5 Viruses for Virology

Question 1

what is the shape of most plant viruses

Answer 1

rod shaped (flexuous or rigid)

Question 2

what is the genome of plant viruses

Answer 2

• 90% of plant viruses have ssRNA genomes

- other 10% are either dsRNA, ssDNA or dsDNA

Question 3

how many polypeptides can nucleic acids in genome code for

Answer 3

nucleic acid is usually enough to code for 3-10 polypeptides

Question 4

virus effect on plants

Answer 4

Virus don’t kill the plant cells - will replicate in the and reduce the yield of the plant

Question 5

what are the classical characteristic symptoms of virus infected plants

Answer 5

classical symptoms are colour deviations – Mosaics/striping in leaves – colour break in flowers

Question 6

what are the effect on plant in virus infect plants

Answer 6

• presence of virus ‘taxes’ the plant – reduces yield
• may also cause dwarfing (Barley yellow dwarf)
• may cause malformations such as leaf curls or tumours

Question 7

effect of virus infecting trees

Answer 7

usually best to destroy infected plants to prevent spread to other trees

Question 8

persistant viral transmission

Answer 8

Is persistant if it gets to the salivary gland

Question 9

non persistant viral transmission

Answer 9

Insect picks the virus up within a few seconds of feeding, will transmit to another plant if it feeds – is non persistant (can only transmit for a few hours) as the virus is on the outside of the plant

Question 10

infection process

Answer 10

• virus must enter plant – usually carried by an insect vector, such as an aphid
• virus must replicate, produce more copies of genome and coat protein
• virus must reassemble
• virus must spread from cell to cell

Question 11

replication for RNA virus

Answer 11

• RNA encodes a replicase which copies RNA into subgenomic RNAs
• subgenomic RNAs encode proteins with the different functions
• different RNAs are packaged into different particles – multipartite viruses

Question 12

what does RNA encode

Answer 12

a large polyprotein, which is then cleaved by a proteinase to produce the peptides with each of the different functions

Question 13

what are the three particles required for infection

Answer 13

RNA1 (in particle 1)
RNA2 (in particle 2)
RNA3 (in particle 3)

Question 14

what does the RNA1 encode

Answer 14

methyltransferase and helicase

Question 15

what does the RNA2 encode

Answer 15

RdRp (RNA dependant RNA polymerase)

Question 16

what does RNA3 encode

Answer 16

contains the movement and coat protein genes, the latter being produced from a subgenomic RNA

Question 17

what is a strand of RNA encapsidated by

Answer 17

RNA is generally encapsidated in multiple copies of a single polypeptide

Question 18

Self-assembly

Answer 18

• Assembly is a spontaneous event – put RNA and protein in a test tube, they will interact to form a virus of the correct structure
  Spontaneously assemble – can dislocate to form a helical structure (virus binds inside – packs inside)

Question 19

Virus spread within plants

Answer 19

part of replication, virus produces a movement protein
Migrates to plasmodesmata (narrow channels between plant cells), widens them to facilitate movement of virus between cells
Virus enters the phloem, allowing long distance movement to cells in the rest of the plant

Question 20

How do plants defend themselves against viruses

Answer 20

Resistance genes that encode proteins which recognise viral proteins and elicit a defence response in the plant – the hypersensitive response
Post-transcriptional gene silencing
But viruses have counter defence mechanisms themselves

Question 21

Defence against virus

Answer 21

when replicates will go through ds intermediate form recognised by dicer proteins – chip into small interfering ribonuclear protein
siRNPs degrade the RNA