Lecture 2 - Viral Agents 1: Nature and Classification of Viruses

Question 1

What is a virus?

Answer 1

An obligate intracellular molecular parasite

Question 2

Eukaryotic organelle that poliovirus is a similar size to

Answer 2

Ribosome (20nm). Polio virion is 30nm

Question 3

Range of virion sizes

Answer 3

20nm to 300nm

Question 4

Negative staining stains

Answer 4

Potassium phosphotungstate

Ammonium molybdate

Question 5

Qualities of good negative stains for viral EM
1)
2)

Answer 5

1) Highly soluble (so don’t crystallise as they dry)

2) Electron-dense

Question 6

Cryo-electron microscopy advantage

Answer 6

Transmission EM without electron-dense negative stain.

Therefore no distortion from sample prep

Question 7

Cryo-electron microscopy
1)
2)
3)

Answer 7

1) Prepare virus by freezing in vitreous (uncrystallised) water
2) Electron microscopy at -160C
3) Two-dimensional projections (Fourier transformations) assembled into a three-dimensional image (inverse Fourier transform)

Question 8

What is cryo-electron microscopy useful for?

Answer 8

Imaging viruses with a labile envelope

Question 9

What are X-ray crystallography images constructed from?

Answer 9

Electron-density maps

Question 10

Protein subunits of capsid

Answer 10

Capsomeres

Question 11

Nucleocapsid

Answer 11

Proteins most closely associated with nucleic acid

Question 12

Matrix

Answer 12

Some viruses have a protein layer beneath the envelope that connects the capsid and envelope glycoproteins

Question 13

Capsid shapes
1)
2)
3)

Answer 13

1) Helical symmetry
2) Icosahedral
3) Complex symmetry

Question 14

Icosahedral symmetry

Answer 14

20 faces

Three axes of symmetry - 2-fold, 3-fold, 5-fold

Question 15

Examples of viruses with icosahedral symmetry
1)
2)
3)

Answer 15

1) Adenovirus
2) Herpesvirus
3) Pappilomavirus

Question 16

Advantage of a lipid envelope

Answer 16

Can leave host cell without killing it

Question 17

What are viruses divided into?

Answer 17

Family, genera, species

Question 18

How are viral families distinguished?

Answer 18

Kind of genome, genome arrangement, replication strategy, virion morphology (capsid symmetry, virion size, envelope)

Question 19

What do viral family names end in?

Answer 19

-viridae

Question 20

What ends in -viridae?

Answer 20

Viral family names

Question 21

How are viral genera distinguished?

Answer 21

Size of genome, genome sequence differences, number, size of proteins, serological reactivity, host range and disease

Question 22

What do viral genus names end in?

Answer 22

-virus

Question 23

What ends in -virus?

Answer 23

Names of viral genera

Question 24

How can DNA and RNA viruses be distinguished?
1)
2)
3)

Answer 24

1) Infect cells in presence of 14C-thymidine and 3H-uracil.
2) Purify viral particles produced in cells
3) Use a scintillation counter (radiation detector) to determine whether a virus contains 14C-thymidine (DNA) or 3H-uracil (RNA)

Question 25

How to determine whether a virus contains ssRNA or dsRNA
1)
2)
3)
4)
5)

Answer 25

1) Label viral RNA during growth
2) Extract viral RNA from purified particles
3) Divide into two portions. Add RNase A/T1.
4) RNase A/T1 only digests ssRNA
5) Trichloroacetic acid (TCA) is used to precipitate RNA. Only RNA polymers will precipitate (if ssRNA has been digested, won’t precipitate)

Question 26

Way to solubilise proteins and lipids in viral purification

Answer 26

Sodium dodecyl sulphate plus or minus proteinase K

Question 27

Viral purification method
1)
2)

Answer 27

1) Solubilise proteins and lipids (sodium dodecyl sulphate/proteinase K)
2) Phenol extraction

Question 28

Molecule separations in phenol extraction
1)
2)

Answer 28

1) Aqueous phase - Nucleic acids

2) Phenol phase - Proteins

Question 29

Do all viruses cause disease?

Answer 29

No. Most infections are subclinical

Question 30

Example of DNA helical-symmetrical virus

Answer 30

None. No DNA helical-symmetrical viruses exist

Question 31

Modified Koch's postulates for viruses
1)
2)
3)
4)

Answer 31

1) Virus should have regular association with disease in question
2) Virus must be characterised (isolated from cell culture, genetically or immunologically distinguished from other viruses)
3) Clinical syndrome should be reproducible in animals or human volunteers
4) Virus should be reisolated from test subject

Question 32

Examples of enteric viruses

Answer 32

Rotavirus
Calicivirus
Some adenoviruses

Question 33

Examples of respiratory viruses

Answer 33

Orthomyxovirus
Rhinovirus
Some paramyxoviruses, coronaviruses, adenoviruses

Question 34

Arboviruses

Answer 34

Infect insects, which then infect humans

Question 35

Viruses which infect insects and then humans

Answer 35

Arboviruses

Question 36

Hepatitis viruses that are spread enterically

Answer 36

A and E

Question 37

Hepatitis viruses spread sexually or through blood

Answer 37

B, C, D