Virology

Question 1

What are 4 host response evasion strategies of parasites? Provide an example for each.

Answer 1

1. elicit minimal response
   - herpes simplex virus remains latent in host cells for long periods without causing pathology
2. evade effects of response
   - mycobacteria survive within a granuloma created to localize and destroy infection
3. depress host’s defense
   - HIV destroys T cells
   - Malaria (Plasmodium depresses immune response)
4. antigenic change
   - viruses, spirochetes, and trypanosomes change surface antigens to avoid recognition by hosts

Question 2

Define interferons. What are the 3 types and their functions?

Answer 2

Glycoproteins produced by many cells; especially leukocytes.

3 types:

(1) IFN-alpha (20 types); (2)IFN-beta (2 types) - both inhibit viral protein synthesis & activate leukocytes to kill viruses
(3) IFN-gamma (3 types) - up regulate MHC I, II expression & antigen presentation; activate NK and Tc cells to kill virus-infected host cells.

Question 3

What are characteristics of interferons? (4 main ones discussed in class)

Answer 3

1. cell-specific in both production and effects but are virus non-specific
2. induced by presence of ds-RNA
3. IFN inhibits translation of viral mRNA, and viral replication
4. IFN induces common features of viral infection (flu-like symptoms: fever myalgia, headache)

Question 4

Gram positive vs gram negative

Answer 4

gram position - thicker cell wall (purple); eg. E coli

gram negative - thinner cell wall but has an outer membrane (pink), eg. staphylococcus aureus

Question 5

What are the resistant organisms discussed in class?

Answer 5

spores (endospores), prions, myobacteria

Question 6

What is the difference between sterilization, disinfection, and antisepsis? Provide methods of each and what may/may not be killed.

Answer 6

sterilization - total destruction of ALL microorganisms; autoclave, dry heat, filtration, radiation, ethylene oxide gas.

disinfection - destruction of MOST organisms; pasteurization, used for heat-sensitive instruments; spores and myobacteria may not be killed

antisepsis - use to kill most organisms on skin or in tissue; kills most organisms including myobacteria but not spores; alcohols have a drying action and most effective in presence of water (thus 70% alc better than 95%); iodophors (iodine); chlorhexidine (slower killing but residual action, chlorine = powerful antioxidant); phenolic compounds (higher phenol coefficient = more efficient in killing microrganisms); quaternary ammonium compounds

Question 7

How are new strains of prokaryotes created? Describe each and their mechanisms.

Answer 7

mutation: genetic drift (small changes) that may be spontaneous or damage-induced (mutations=BS, FS, insertions, deletions, silent, null); e.g. MIC/MBC
reassortment: genetic shift (large changes) that can occur in some virus and all eukaryotes; requires segmented genomes; can be homopolyploidy (mixing between same strains) or heteropolyploidy (mixing between different strains to get a hybrid)

Question 8

What are the four families of RNA virus with fragmented genomes that can undergo reassortment? Reassortment is a mechanism of ____?

Answer 8

B - bunyaviridae
O - orthomyxoviridae
A - arenaviridae
R - Reoviridae

genetic shift (may be the initiating factor in many epidemics)

Question 9

For transformation to occur, recipient cells must be _____? What characterizes this (3x)? What are gram positive and gram negative species that can take up exogenous ____/chormosomal DNA?

Answer 9

competent cells

1. bind DNA
2. DNA uptake
3. DNA integration

gram positive: S. pneumoniae, S. aureus, B subtilis
gram negative: N meningitidis, N gonorrhoeae, H. influenzae, E. coli

Question 10

What is a plasmid?

Answer 10

small autonomously replicating circular pieces of dsDNA

Question 11

What are 5 bacterial toxins encoded in a lysogenic phage? Name the bacteria that is associated with each.

Answer 11

A - shigA-like toxin (E. coli)
B - Botulinum toxin (Cl. botulinum)
C - Cholera toxin (V. cholerae)
D - Diphtheria toxin (C. diptheriae)
E - Erythrogenic toxin (S. pyogenes)

Question 12

What are virulence factors produced by plasmids?

Answer 12

1. antibiotic resistance through production of antimicrobial agents (bacteriocins - strain specific antibiotics against other strains of the same species that produced them)
2. toxins (ABCDE)
3. adhesins - allow bacteria to attach to host cells
4. growth factors - act to directly aid bacteria in competing with mammalian host cells for nutrition

Question 13

What is conjugation?

Answer 13

fertility plasmid-facilitated transfer of a plasmid or of host chromosome to a recipient cell; occurs only between strains the same or closely-related species; capable of transferring large pieces of DNA

Question 14

Many plasmids of gram-______ bacteria are conjugative, and carry ___ genes that mediate their own transfer by coding for the production of ______ and enzymes necessary for conjugation.

