Lecture 4 - Virus Replication Cycle

Question 1

What is a resistant cell?

Answer 1

No receptor and cannot be naturally infected by that virus

Question 2

What is a permissive cell?

Answer 2

Susceptible to infection (has a suitable receptor for the virus to attach to) and the capacity to support the full virus replication cycle

Question 3

What is a semi-permissive cell?

Answer 3

Susceptible to infection but does not support the full virus replication cycle

Question 4

What is a susceptible cell?

Answer 4

Has a functional receptor for the virus to bind to and may or may not be able to support the full virus replication cycle

Question 5

What is virus tropism?

Answer 5

Ability of a virus to infect a specific host or a specific type of host cell

Question 6

What is the multiplicity of infection (MOI) and what does it vary between?

Answer 6

• Average number of infectious virus particles per cell

- Varies between cell types (cell types differ in permissiveness)

Question 7

What does an MOI = 1 indicate?

Answer 7

Using a Poisson random distribution curve, MOI=1 indicates that 1/3 of cells will receive one hit (1 virus), 1/3 of cells will receive more than one hit (>1 virus) and 1/3 of cells will receive less than one hit. This means that about 33% of cells will not get infected at this MOI

Question 8

What MOI is required for complete, synchronous infection of the cell monolayer (>99% of cells get infected)

Answer 8

MOI > 5

Question 9

What is the procedure for obtaining a virus growth curve

Answer 9

• Using MOI = 5-10, apply viruses at 4 degrees C (allows for attachment but not entry to synchronise infection)
• Change temperature to 37 degress
• Harvest extracellular supernatant and virus-infected cells at specific time points over hours or days and titre virus in each fraction
• Compare and contrast eclipse and latent phase for enveloped and naked viruses

Question 10

What 2 things are required for virus attachment to a host cell?

Answer 10

Viral-attachment protein (VAP) and a specific cell receptor

Question 11

What occurs during virus penetration?

Answer 11

The virus crosses a cell or endosomal membrane

Question 12

What is the difference in uncoating location for RNA and DNA viruses

Answer 12

RNA viruses uncoat in the cytoplasm

DNA viruses uncoat in the nucleus

Question 13

What 2 ESSENTIAL steps need to occur for successful virus uncoating?

Answer 13

• The stable virus capsid structure needs to be destabilized (metastability)
• Uncoating needs to deliver the virus genome in a replication-competent format to the correct subcellular compartment

Question 14

What is required for a genome to be in a replication competent format?

Answer 14

Nucleic acid plus any essential proteins

Question 15

What is the correct subcellular compartment an RNA virus genome needs to be delivered to?

Answer 15

Cytoplasm

Question 16

What is the correct subcellular compartment a DNA virus genome need to be delivered to?

Answer 16

Nucleus

Question 17

What 2 types of release from the host cell may occur for viruses?

Answer 17

Budding or lysis

Question 18

What are the 3 steps in virus entry and what is the function of entry?

Answer 18

Attachment, penetration and uncoating

Delivery of replication competent form of genome to the correct subcellular compartment

Question 19

What are the 3 steps of viral macromolecular synthesis?

Answer 19

Transcription, translation and genome replication

Question 20

Does viral MMS occur simultaneously or in discrete steps?

Answer 20

Simultaneously

Question 21

What are 2 types of control for viral MMS?

Answer 21

Cis factors and trans factors

Question 22

What are the 2 steps in virus maturation?

Answer 22

assembly and release

Question 23

What property of viruses are entry and maturation dependent on?

Answer 23

Whether the virus is naked or enveloped

Question 24

What is an abortive virus cell infection and what causes it? What permissiveness would these cells have?

Answer 24

No infectious virus progeny are produced because the virus replication cycle is stalled before maturation
Often due to lack of host cell factor required for virus replication
Semi-permissive cells

Question 25

What is a productive virus cell infection and what does it look like?

Answer 25

Production and release of infectious progeny virions | Often CPE are observed

Question 26

What is a latent virus cell infection?

Answer 26

Persistence of viral genome in host cells with no detectable virus production, often subject to reactivation

Question 27

What is a persistent virus cell infection? Is it lytic or non-lytic? Would you expect to observe cytopathic effects in a persistent infection?

Answer 27

Productive, long-term infection generating infectious progeny virions. Non-lytic infection Frequently CPE are absent

Question 28

What is a transforming virus cell infection and what can it cause? What is the permissivity of these cells?

Answer 28

Little or no virus production, altered cell growth and morphology that may lead to cancer.

Question 29

What are the characteristics of an acute viral infection?

Answer 29

Usually rapid and self-limiting (or cleared by host response) but can be fatal

Question 30

What is an oncogenic virus?

Answer 30

Viruses that induce cell transfomation and cancer

Question 31

What are 3 examples of intrinsic host defenses to viruses and what they do

Answer 31

- Autophagy disrupts intracellular virus replication - Apoptosis kills virally infected cells - microRNAs inhibit virus replication

Question 32

What are 3 examples of innate host defense mechanisms?

Answer 32

- Complement - Natural killer cells - Antigen presenting cells

Question 33

What are 3 ways host cells can sensethe presence of a virus?

Answer 33

- virus binding and activation of host cell receptors - activation of nucleic acid receptors on endosomes (TLR) - nucleic acid-binding proteins in the cytoplasm (RIG-1, PKR)

Question 34

What is the result of activating virus-sensor pathways?

Answer 34

Lead to production and release of soluble immune mediators (cytokines, chemokines) which recruit and activate specific adaptive immune responses