Viruses

Question 1

What are viruses? (3 words)

Answer 1

obligate intracellular parasites

Question 2

Viruses are massively variable TRUE/FALSE

Answer 2

TRUE

Question 3

What can viruses not do?

Answer 3

They cannot replicate themselves
They cannot make energy or utlilise nutrients
They can not synthesise proteins, nucleic acids or anything
They depend entirely on the cell they infect for all of these functions
No endogenous metabolism

Question 4

How are viruses classified?

Answer 4

See diagram in lecture

• RNA or DNA?
• SS or DS?
• Antisense or sense?
• Reverse transcriptase?

Question 5

What is a vision?

Answer 5

The infectious stage of the virus lifestyle

Question 6

What is the simple structure of a virus?

Answer 6

nucleic acid surrounding by protein

Question 7

What is a capsid?

Answer 7

The protein that surrounds the nucleic acid to make a virus. They can be simple or complex. 95% are helical, however there are no human viruses with helical capsids.

Question 8

What is an icosahedral capsid?

Answer 8

20 equilateral triangles arranged in a sphere

Question 9

What is the simplest possible viral lifecycle?

Answer 9

enter
replicate
assemble
release

Question 10

What do retroviruses do?

Answer 10

Make a DNA reverse transcript of their RNA genome before they do anything else. This is then integrated into the chromosomal DNA. Infection is therefore irreversible

Question 11

What are the 3 phases of the one-step growth curve?

Answer 11

Eclipse - Infectious furs is absorbed onto cells, can’t tell if someone is infected
Log phase - exponential increase in the no. of viruses
Plateau - virus bursts out of the cell

Question 12

Viruses that replicate by bursting cells tend to be……

Viruses that bud chronically from sick cells tend to …

Answer 12

• naked particles, that is just a capsid with spikes

- pick up a membrane as they leave so are called enveloped (looks like a virus in a baggy plastic bag)

Question 13

What is tropism?

Answer 13

The binding of a virus to a specific surface receptor so that it can infect the cell.
This is why viruses tend to be species and organ specific

Question 14

Describe the tropism of HIV.
What does it bind to?
How does this lead to disease?

Answer 14

HIV binds to two molecules on the cell surface:
- CD4
- CCR5, a chemokine receptor
HIV infects cells which express these receptors - macrophages an CD4+ Tcells
HIV infection leads to the gradual destruction of all cells which express these markers, leading to immune dysfunction and a complex pattern of diseases called Auto Immune Deficiency Syndrome

Question 15

Describe the proteins involved in the attachment of HIV to the cell surface.

Answer 15

GAG’s (glycosaminoglycans) on the surface of the cell are slightly charged, slightly opposite charge to that of the virus. If the cell has CD4 interact with the gp120 protein on the virus. The virus is irreversibly attached. If the cell also has the secondary binding factor (CCR-5) then the virus can gain entry into the cell.

Question 16

What is meant by a virus being from zoonotic origin?

Answer 16

It originally infected animals but evolved to be able to infect humans

Question 17

What makes an ideal virus?

Answer 17

A well adapted virus will not cause any disease to the host
Most catastrophic infections are zoonoses e.g. Ebola
Many are unstable in the new species, are poorly transmitted and so will disappear.

Question 18

What is meant by antigenic drift and antigenic shift?

Answer 18

Antigenic drift is the gradual accumulation of mutations over viral generations.
Antigenic shift - viral ‘ e.g. if a pig cell is infected by human and avian flu, it becomes a mixing vessel and an entirely new virus is released. These are RNA viruses and therefore are unstable so mixing of genomes can occur.

Question 19

What are the proteins of the flu virion surface?

Answer 19

Haemagluttinin (H) - responsible for crosslinking Hb

Neuraminidase (N)

Question 20

What is VEHCS?

Use polio as an example

Answer 20

Virally Encoded Host Cell Shutoff - the mechanism of how a virus controls its host, modulation of host-cell machinery
Polio virus
- cap is removed from ribosome subset. This is required for host protein synthesis but not for viral protein synthesis
- Shortly after infection there is a sharp decrease in cell synthesis (no cap, no mRNA, no host protein)
- Start of viral protein synthesis (no cap, no problem, polio mRNA doesn’t need it
- Virus assembly
- Release

Question 21

How does Herpes Simplex Virus evades the immune system and causes a latent infection?

Answer 21

• HSV leads to skin lesions and spread to new hosts. However a successful immune response to infections eliminates the virus from the skin
• HSV evades the immune response by becoming quiescent
• In latent infection the HSV genome resides int he nucleus of the host cell and produces NO proteins, just controlling mRNA transcripts called Latency Associated Transcripts (LATs).
• The LATs of HSV produce a spectrum of microRNA which act to control the host cell, even though the virus produces no protein.
• Immune recognition requires protein, latent HSV doesn’t make any

Question 22

What is HERV’s?

How is it adapted?

Answer 22

Human Endogenous Retrovirus
The best adapted viruses encorporate themselves into the germ-line by getting into the gametes. They become ‘self’ in the second generation.
Mammals have adopted HERV’s to make syncitiotrophoblast, make using envelope protein of HERV-W.
It is required for reproduction

Question 23

What are the 4 courses of viral infection?

Answer 23

Acute infection e.g. rhinovirus
Persistant infection, smouldering (no human one)
Persistant infection, latent e.g. HSV
Persistant infection, slow e.g. HIV, Measles virus SSPE

Question 24

What is the prodromal response to infection?

Answer 24

Most common symptoms of viral infection are due to the general effects of innate immune responses.
E.g.
fatigue, fever, sleep
bone ache (haematopoesis)

Question 25

What causes the spots in measles?

Answer 25

cell-cell fusion