Answer 14

gram negative
tra gene
sex pili

Question 15

What are the steps of conjugation?

Answer 15

1. extrusion of a sex plus
2. adhesion of the tip of the sex plus to the outer membrane of a gram-negative cell wall; cells are now bound together at a point of direct envelope-to-envelope contact
3. plasmid will undergo transfer replication
   - one parental strand of DNA is broken and passed into recipient while the other remains circularized and in the donor
   - complementary strands are synthesized in both donor and recipient cells
   - ligase re-circulized daughter DNA plasmids after completion of transfer replication

Question 16

Define an episome.

Answer 16

a plasmid that is capable of integrating into the chromosome

Question 17

Integration of the plasmid into the bacterial chromosome is dependent upon ____?

Answer 17

homology between the two

Question 18

Plasmid that are able to mobilize chromosomal transfer are called ____?

Answer 18

sex factors or F (fertility) factors (cells that contain sex factor F are designated F+)

Question 19

What is produced if the F-plasmid is integrated into the chromosome?

Answer 19

Hfr donor cell (high frequency recombination)

Question 20

F+ transfer of genetic material includes what?

Hfr transfer of genetic material includes what?

Answer 20

1. plasmid only

2. plasmid and some chromosomal genes

Question 21

Gram-negative plasmids that contain antibiotic resistance genes are called?

Answer 21

R factors

Question 22

What is transduction?

Answer 22

genetic transfer that occurs in both gram positive and gram negative bacteria in which a fragment of DNA is carried to the recipient cell in a virus (bacteriophage) produced by a donor cell

Question 23

Define a prophage. Name the result of the process that defines a prophage.

Answer 23

a bacterial virus that is latently infecting the host, often integrating into the chromosome (process known as lysogenization which results in change in phenotype of the host cell called lysogenic conversion)

Question 24

Define restricted (specialized) transduction.

Answer 24

when a transducing phage only carries segments of DNA that are immediately adjacent to a site of prophage attachment; these phages can integrate their own DNA into the bacterial chromosome - occasionally, upon induction, phage DNA excision is imprecise and resulting piece of excised phage contains some of the host bacterial genome; it will then replicate and lyse host cell, releasing “defective phage”

Question 25

What is recombination? What viruses can it occur in? Dependence on homology distinguishes what?

Answer 25

breakage and reunion of homologous regions of donor and recipient double-stranger nucleic acid molecules that have been brought into close proximity following transformation, conjugation, or transduction to produce a new genotype. occurs in almost all groups of DNA-containing viruses; Picornaviridae are the only known viruses that exhibit RNA recombination. distinguishes legitimate recombination from illegitimate recombination

Question 26

When can genetic recombination occur in viruses?

Answer 26

only when two viruses simultaneously infect the same cell

Question 27

What are the steps of DNA recombination?

Answer 27

1. homologous DNA strand pair 2. breakage of two strands 3. broken segments are then reciprocally joined 4. crossover point or Holliday junction is laterally displaced 5. the other strings are broken and partially digested 6. localized repair and reciprocal joining occurs 7. the resultant is recombinant DNA molecules that exhibit heterozygosity at the site of crossing over

