virus infection

Question 1

what are the methods of transmission in plant viruses?

Answer 1

• mechanical
• vegetative propagation
• vectors

Question 2

what is required for viruses to infect plants?

Answer 2

a mechanical breach of the integrity of the cell wall in order to directly introduce a virus particle into the cell

Question 3

how do some viruses utilise the plasmodesmata?

Answer 3

normal plasmodesmata are too small to allow passage of virus particles and genomic nucleic acids, but many plant viruses have evolved specialised movement proteins to modify the plasmodesmata

Question 4

give an example of a virus which utilises the plasmodesmata to infect plants

Answer 4

TMV - produces 30-K protein which causes the plasmodesmata to begin to transport genomic RNA from infected cells to neighbouring cells

Question 5

what are the symptoms of virus infection in plants?

Answer 5

• growth retardation
• distortion
• mosaic patterning on leaves
• yellowing
• wilting

Question 6

what do the symptoms of virus infection in plants result from?

Answer 6

necrosis
hypoplasia
hyperplasia

Question 7

what is hypoplasia? (plants)

Answer 7

localised retarded growth that frequently leads to mosaicism

Question 8

what is hyperplasia? (plants)

Answer 8

excessive cell division or growth of abnormally large cells, resulting in the production of swollen or distorted areas of the plant

Question 9

what does the hypersensitive response refer to?

Answer 9

• the synthesis of a range of new proteins - in response to virus
• increase in production of cell wall phenolic substances
• release of active oxygen species
• production of phytoalexins
• accumulation of salicylic acid

Question 10

what are the characteristics of systemic resistance to virus infection in plants?

Answer 10

these plants have a tendency to increase local necrosis, producing substances such as proteases and peroxides to prevent systemic spread

Question 11

how is Tobacco N gene an example of how a plant may have systemic resistance to viral infection?

Answer 11

• this gene encodes a cytoplasmic protein with a nucleotide binding site that interferes with TMV replicase
• when present in plants, this gene causes TMV to produce a localised necrotic infection rather than systemic mosaicism

Question 12

how are virus-resistant plants created?

Answer 12

by production of transgenic plants expressing recombinant virus proteins or nucleic acids that interfere with virus replication without producing the pathogenic consequences of infection

Question 13

which virus components can be used to create a virus-resistant plant?

Answer 13

• virus coat proteins
• intact or partial virus replicase that interfere with genome replication
• antisense RNAs
• defective virus genomes

Question 14

what is the importance of the humoral immune response to virus infection?

Answer 14

permits clearance of the virus from the body

Question 15

what is the importance of serum neutralisation in the humoral immune response?

Answer 15

stops the spread of virus to uninfected cells and allows other defence mechanisms to mop up infection

Question 16

which three classes of antibody are induced by viral infection?

Answer 16

IgM
IgG
IgA

Question 17

what is IgM?

Answer 17

a large, multivalent molecule that is effective at cross-linking large targets such as bacterial cell walls

Question 18

what is the importance of IgA?

Answer 18

important for initial protection from viral infection

-secretory IgA is produced at mucosal surfaces and results in ‘mucosal immunity’

Question 19

give examples of the mechanisms which may cause direct neutralisation of viruses by antibodies

Answer 19

• conformational changes in the virus capsid

- blocking of virus receptor binding by steric hindrance

Question 20

what is meant by ‘opsonisation’?

Answer 20

the secondary consequence of antibody binding where there is phagocytosis of antibody coated target molecules by mononuclear cells or polymorphonuclear cells

Question 21

give an example of where opsonisation of virus may actually enhance virus uptake

Answer 21

HIV - causes increase in uptake of virus by macrophage

Question 22

how do observations made on cogenital defects in cell-mediated immunity provide evidence of the importance of cell-mediated immunity in the control of virus infection?

Answer 22

-congenital defects result in predisposition to viral infections rather than bacterial infection

Question 23

which three effects does cell-mediated immunity rely on?

Answer 23

• non-specific killing by NK cells
• specific cell killing (mediated by cytotoxic T lymphocytes)
• antibody dependent cellular cytotoxicity

Question 24

what is the role of natural killer cells?

Answer 24

provide a rapid first line of defence in the early stages of infection

Question 25

what is the distinguishing feature of antibody-dependent cellular cytotoxicity?

Answer 25

the mechanism is dependent on the recognition of antigen on the surface of the target celery means of an antibody on the surface of the effector cell

Question 26

what is necrosis marked by?

Answer 26

non-specific changes such as disruption of the plasma membrane and nuclear envelope, rupture of membrane bound organelles e.g. mitochondria and lysosomes, cell swelling, fragmentation of DNA/RNA etc.

Question 27

by which routes can viruses enter the body?

Answer 27

- skin - mucosal membranes - alimentary canal - respiratory tract

Question 28

what is the advantage to the virus of using a secondary vector?

Answer 28

the natural environment is a barrier to infection as most viruses are sensitive to heat, drying and UV light

Question 29

what is horizontal transmission?

Answer 29

host-host transmission | direct, relies on high rate of infection to maintain virus population

Question 30

what is vertical transmission?

Answer 30

the transmission virus from one generation of hosts to the next, can occur by infection of the foetus before, during or after birth. relies on long term persistence of the virus in the host rather than rapid propagation and dissemination of the virus

Question 31

what is the significance of local infection?

Answer 31

virus must initiate an infection by entering a susceptible cell. this initial interaction determines whether an infection will remain localised or become systemic

Question 32

by which ways can infection spread via the bloodstream?

Answer 32

- direct inoculation via arthropod vectors, blood transfusions/IV drug use - free travel of virus in the plasma (togavirus, enterovirus) - association with blood cells

Question 33

what is the importance of tissue tropism and what dictates it?

Answer 33

controls spread. tissue tropism is controlled by route of infection, but also interaction of a virus attachment protein with a specific receptor molecule on the surface of the cell

Question 34

what is antigenic drift?

Answer 34

gradual accumulation of minor mutations (e.g. nucleotide substitutions) in the virus genome, which results in subtly altered coding potential and antigenicity

Question 35

what is antigenic shift?

Answer 35

a sudden and dramatic change in the antigenicity of a virus due to reassortment of the segmented virus genome with another virus genome of a different antigenic type

Question 36

what are the two main requirements for long term persistence of virus infections?

Answer 36

- regulation of lytic potential | - evasion of immune surveillance

Question 37

what is abortive infection?

Answer 37

when a virus infects the cell but cannot complete the full replication cycle

Question 38

what is meant by the term 'latent infection'?

Answer 38

when the virus down regulates its gene expression and enters an inactive state with strictly limited gene expression and without ongoing replication