Lecture 17: Classification of Viruses

Question 1

What are the main criteria for the classfication of viruses?

Answer 1

• The type of nucleic acid
• The capsid symmetry
• The presence or absence of a lipid envelope

Question 2

What are the 2 types of unconventional viruses?

Answer 2

Prions and virions

Question 3

Is it true that viruses can be classified based on the diseases they cause?

Answer 3

Yes

Question 4

How can viruses be classified based on the diseases they cause?

Answer 4

They can be classified as..

• Hepatitis viruses
• Common cold viruses
• Respiratory viruses
• Enteroviruses
• Sexually transmitted viruses

Question 5

What are the disadvantages of classifying viruses based on the diseases they cause?

Answer 5

• Some viruses will be focused on, whilst some will end up getting ignored
• A single virus may cause more than one type of disease
• Viruses infect more than one host and sometimes with different manifestations in each host

Question 6

Is it true that some viruses can be classified according to the host that they infect?

Answer 6

Yes i.e (human viruses, animal viruses, bacterial viruses (bacteriophages), plant viruses, insect viruses)

Question 7

When is it a good idea to classify viruses based on the host they infect?

Answer 7

• WIth viruses that have restricted host range
• With viruses that infect a small range of hosts

Question 8

What is a problem with classifying viruses based on the host they infect?

Answer 8

• Because some viruses can be grouped even further beyond this

Question 9

Is it true that viruses can be classified based on their structure?

Answer 9

Yes

Question 10

How can viruses be classified based on their structure?

Answer 10

• The nature of the nucleic acid (whether nucleic acid is DNA or RNA; single or double stranded DNA; linear or segmented; if single stranded, if negative or positive polarity).
• Capsid symmetry (icosahedral, helical or complex
  ).
• Presence or absence of lipid envelope

Question 11

Is it true that influenza strains can have different genomes to one another?

Answer 11

Yes

Question 12

Which strand of RNA can be translated immediately by the ribosomes into proteins?

Answer 12

+ sense RNA

Question 13

Describe the -ve RNA

Answer 13

• Complementary to the +ve RNA strand.
• Has to be replicated by RNA-dependent RNA polymerase (RdRp) into +ve RNA before translation

Question 14

What type of capsid does smallpox have?

Answer 14

A complex one

Question 15

Is it true that there are more human RNA viruses than human DNA viruses?

Answer 15

Yes

Question 16

Can viruses be classfied based on taxonomy?

Answer 16

Yes

Question 17

How can viruses be classified based on taxonomy?

Answer 17

• Order
• Family
• Subfamily
• Genus
• Species

(X No Kingdoms, Phyla, Classes
)

Question 18

What committee are new viruses currently classified by?

Answer 18

The International Committee on Taxonomy of Viruses (ICTV), founded in the late 1960s,

Question 19

What are the characterstics for ICTV?

Answer 19

• Host range (Eukaryote or prokaryote, animal, plant)
• Morphological features of the virion (enveloped, capsid symmetry)
• Nature of the genome nucleic acid (DNA or RNA, single stranded or double stranded, positive or negative sense, etc.)
• Additional features may allow subdivisions allocation including:
  >length of the tail of a phage
  >presence or absence of specific genes in the genomes of similar viruses..
• (Phylogenetic trees may be established using nucleotide sequencing..
  )

Question 20

Who developed the Baltimore Classification of Viruses?

Answer 20

David Baltimore

Question 21

Who won a nobel prize for the discovery of reverse transcriptase?

Answer 21

David Baltimore

Question 22

What did David Baltimore realise about all viruses?

Answer 22

That they all must be able to to make mRNA, as this will be read by the host cell’s ribosomes

Question 23

How many classes does the Baltimore system of classification classify viruses into?

Answer 23

7

Question 24

Baltimore Classification of Virues

Answer 24

1) Double stranded DNA viruses

2) Single stranded DNA

3) Double stranded RNA

4) Single stranded RNA. RNA is +ve sense.

5) SIngle stranded RNA that is -ve sense (can’t immediately be translated into proteins)

6) Single stranded RNA that is positive sense, replciation is through a DNA intermediate.

7) Double stranded DNA, where one strand is incomplete. (hepatitis viruses)

Question 25

What are the 2 types of unconventional viruses?

Answer 25

Prions and viriods

Question 26

Describe viriods

Answer 26

• Infect plants
• Who won a nobel prize for the discovery of reverse transcriptase?
• Not surrounded by protein or capsid component
• The smallest self-replicating pathogens known.
• They appear as rod-shaped or dumb-bell-shaped molecules
• Can cause Potato Spindle Tuber Viroid (PSTVd), which is a pathogen of potatoes that results in stunting of the plant and malformation and cracking of tubers.

Question 27

Describe prions

Answer 27

• Agents of a number of diseases characterized by slow, progressive neurological degeneration that are fatal.
• Diseases give the brain a spongy appearance
• Cause mad cow disease in cows
• Cause misfolding of proteins. This reduces the solubility of proteins.
• Replicate slowly within their hosts.
• Has no nucleic acid