Virus Structure and Function

Question 1

What are the basic properties of a virus?

Answer 1

1) Submicroscopic, obligate intracellular parasites
2) Not alive, don’t grow or undergo division
3) in host cell, genome is replicated — directs synthesis of viral components => progeny
4) Produced from self-assembly of newly-synthesized components

Question 2

What are the strategies that viruses employ for survival?

Answer 2

• House DNA/RNA genome in capsids
• Genome contains all info to initiate/complete infectious cycle
• Establish relationship w/ host — can be benign to lethal

Question 3

What are the two means of classifying viruses?

Answer 3

1) The classical system

2) The baltimore system

Question 4

Describe the classical system of viral classifcation

Answer 4

• Grouped according to their shared physical properties

1) DNA vs RNA
2) Helical vs icosahedral capsid
3) Naked vs enveloped capsid
4) Dimensions of virion/capsid

Question 5

Describe the baltimore system of viral classification

Answer 5

• Based on Central Dogma (DNA => RNA => Protein)
• Says all viruses are parasites of host mRNA translation machinery
• Says viruses must produce mRNA to decode their genomes

OVERALL: this system classifies according to how they produce mRNA

Question 6

What are the main structural characteristics of virus particles?

Answer 6

• Genome

- Capsid

Question 7

What are the seven classes of viral genomes?

Answer 7

1) dsDNA
2) Gapped circular DNA
3) ssDNA
4) dsRNA
5) ss(+)RNA
6) ss(+)RNA w/ DNA intermediate
7) ss(-)RNA

Question 8

Describe the protein expression strategies and replication strategies of dsDNA

Answer 8

• Conventional form of DNA replication/protein expression

Question 9

Describe the protein expression strategies and replication strategies of gapped circular DNA

Answer 9

Structure: part DS, part SS

Question 10

Describe the protein expression strategies and replication strategies of ssDNA

Answer 10

a

Question 11

Describe the protein expression strategies and replication strategies of dsRNA

Answer 11

a

Question 12

Describe the protein expression strategies and replication strategies of ss(+)RNA

Answer 12

• Translated directly into protein by host ribos (the genome is essentially all mRNA)
• Must be translated before replication/mRNA synthesis occurs

Question 13

Describe the protein expression strategies and replication strategies of ss(+)RNA w/DNA intermediate

Answer 13

aka: RETROVIRUS

• When infected, this RNA is converted to dsDNA by virion enzyme (reverse transcriptase)
• dsDNA intermediate integrates into host DNA and becomes permanent part of host genome (a PROVIRUS)

Question 14

Describe the protein expression strategies and replication strategies of ss(-)RNA

Answer 14

• Genome cannot directly make protein
• Must be copied first to make (+) strand mRNA (only the + strand can be translated!!)
• You need an RdRp in the virus to do that

Question 15

Describe a typical, generalized replication cycle of a virus

Answer 15

• NOT logarithmic
• more like a BURST! (one second you have no virus, then the next you got a shit ton)
• Think eclipse and latent phase

1) Entry: membrane fusion
2) Entry and uncoating: fusion of viral envelope w/ cell membrane
3) Uncoating
4) Gene Expression/Translation
5) Replication
6) Assembly of new particles!

Question 16

Describe the general rules of RNA genomes

Answer 16

• Cells have no RNA-dependent RNA poly to replicate RNA viral genomes or to make mRNA from RNA
• RNA viral genomes encode novel RdRps (THIS IS IMPORTANT!!)
• Polys produce both RNA genomes and mRNA from RNA templates
• mRNA produced is readable by host ribosomes!!!

Question 17

What is a capsid?

Answer 17

• Protect genome
• Serve as a specific genome delivery device
• they assemble from components made during infection
• CAN BE surrounded by lipid envelope

Two types:

1) Helical
2) Icosahedral

Question 18

What is the eclipse phase?

Answer 18

a

Question 19

What is the latent phase?

Answer 19

a