The nature of viruses

Question 1

What is the basis of the Baltimore classification system?

Answer 1

Based on genome & mode of replication

Question 2

List and describe the Baltimore classification groups.

Answer 2

Class:
1: dsDNA
2. ssDNA
3. dsRNA (segmented)
4. ssRNA +sense (already an mRNA)
5. ssRNA -ve sense (needs to be converted to +ve)
6. ssRNA +ve sense w/ dsDNA intermediate (before replication). MEANS: RNA + reverse transcriptase => DNA => mRNA
7. dsDNA w/ +ve sense (ssDNA). undergo reverse transcriptase => mRNA

Question 3

Describe the typical structure of (enveloped) viruses

Answer 3

• capsid (protein coat) made up of capsomeres (w/ nucleic acid inside)
• envelop (membrane)
• protein projections (for attachment on host cell)

Question 4

List and describe the typical replication life cycle of viruses. (from infect cell -> release virions)

Answer 4

1. Attachment to specific host cell’s receptor on membrane
2. Penetration: enter host cell & “uncoats” capsid
3. Replication of viral genome: production of viral mRNA
   & early viral proteins (non-structural> assist in replication)
4. Production of late viral (structural) proteins
5. Assembly of the progeny virions
6. Release of virions

Question 5

2 ways viruses cause disease?

Answer 5

1. Replication w/in host cell => damage in cell
2. Damage form host defenses in its attempt to remove virus from infected cells

Question 6

Describe 3-5 types of cytopathic effect (CPE).

Answer 6

• low immunity: destruction of T helper cells & adaptive I.Resp
• transformation of cells: pre-cancerous-like changes
• inclusion bodies: aggregation of mature viral particles
• syncytia: cell fusion
• lysis: stop host cell from making protein -> lysis

Question 7

List the events leading to a viral infection.

Answer 7

1. Invade host
2. Replicate in cells of inoculation
3. Overcome local defences
4. Spread to other cells & other areas
5. Replicate again
6. Exit from host in lrg no. to infect another host

Question 8

Which immune cell type is most responsible for fighting viral infections?

Answer 8

Cytotoxic T cells > destroy cells w/ virus Antigenic markers (presented by Clas 1 MHC)

Question 9

How can viruses evade the immune system? (7)

Answer 9

• poorly immunogenic (stimulate IR)
• Not displaying enough of their antigen on the surface
• Inactivate B cells, T cells, macrophages
• Interfere w/ expression/transport of MHC proteins
• excess antigen production > neutralise antibodies
• mutations - change antigens of virus
• infection to foetus before immune system has developed

Question 10

What is the incubation time for Rabies?

Answer 10

28-140 days (3-12wks)

Question 11

What is “RO” and what does it describe?

Answer 11

a) Ro = mean of 2dary cases generated by one primary case in a susceptible community
b) Reproduction number (Ro): infectiousness of a virus