virus diversity, structure and classification L13

Question 1

DEFINTION of virus

Answer 1

nucleoprotein complex that infects cells and uses cells metabolic processes to replication

smallest infective agents known

metabolically inert - no activity outside hist cell

most are highly species specific

Question 2

how to see viruses

Answer 2

electron microbe or x-ray is really small, between 10-400 nm

Question 3

viral morphology general

Answer 3

structural components
- nucleocapsid
- envelope (either + or - )

genetic content
- nucleic acid
- DNA or RNA
- ss or ds
- linear or circular
- segmented

Question 4

capsid morphology

Answer 4

capsid is the protein shell made up of capsomeres that protect the viral genome and help virus attach to host cell

capsomere arrangement
- polyhedral
- helical
- complex

Question 5

enveloped viruses

Answer 5

can either have naked or enveloped

• enveloped have a lipid layer around the caspid, , which can contain viral glycoproteins to help bind to host cell surfaces

Question 6

viral envelope morphology

Answer 6

components
- phospholipids
- proteins
- glycoprotein spikes - bind to receptor on host cells so determine which cells can viruses infect (tropism) and also can be a target for vaccines

Question 7

viral nucleic acid morphology

Answer 7

DNA or RNA

can either be circular or linear

can have multiple genomic segments

can be single stranded or double stranded

Question 8

viral classification

Answer 8

this is based on molecular biology of genome and biophysical structure

Question 9

polyhedral and helical viruses

Answer 9

polyhedral
these are also called icosahedral
these have an envelope and DNA, and are symmetrical

helical, have envelop and DNA but are not symmetrical, cylindrical

Question 10

baltimore classification

Answer 10

explained the relationship between the viral genome and mRNA
there are 7 classes
- includes whether it is DS or SS, if it is positive or negative sense or if reverse transcriptase or uses DNA or RNA

+ssRNA = same sense as mRNA, can. be used directkly
-ssRNA - complementary to mRNA so needs to be converted
reverse transcriptioase - RNA —> DNA

Question 11

growing viruses

Answer 11

can be grown in tissue culture like human or mouse fibroblasts or epithelial cells

can be grown in fertilised chicken eggs, as live cells are needed to go virus, and fertilised eggs are controlled, sterile, nutrient rich environment and cheap and easy to produce

Question 12

viral quantification - plaque assay

Answer 12

the number of infectious virus particles in a sample by counting clear plaque zones

plaque count x (1/dilutioin) = plaque forming units. (PFU)
PFU/ML = PFU divided by volume of virus added

Question 13

viral quantification - CPE

Answer 13

cytopathic effect - visible changes or damage to host cells, cell rounding , detachment, fusion of cells or lysis

• TCID 50 = tissue culture infectious dose 50%, the quantify of virus required to produce signs of infection in 50% of cultures

this is less precise than plaque assay

Question 14

one step growth curve

Answer 14

used to study replication cycle of viruses in single round of infection, helps to understand viral replication, latent period, burst size etc

lag phase - virus attaches to cells but not yet replicating - latent phase, time from infection until first viral particles are produced

log phase - virus replication accelerates, increase in viral particles

plateau phase - virus reaches peak concentration and host cell is near lysis - burst size - max number of virus particles produced per cell - peak

decline phase - viral particles are released and decrease as host cells die or get depleted