Question 28

What is transposition

Answer 28

``` illegitimate recombination (non homologous) - plasmid-borne genes can move and integrate into the host chromosome especially when facilitated by a selective pressure mechanism has been demonstrated in bacteria using drug-resistance genes and is supported by evidence of "pathogenicity islands" in uropathogenic E. coli, H pylori, etc ```

Question 29

Define complementation.

Answer 29

Two copies of expressed genes in one cell. A functional gene will cover a deficiency caused by a mutant gene.

Question 30

Define phenotypic mixing.

Answer 30

two viruses in one cell, progeny viruses may have combined attributes from both parental viruses

Question 31

What are tests to determine antibiotic susceptibility?

Answer 31

Diffusion tests - used to identify antibiotic resistant bacteria Dilution tests - determine the MIC (minimum inhibitory conc.) or MBC (minimum bactericidal conc.); useful for treatment - MIC, no growth in inoculum but will grow on plate - MBC, no growth in inoculum or plate

Question 32

Virus, Prion, DNA/RNA Viral Genomes, or Bacterial Genomes?: 1. reproduce by duplication > fission 2. aberrant proteins 3. unsegmented 4. unsegmented or segmented 5. single- or double-stranged chromosome 6. obligate intracellular parasites 7. filterable 8. reproduce by assembly 9. linear chromosome 10. depend on host processes

Answer 32

1. bacterial genomes 2. prions 3. bacterial genomes 4. DNA/RNA viral genomes 5. DNA/RNA viral genomes 6. virus 7. virus 8. DNA/RNA viral genomes 9. DNA/RNA viral genomes 10. virus

Question 33

What are the capsomer subunits of the nucleocapsid (capsid, protein coat) of viruses called? These subunits are made up of ______.

Answer 33

pentamers (12) and hexamers which are made up of protomers

Question 34

All icosahedral capsids have __ corners each occupied by a pentane capsomer and __ trianglular faces, each containing __ hexon capsomers.

Answer 34

12, 20, 20

Question 35

How are helical and complex viruses structurally different from icosahedral?

Answer 35

are made up of only protomers, protomers are not arranged into capsomers

Question 36

Describe the envelop and spikes.

Answer 36

envelope - exterior to capsid, of host origin, wet transmission. spikes - glycoprotein projections through envelope, viral attachment sites

Question 37

Define viral propagation. What is the viral propagation cycle at the cellular level (5 steps)?

Answer 37

entrance to the human host. cycle: adsorption, penetration, uncaring (virus disappears, eclipse phase, free nucleic acid to reproduce itself), replication of nucleic acid, maturation and release

Question 38

What is require for adsorption to occur? Is the process specific or non-specific?

Answer 38

requires interaction between unique viral surface proteins and highly specific host cell surface receptor sites; highly specific process for viral cell invasion, tissue specific

Question 39

What are the 3 processes of penetration?

Answer 39

1. enveloped viruses: membranes fuse releasing the nucleocapsid into the cell 2. naked viruses: nucleocapsid binds to membrane receptor sites, then either digests thru the membrane or is engulfed and enters within a vesicle 3. both naked and developed viruses can be ingested by phagocytic cells and enter the cell within a vesicle

Question 40

Describe the process of uncoating.

Answer 40

cellular proteolytic enzymes digest the capsid away, occurs in the cytoplasm, coincident with the eclipse phase (the time of viral replication)

Question 41

Where do most DNA viruses replicate? What is the one exception? What occurs in early transcription? Late transcription?

Answer 41

in the nucleus; poxviruses replicate in cytoplasm Early transcription: synthesize viral proteins, some may produce dsRNA which will induce interferons Late transcription: viral capsid proteins made (usually cytoplasm, sometimes in nucleus)

Question 42

Where do the majority of RNA viruses replicate? What are the 2 exceptions?

Answer 42

cytoplasm; except orthomyxoviruses and retroviruses

Question 43

What must the RNA viral genome encode that is absent in eukaryotic cells? The ssRNA that is related by uncaring will act as either...? (x4)

Answer 43

RNA-dependent RNA polymerase act as either: 1. mRNA (+) to synthesize viral-coded proteins 2. a template (-) to synthesize mRNA 3. become encapsulated, resulting in mature progeny virions

Question 44

For naked viruses, maturation consists of what two main processes? Where does maturation occur? What is released rapidly after maturation? What is usually NOT related rapidly after maturation. Describe this latter answer.

Answer 44

1. assembly of capsid 2. capsid association with nucleic acid occurs at the site of nucleic acid replication (DNA-viruses in nucleus, RNA-viruses in cytoplasm) RNA viruses released rapidly after maturation. DNA viruses usually NOT released rapidly, usually accumulate in large numbers at site of maturation (nucleus) forming inclusion bodies. Inclusion bodies may be related when cell autolysis or extruded without lysis

Question 45

Describe the process of release in enveloped viruses.

Answer 45

bud out of cytoplasmic or nuclear membrane envelope, antigenic mosaic of viral and host proteins stud the envelop, slow/continuously relate by budding; cell is not lysed; little intracellular accumulation of virus occurs; inclusion bodies are uncommon

Question 46

Describe the process of release in complex viruses.

Answer 46

tend to have 2 membranes with one around the nucleic | have inclusion bodies, released by cell lysis

Question 47

# Define the lytic cycle. Describe latency. What can the basis of latency be attributed to (3x)

Answer 47

Normally, reproduction of a virus at a rapid rate and large quantity. Acute, destructive, terminal, common. Rarely, virus enters the cell and persists in cell with no detectable effect on host cell. May be activated by systemic shock. Chronic. Latency has been attributed to: lysogenization (when a signal stimulates the virus to replicate), lack of pathogenicity of virus, suppression of reproduction by immune system.

Question 48

What is one outcome of a viral propagation?

Answer 48

transformation of a host cell into rapidly growing cells that can form tumors; can be benign or malignant (papillomavirus type 16 and 18)