Viruses

Question 1

What are Viruses?

Answer 1

Infectious obligate intracellular parasites.

Question 2

What are viruses that only infect bacteria?

Answer 2

Bacteriophages

Question 3

Describe the genetics of viruses:

Answer 3

DNA or RNA; genome replicated in host to produce more viral components.

Question 4

How are viruses classified?

Answer 4

Baltimore system: depends on use of ds/ss DNA/RNA

Question 5

What is the difference in viral polymerases? What does this lead to?

Answer 5

Lack of proof-reading capacity, so increased rate of mutations

Question 6

Describe viral replication:

Answer 6

1. Viral coat proteins bind to host cell surface receptors
2. Coat fuses w/ plasma membrane, Genome released into cytoplasm
3. Regulatory proteins then structural (coat) proteins synthesised
4. Genome replicated
5. Proteins and genome assemble and then bud out of cell

Question 7

What is a viral tropism?

Answer 7

Predilection of virus to infect certain tissues and not others as defined by receptors on surface (susceptibility of cell), ability to complete replication (permissivity) and whether virus can reach tissue (accessibility)

Question 8

What is the HIV tropism?

Answer 8

Cell needs CD4+ (and CCR5+/CXCR4+)

Question 9

What is viral oncogenesis?

Answer 9

Viruses may encode oncogenes that interfere with cell cycle to promote S phase and enhance own replication; may also inhibit TSGs; many tumours caused by viral infection and so contain viral DNA

Question 10

What is zoonosis?

Answer 10

Route of transmission involving insects/animals
Human viruses may recently arise from animals e.g. SARS (emerged from bats via civets) and HIV (HIV1 from chimpanzees, HIV2 in sooty mangabeys - acquired mutations to infect humans/evade immunity)

Question 11

What is prophylaxis?

Answer 11

Preventing disease before aetiological agent acquired, by vaccination or giving drug before infection

Question 12

How might a virus be attenuated?

Answer 12

1. Pathogenic virus isolated and cultured in human cells; after some time used to infect monkey cells, and over time mutations are acquired to grow well in monkey cells; upon re-infection of humans, the virus does not replicate or infect well as adapted to monkeys
2. Hypothesis driven reverse genetics could be used to engineer in weakness to viruses, attenuating the strain

Question 13

What are the pros of Attenuated and inactivated vaccines?

Answer 13

Live: Rapid, broad, long-lived immunity
Inactivated: Safe, can be made from wild-type virus

Question 14

What are the cons of attenuated and inactivated vaccines?

Answer 14

Live: Requires attenuation, which could reduce level of immunity. May revert to original if mutates
Inactivated: Boosters required, and high doses required

Question 15

What are Nucleoside analogues?

Answer 15

Similar structure to nucleosides that lack a 3’ hydroxyl group, so chain terminated as phosphodiester bond formation prevented

Question 16

What type of drug is acyclovir?

Answer 16

A nucleoside analogue:
Only activated inside virus infected cells, and has a much higher affinity for viral DNA polymerase vs host cell - resistance rare (maps to thymidine kinase); given as pro-drug which is un-phosphorylated - only viral thymidine kinase (e.g. In herpes) can form monophosphate precursor that host cell turns to triphosphate - SO only included in viral genome

Question 17

What is antigenic drift?

Answer 17

Gradual evolution of virus driven by antibody selective pressure - necessitates yearly update to vaccine