principals of virology

Question 1

the extracellular form of a virus is called a

Answer 1

virion

Question 2

where do viruses undergo replication?

Answer 2

inside the host cell– thus viruses are obligate IC parasites

Question 3

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

Answer 3

the protein coat that protects the genome

1. icosahedral (structure formed from 20 triangular faces)
2. helical

Question 4

nucleocapsid

Answer 4

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

Question 5

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

Answer 5

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

Question 6

positive strand vs negative strand RNA

Answer 6

positive strand: same sense as viral mRNA

negative strand: complementary to viral mRNA

Question 7

tropism

Answer 7

preferred cell type for replication

Question 8

how can virions be visualized directly?

Answer 8

electron microscope

Question 9

8 steps of viral replication:

Answer 9

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

Question 10

3 usual methods for viral entry into the cell

Answer 10

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

Question 11

a host cell can be permissive or non permissive leading to

Answer 11

a productive or non-productive infection respectively

Question 12

6 stages of viral infection

Answer 12

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

Question 13

ifxn of the host org can be (3 things)

Answer 13

acute, chronic, or latent

Question 14

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

Answer 14

1. the virion (ab-mediated neutralization)

2. infected cells (cytolytic T-cells (CD8+ CTLs- using Class I MHC)

Question 15

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

Answer 15

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

Question 16

hemagglutination?

Answer 16

hemagglutination– where viruses cross-link and agglutinate RBCs

Question 17

2 ways infectivity can be measured

Answer 17

1. ability to infect animals and cause disease

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

Question 18

quantitative assays of viruses (3)

Answer 18

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

Question 19

TCID50

Answer 19

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

Question 20

LD50

Answer 20

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

Question 21

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

Answer 21

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

Question 22

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

Answer 22

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

Question 23

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

Answer 23

evidence of prior infection or prior exposure to viral Ags

Question 24

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

Answer 24

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

Question 25

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

Answer 25

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

Question 26

defining characteristics of viruses (4)

Answer 26

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…

Question 27

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

Answer 27

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

Question 28

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

Answer 28

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

Question 29

how can virus infection be detected?

Answer 29

by the presence of an immune response, like Abs

Question 30

2 ways in which infection can cause disease

Answer 30

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)