Viral Properties

Question 1

What technique is used to prove that a virus causing a particular disease?

Answer 1

Koch’s Postulates

Question 2

Define virus.

what does obligate mean?

Answer 2

Infectious obligate intracellular parasites

• cannot complete its life-cycle without being inside a host cell

Question 3

What is the average size of a virus?

Answer 3

100nm

Question 4

What are the two broad types of virus morphology?

Answer 4

Non-enveloped
– protein capsid
- more symmetrical
- adenovirus, picornavirus, calicivirus

Enveloped
– lipid envelope membrane derived from host membrane

• pleomorphic - have lots of different shapes

Question 5

What is the main classification of viruses and what is it based on?

Answer 5

Baltimore Classification – based on the genome

Question 6

What are the different groups under this classification?

Answer 6

DNA Viruses (double stranded, single stranded)

RNA Viruses (positive sense, negative sense, double stranded)

DNA and RNA Viruses they follow a different cycle for making it (retroviruses, double stranded DNA (RT))

Question 7

What is the difference between positive sense RNA and negative sense RNA?

Answer 7

• Positive sense RNA can be translated straight away
• Viruses with a negative sense strand copy need to be transcribed back to positive sense
• Viruses with a negative sense strand need to contain the necessary enzymes and machinery to do this

Question 8

What are some common features among RNA viruses and retroviruses?

Answer 8

• use their own polymerase to replicate
• these are error prone and don’t have proofreading
• leads to a high mutation rate
• they have small unstable genomes
• They often use complicated coding strategies

Question 9

What are some common features among DNA viruses?

Answer 9

• Larger because DNA is more stable so there is space for accessory genes which gives them advantages
• Some viruses have their genome in one long strand, others have it in several little pieces - SEGMENTED GENOME

Question 10

What are the good and bad aspects of having a segmented genome?

Answer 10

Good – allows an opportunity for exchanges of genetic material and fast evolution

Bad – all the segments need to be assembled when the virus leaves the cell

Question 11

Describe the replication cycle of HIV-1.

Answer 11

1. virus is on the outside
2. GP120 receptors on the HIV bind to CD4 receptors on T cells and bind to co-receptors (CCR5 or CXCR4) allowing the membranes to fuse and the viral contents to enter the cell
3. Some copies of the virus genome gets replicated
4. Some gets reverse transcribed to viral DNA, which is integrated into the host genome.
5. It is then transcribed and translated into proteins
6. The proteins and copies of the genome then assemble to form new virus particles, which exit the cell.

Question 12

What is the cytopathic effect?

Answer 12

Death of a cell as a result of being infected by a virus

Question 13

How can viral plaques be used to quantify the amount of virus in a sample?

Answer 13

Plaque Assay – the virus undergoes serial 10-fold dilutions and is then spread on a monolayer of susceptible cells
A plaque will appear where an individual virus has killed some cells
The number of plaques can be counted and scaled up to quantify the amount of virus in a sample

Question 14

What are two other ways of detecting the presence of virus in a sample?

Answer 14

Syncytia formation

Immunostaining

Question 15

What are the three phases of growth of a virus?

Answer 15

Eclipse- cant see the virus

Logarithmic - logarithmic increase in the number of viruses in the sample

Cell Death- cells start dying

Question 16

What are the five techniques used to diagnose a viral infection?

Answer 16

detecting viral genome
- PCR

detecting viral antigen

• Indirect Fluorescence Antibody (IFA)
• ELISA

detecting viral particles
- Electron Microscopy
Haemagglutination Assay

detecting virus cytopathic efect in cultured cells
- virus isolation

detecting antibodies to virus
- serology

Question 17

what are kochs postulates?

Answer 17

• The microorganism must be found in large numbers in all diseased animals, but not in healthy ones.
• the organism must be isolated from the culture and grown alone
• when the isolated organism is injected into other organisms it must produce the same disease
• suspected microorganisms must be retrieved and compared to the first host and proved to be identical

Question 18

what is a positive strand?

what is a negative strand?

Answer 18

positive = sense strand which can be translated directly

negative = antisense strand

Question 19

how does the virus replication system work?

Answer 19

• virus on outside where it is inert
• virus has viral attachment protein on its coat , this binds to the receptor on the host cell surface
• this allows the virus to dock down onto the cell surface
• Some viruses enter the cell by fusing with the host cell membrane and injecting their DNA or RNA into the cytoplasm
• Other viruses enter via a series of vesicles such as endosomes

-viral genome is in the cell, it
must be made into messenger RNA if it isn’t already

at the same time the virus is replicating its own genome using a polymerase

Question 20

why does the cytopathic effect happen?

Answer 20

• The cytopathic effect could be due to the virus taking
  over your genetic machinery so no more proteins are produced
• Death is probably due to apoptosis
• the virus needs to compete aggressively with the host cell protein synthesis

Question 21

what is a plaque?

Answer 21

the result of an individual virus
infecting one cell and then infecting other cells

due to the death of a cell the virus makes a plaque on the cell monolayers

Question 22

how does looking at syncytia work?

Answer 22

• some viruses undergo syncytia formation

- this is when a big bundle of cells have stuck together in the middle

Question 23

how does immunostaining the infected cells work?

Answer 23

• viruses produce new things inside the cells
• if we think someone has a virus put the sample into a cell and wait for the unique protein to be made
• You then use an antibody which can detect that protein

Question 24

how to propagate viruses?

Answer 24

• We can passage viruses in the lab by providing permissive cells
• Viruses may accumulate mutations that adapt them to the new host
• this might lead to attenuation