Lecture 17. Classification

Question 1

What are the disadvantages of classifying viruses based on diseases?

Answer 1

Focuses on some viruses and ignores others
A single viruses may cause more than one disease (e.g chickenpox)
Viruses infect more than one host

Question 2

What are examples of classifying viruses based on diseases?

Answer 2

Hepatitis viruses
Common cold viruses
Respiratory viruses
Enteroviruses

Question 3

What is classification based on host?

Answer 3

To group viruses according to the host that they infect: human viruses, animal viruses, bacterial viruses (bacteriophages), plant viruses, insect viruses.

Question 4

What are examples of viruses that have restricted host range?

Answer 4

Hepatitis B virus (HBV), Human Papillomavirus (HPV)

Question 5

What is an example of a virus that infect a small range of hosts?

Answer 5

Poliovirus

Question 6

What are examples of viruses that infect very different species?

Answer 6

Rabies, influenza

Question 7

What are examples of viruses grouped based on the nature of the host cell?

Answer 7

Herpesviruses, alphaherpesvirsues, betaherpesviruses, gammaherpesvirsues

Question 8

What are the main criteria for classification based on virus particle morphology and nucleic acid?

Answer 8

Nucleic acid
Type of nucleic acid (DNA or RNA). Single or double stranded. Linear, circular, single molecule or segmented (Influenza). If single-stranded: negative or positive (polarity)
Capsid symmetry (icosahedral, helical or complex)
Presence or absence of lipid envelope

Question 9

When was the International Committee on Taxonomy of Viruses (ICTV) founded?

Answer 9

Late 1960s

Question 10

What is the Systemic Taxonomy Scheme?

Answer 10

Order
Family
Subfamily
Genus
Species

Question 11

What isn’t included in the Systemic Taxonomy Scheme?

Answer 11

Kingdoms, Phyla and Classes

Question 12

What are characteristic for ICTV?

Answer 12

Host range (eukaryote or prokaryote, animal, plant…)
Morphological features of the virion (enveloped, capsid symmetry…)
Nature of the genomic 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 and presence or absence of specific genes in the genomes of similar viruses
Phylogenetic tress may be established using nucleotide
sequencing

Question 13

What is the Baltimore Classification of Viruses?

Answer 13

Thousands of different viruses
Cause different number of viral infections and diseases
All viruses must make their mRNA
mRNA will be read by the host ribosome to produce the viral proteins
No exception to this

Question 14

What are the 7 viral genome types in the Baltimore classification system?

Answer 14

i) dsDNA
ii) ssDNA (+ve strand)
iii) dsRNA
iv) ssRNA (+ve strand)
v) ssRNA (-ve strand)
vi) ssRNA → ssDNA (-)
vii) gapped dsDNA

Question 15

What are viroids?

Answer 15

Novel agents of disease in plants
Contains a single circular ssRNA molecule as infectious materials
No protein component
Viroid genomes range in size from 220 to 400nt
The smallest self-replicating pathogens known
Up to 70% of the nucleotides in the genome RNAs are base-paired
They appear as rod-shaped or dumb-bell-shaped molecules

Question 16

What are many viroids?

Answer 16

Important agricultural pathogens, others replicate without causing symptoms

Question 17

What is an example of a viroid?

Answer 17

Potato Spindle Tuber Viroid (PSTV)
Pathogen of potatoes
Causes stunting of the plant and malformation and cracking of tubers

Question 18

What are prions?

Answer 18

Agents of a number of diseases characterised by slow, progressive neurological degeneration that are fatal
The diseases are associated a spongy appearance of the brain
No nucleic acid has yet been found in association with infectious material