Virus Replication

Question 1

Describe virus replication.

Answer 1

-viruses dont have genetic capability to multiply by division
-virus hijacks & utilizes host cell machinery to make its proteins & nucleic acid for next gen of virus
-‘assembly line’

Question 2

Describe a permissive cell VS a non permissive cell.

Answer 2

1. Permissive
   -virus able to replicate in cell
2. Non permissive
   -cells where a factor(s) necessary for viral reproduction is not present

Question 3

Describe multiplicity of infection. (MOI)

Answer 3

-# of virions added per cell during infection

Question 4

Describe the latent period VS eclipse period of the one step virus growth curve.

Answer 4

1. Latent
   -after uncoating & before appearance of extracellular new virus particle
2. Eclipse
   -after uncoating & before appearance of intracellular new virus particle

Question 5

Describe the absorption, eclipse period, latent period, & burst size of the one step virus growth curve.

Answer 5

1. Absorption
   -virus attach to and enter cell
   -titer of free virus in medium declines
2. Eclipse
   -time interval between uncoating (disappearance of viruses) & appearance, intracellularly, of 1st infectious progeny virions
3. Latent
   -time before new infectious virus appears in medium (uncoating -> prior to release of 1st extracellular virions)
   -no extracellular virions detected
4. Burst size
   -# of infectious virions released per average cell

Question 6

Describe the steps of virus replication.

Answer 6

1. Attachment
2. Penetration
3. Uncoating
4. Synthesis of viral components (nucleic acid & protein)
5. Assembly & maturation
6. Release in lg #s

Question 7

Describe virus attachment receptors.

Answer 7

-virus attachment to receptors on host cells are SPECIFIC
-each virus has its own specific receptors on specific host cells

Question 8

Describe the attachment of viruses to host cell surfaces.

Answer 8

-mediated by integration between virus & complimentary receptor on host cell surface
-cells that lack receptor escape being infected by viruses
-binding to cell receptor not sufficient in some cases for infection = additional cell surface molecule or co receptor is needed for entry
-some viruses use more than one host cell receptor (ex. HIV)

Question 9

Describe the different types of virus penetration & uncoating in host cell for non enveloped viruses.

Answer 9

1. receptor mediated endocytosis
   >clathrin mediated endocytosis
2. pore mediated penetration of viral genome into host cell cytoplasm
   >create pore in host membrane

Question 10

Describe the different types of virus penetration & uncoating in host cell for enveloped viruses.

Answer 10

1. receptor mediated endocytosis (pH dependent fusion protein)
   >fusion of virus membrane w host endosomal membrane release viral genome
   >fusion protein requires low pH for activation in endosome -> virus envelope fusion w endosomal membrane
2. surface membrane fusion (pH independent fusion protein)
   >fusion of virus envelope w host cell membrane on surface of host

Question 11

Describe other uncommon methods of entry.

Answer 11

-antibody mediated attachment & penetration -ex. FIPV
>FIPV enters host macrophage by attachment of spike protein to CD13 receptor

Question 12

Describe virus uncoating.

Answer 12

-release of viral genome in host cell
-virion can no longer be detected
-loss of infectivity of virions

Question 13

Describe the 2 functions of a parent virus.

Answer 13

1. Multiple copies for new viruses
2. Viral proteins for capsid & successful replication

Question 14

Describe reverse transcriptase.

Answer 14

-conversion of viral RNA to cDNA during virus replication

Question 15

Describe the processing of primary RNA transcript (pre-mRNA).

Answer 15

-viral mRNA must conform to the requirements of host cell translation system
>ex: host cell recog mRNA & translate same
-series of modifications:
>capping -> addition of poly A tail -> splicing
-after processing mRNA is translated in cytoplasm
-viral mRNA made in nucleus also exported to cytoplasm

Question 16

Describe the steps of primary RNA transcript (pre-mRNA).

Answer 16

1. Capping
   -add 7-methylguanosine to the 5’ end of RNA
2. Addition of 3’polyadenylated tails
3. Splicing
   -RNA splicing to remove introns & joins exons in primary transcript
   >exon = portion of gene that codes for AA
   >intron = portion of gene that doesnt code for AA

Question 17

Describe the two types of splicing.

Answer 17

1. Constitutive
   -every intron spliced out
   -every exon spliced in
2. Alternative
   -all intron spliced out
   -only selected exons spliced in
   -result: mRNAs w diff coding info from a single gene

Question 18

Describe the 2 types of viral mRNA.

Answer 18

1. Monocistronic
   -mRNA that encodes one polypeptide
2. Polycistronic
   -mRNA that encodes several polypeptides

Question 19

Describe assembly & maturation.

Answer 19

-assembly of virus genome & proteins into new virions follow a specific order
-all components (nucleic acid & proteins) packages to form mature virions
-may take place in:
>nucleus
>cytoplasm
>PM (enveloped viruses)

Question 20

Describe the release of progeny virions in naked VS enveloped virions.

Answer 20

1. Naked virions
   -lysis of host cell -> virus release
   -cant exit host cell by budding bc lack envelope
2. Enveloped virions
   -budding thru PM

Question 21

Describe exocytosis.

Answer 21

-viruses mature by budding thru membrane of GA or ER
-vesicles containing virus migrate to PM & released by exocytosis
-viruses that acquire envelope while budding from ER, GA, or nuclear/nucleus membrane leave host cell by exocytosis

Question 22

Describe replication of retroviruses.

Answer 22

1. Reverse transcriptase
   -synthesizes RNA into DNA
2. Integrate
   -integrates viral DNA into host genome

Question 23

Describe the different types of cell to cell spread of viruses.

Answer 23

1. Extracellular spread
2. Intracellular spread
   -results in rapid virus:
   >dissemination
   >evasion of IS
   >persistent infections
3. Nuclear spread of virus genome