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Flashcards in principals of virology Deck (30):

the extracellular form of a virus is called a



where do viruses undergo replication?

inside the host cell-- thus viruses are obligate IC parasites


capsid what is it and what are the 2 general forms?

the protein coat that protects the genome
1. icosahedral (structure formed from 20 triangular faces)
2. helical



the capsid (protein coat that protects the genome) + the genome


what criteria (6) are used to divide viruses into families

1. type of genomic nucleic acid (RNA or DNA; ss or ds; + or - for ss)
2. symmetry of the capsid (helical, icosahedral, complex)
3. presence/absence of an envelope
4. mode of replication
5. tropism: preferred cell type for replication
6. type of pathology


positive strand vs negative strand RNA

positive strand: same sense as viral mRNA
negative strand: complementary to viral mRNA



preferred cell type for replication


how can virions be visualized directly?

electron microscope


8 steps of viral replication:

1. binding- to specific cell surface receptors
2. entry
3. uncoating
4. transcription of mRNA
5. translation of viral proteins
6. replication of the input genome
7. assembly of progeny viral particles
8. egress


3 usual methods for viral entry into the cell

1. fusion of envelope with surface plasma membrane
2. receptor-mediated endocytosis followed by fusion with endosome membrane
3. receptor mediated endocytosis followed by lysis of the endosome


a host cell can be permissive or non permissive leading to

a productive or non-productive infection respectively


6 stages of viral infection

1. entry
2. primary replication at the site of entry
3. spread through the host
4. secondary replication at new sites
5. persistence or clearance by the host immune response
6. transmission or release from the host-org


ifxn of the host org can be (3 things)

acute, chronic, or latent


what are the two components of a viral ifxn that the host imm response can target?

1. the virion (ab-mediated neutralization)
2. infected cells (cytolytic T-cells (CD8+ CTLs- using Class I MHC)


besides looking for the virion with electron microscopy, how else might you detect physical components of virions? (5)

1. Abs specific for viral capsid prots
2. Viral nucleic acids can be detected by hybridization of PCR (polymerase chain rxn)
3. hybridization based assays can include signal amplification assays such as branched chain DNA assay or hybrid capture assay
4. immunoflourescence can be used to detect viral proteins
5. hemagglutination-- where viruses cross-link and agglutinate RBCs



hemagglutination-- where viruses cross-link and agglutinate RBCs


2 ways infectivity can be measured

1. ability to infect animals and cause disease
2. ability to infect cultured cells and cause cytopathic effect (CPE)


quantitative assays of viruses (3)

1. negative staining and observation with electron microscopy
2. end-point dilution-- virus is diluted until 1/2 of the cultures (TCID50) or animals become infected (LD50)
3. Plaque assay: area of localized cytpathic effect



tissue culture infectious dose for 50% of cultures-- the virus is diluted until only 1/2 of the cultures become infected to quantify the
infectivity of a virus



lethal dose for 50% of test animals-- the virus is diluted until only 1/2 of the animals become infected to quantify the
infectivity of a virus


what is a particle to PFU (plaque forming units) ratio, and what does it tell us?

a measure of the fraction of virus particles in a given sample that can complete an infectious cycle-- for animal viruses it is always much greater than one (usually 10-10,000) so following a virus particle doesn't mean we're following the real infection pathway


how do we detect virus-specific antibodies? (4)

1. Enzyme-linked immunosorbent assay (ELISA)
2. western blot assay
3. hemagglutination inhibition
4. latex bead agglutination assay


what does the presence of virus-specific Abs tell us?

evidence of prior infection or prior exposure to viral Ags


how can you look for genotypic markers for viruses in isolates? (3)

1. direct sequencing
2. analysis of restriction endonuclease cleavage site polymorphisms
3. PCR


genotypically, how do viruses such as influenza and measles differ from viruses like HIV and herpes simplex virus (HSV) differ in the population

influenza and measles show only one strain prevalent in the population at one time, whereas nearly every unrelated isolate of HIV and HSV can be differentiated. Therefore, you can trace the spread of viruses like HSV and HIV


defining characteristics of viruses (4)

1. obligate IC parasites-- can only replicate in living cells
2. Nucleic Acid and protein complex, sometimes with a lipid membrane as an envelope
3. Use host cell biosynthetic and energy generating machinery
4. Have a simple life-cycle:
-virion = EC particle ---> enters cell ---> disassembled for IC replication ---> virus particles released ---> virion = EC particle...


how do we detect physical components of virion or viral Ags? (5)

1. Electron microscope (EM) to detect virions
2.PCR or hybridization to detect nucleic acid
3. enzyme assay to detext viral enzymes
4. immunological methods to detect viral Ags (ie immunoflourescence)
5. Hemagglutination-- coss linking of RBCs detects virions


what are the different forms in which a virus can be detected (4)?

1. proteins
2. nucleic acids
3. virions
4. infectivity


how can virus infection be detected?

by the presence of an immune response, like Abs


2 ways in which infection can cause disease

1. by killing or damaging cells (Norovirus, rotavirus and influenza)
2. by the resulting immune response (SARS- severe acute respiratory syndrome, Herpes simplex virus (HSV) encephalitis, West Nile encephalitis)