomyxo, Rhabdo) - "-" is nRNA/template ssRNA-RT (Retro) dsDNA-RT (Hepadna)
Steps of viral replication cycle
Asorption Penetration Uncoating -> eclipse period Synthesis Assembly -> ends eclipse Maturation (usu involves enzyme) Release
Requirements for viral growth
Cell:
- machinery (ribosomes, tRNA), ATP, precursors (nucleotides, amino acids), various enzymes, transport pathways (lysis, vesicle, etc)
Virus:
- genes for virion
- non-virion genes -> replication enzymes, manipulation of host cell
Susceptible vs permissive
Describe host cell infectability
Susceptible
- presence of host cell surface receptor (adsorption and entry)
Permissive - can virus replicate after entry
- innate cellular defenses can be overcome
- machinery for replication cycle (synthesis, transport)
Viral challenges
- must encounter host cells
- must evade physical defenses (skin, mucous, etc)
- must overcome defense mechanisms (intrinsic, innate, adaptive)
Protein coat structure
Symmetrical
Identical capsomeres
Helical Icosahedral (like geodesic dome)
Nucleocapsid
= nucleic acid + capsomer protein coat
Enveloped viruses
Lipoprotein membrane around nucleocapsid
Viral antigens
Only proteins!
Unenveloped nucleocapsid -> capsomer
Enveloped -> envelope proteins
Detection methods
Detection - clinical methods - cytopathic or transforming effect - inclusion bodies - plaques - syncytia Cultivation is slow, labor intensive, still doesn't provide specific identity
Cytopathic effect
Viral infection -> change in size, shape, motility, attachment
-> lysis
Visualize with light microscopy as infection spreads
Transforming viruses
Abnormal proliferation -> piles or mountains from single layer culture
- aka tumor viruses
Inclusion bodies
Sites of viral replication
Requires histologic staining
Location may be characteristic (cytoplasm vs nuclear)
Plaque formation
Zone of dead cells from cytopathic virus
PFU = plaque forming unit = only quantitative method for measuring virions in sample
Syncytia
Virus envelope includes “fusion” protein ->
large multinucleated cell
Ex RSV
Viral identification
Usually relies on detection of virion:antibody match
- neutralization
- complement fixation
- hemagglutination (not all viruses)
- fluorescent antibody
- radioimmunoassay, ELISA, etc
Diagnosis of viral disease
Antibodies
- isolate virus -> interaction with known antisera (Ig)
- Seroconversion - acute (prior) vs convalescent antibody levels
= may have “window period” - active infection but not seropositive (Hep B, HIV)
Rapid - presence of viral components
- protein antigens (via antibody)
- nucleic acid (via PCR) - works during window period
Neutralization
Tight binding of antibody:virion surface
- high affinity, essentially irreversible dt repeating epitopes
- > can’t adhere/enter cells
- doesn’t impact intracellular multiplication
- ex local IgA in resp or GI
- > also use for diagnosis (fewer PFUs)
- only works for infectious virions
Simple nucleocapsid viruses
All icosahedral Stable in env't (don't require direct contact for transmission) RNA -> assembled in cytoplasm DNA -> assembled in nucleus (enveloped are different...)
Host range
Species that can be infected
Determined by adsorption (cell receptors)
Ex - polio -> humans and higher primates
Tissuetropism
Range of tissue that can be infected
