nature of viruses

Question 1

Virus structure

Answer 1

Virion - nucleic acid (genome) surrounded by capsid = nucleocapsid
some have lipid membranes (envelopes) derived from budding of
nucleocapsid through host cell membrane

Question 2

Capsid structure

Answer 2

Repeating protein subunits capsomers arranged symmetrically -
can be identical subunits (product of single gene suficient to construct coat)

Question 3

Positive vs negative strand RNA genome

Answer 3

Positive - if RNA is mRNA (can be directly translated into viral proteins)
Negative - complementary to mRNA - mRNA is obtained by transcribing first

Question 4

Coding efficiency (4)

Answer 4

1. Densely packed genes
2. Small intergenic spaces with few non-coding spaces
3. Overlapping reading frams, use of same nucleic acid to code for >1 protein
4. RNA splicing

Question 5

What is the size of viral genomes limited by?

Answer 5

Error prone nature of RNA polymerases - if genome too big then polymerase creates too many mutations

Question 6

arboviruses

Answer 6

transmitted by biting insects

Question 7

What is current classification based on?

Answer 7

Genetic relatedness so will have similar replication strategies

Question 8

Detail the three phases of virus lifecycle and give examples at each
stage e.g. specific example of a virus binding

Answer 8

1. Adsorption and penetration
   - Viruses bind to specific receptors on host cell
   e.g. HIV envelope protein g120 binds to CD4 on T cells and chemokine
   co-receptor
   - Fusion of virus and cell membranes: can either be at cell membrane
   or endocytosis first and viral membrane and cell membrane have to
   get close
2. Eclipse phase
   - highly regulated - temporal and quantitative - capsid replicated
   later
   - Needs to form own polymerase or use hosts
   - formation of new viral proteins and replication of nuclei acids
   -so called because no infectious virus present in cell -> has been
   disassembled and viral proteins are being made
3. Assembly and release
   - production of new infectious virus particles
   -lysis

Question 9

What are the cases for fusion at the plasma membrane vs
endocytosis first? Give a specific example of how membrane fusion
occurs

Answer 9

For enveloped viruses fusion occurs - at membrane with neutral pH
and low pH after endo.
HIV: binding of gp120 with CD4 causes a conformational change to
bring membranes close
Influenza: binding to sialic acid -> endocytosis and acidification -> brings
viral envelope closer to vesicle and insert of HA AA causes membrane
disruption and fusion

Question 10

Receptors used for influenza virus binding and EBV

Answer 10

haemagglutinin (HA) binds to cell surface sialic acid (which is present
on almost all cells)
gp340 with CD21

Question 11

Latent period - HSV in human fibroblasts

Answer 11

Time taken to form new particles - 10h

Question 12

Mean burst size - HSV in human fibroblasts

Answer 12

Average yield of virus particles - 50

Question 13

Non-enveloped viruses penetration and example

Answer 13

1. Binding of birus to receptor
2. Conformaitonal change of virus causing disruptionof host membrane
3. Transfer of capsid and nuclei acid to cell
   Poliovirus