Virus Structure and Classification

Question 1

What is the family name and genus name end with for viruses?

Answer 1

Family name: end with -viridae
Genus name: end in virus

Question 2

What are the 5 factors we classify viruses?

Answer 2

Nature of viral genome (genome composition, geneone structure)
Structure of viral capsid
Presence or absence of an envelope
Morphology
Replication strategy

Question 3

What is the nature of viral genome?

Answer 3

Nucleic acid structure—> DNA or RNA
DNA–> ds DNA viruses, ss DNA viruses—> some ds DNA can be circular
RNA—> ss RNA viruses, ds RNA viruses
ss RNA—> Positive sense ss RNA, same sense as mRNA, Transalted directly to protein
—>Negative sense ss RNA, Need to synthesis+ve sense RNA before viral proteins can be translated

Question 4

What is the difference between positive and segative sense ss RNA?

Answer 4

Positive: can start translation instantl—> no not carry RNA polymerase
Negative need to synthesis before translation occurs—> carry RNA polymerase

Question 5

Why some ds and ss RNA can be segmentated?

Answer 5

Its segmentated too prevent instability and degeneration

Question 6

How do we name ds and ss DNA viruses?

Answer 6

Poxviridae=ds DNA
Paaroviridae=ss DNA

Question 7

Name a ds RNA virus and how many segments does it have?

Answer 7

Reoviridae 10,18 segments

Question 8

Name 2 +ve ss RNA (name some properties to one of them)

Answer 8

Reteoviridae (diploid and reverse transcribing)
Picornaviridae

Question 9

Name a -ve ss RNA non segmented virus

Answer 9

Rhabdoviridae

Question 10

Name a -ve ss RNA segmentated virus and name a property of it?

Answer 10

Orthomyxoviridae 6-8 segments

Question 11

RNA viruses need a RNA polymerase to copy RNA genome, why does this make it error prone and what are the consequences of this?

Answer 11

Rna dependent and polymerase has no proof reading cpability
Consequences of this:
- RNA viruses are more variable- within a species of virus are more subtypes/serotypes
- Can evolve rapidly if need to: if zoonotic can adapt rapidly

Question 12

Name 2 DNA families

Answer 12

Asfraviridae
Herpesviridae

Question 13

What are the 3 capsid types:

Answer 13

Icosahedral
Helical
Complex

Question 14

Give properties of the Icosahedral capsid

Answer 14

12 vertices
20 triangular sides
composed of capsomers
-penton and hexon capspmers
Smaller means stronger

Question 15

Give 2 examples of non enveloped icosahedral viruses and their properties

Answer 15

Parvoviridae: 18-26 nm in diameter, capsid consists pf 12 capsomers, 60 copies of a single protein VP2
Adenoviridae: capsid is built from 252 capsomers, 240are hexavalent, 12 are pentavalent, each capsomer contain 1-4 different proteins

Question 16

What are helical capsids?

Answer 16

Capsid protein are arranged in a spiral configuration around a single axis
structural unit is one capsid protein
single capsid proteins are arragned as a helix around the genome
ALL ANIMAL VIRUSES WITH HELICAL SYMMETRY ARE ENVELOPED

Question 17

Give 2 examples of enveloped helical capsid viruses and their properties?

Answer 17

Paramyxoviridae: helical nucleocapsid containing ss RNA, roughly spherical, can be much larger and more pleomorphic (rabies/nipah/hendra)

Rhabdoviridae: approx 180 nm and 75 nm wide, bullet shaped virion, spike-like projection on surface, nucleoprotein encases the RNA genome (rabies)

They all have very irregular shapes

Question 18

What are the properties of a complex capsid?

Answer 18

Some of the large viruses have capsid structures that are more complex
Poxviridae
>100 proteins
Neither helical or icosahedral structure
enveloped, brick-shaped or ovoid virion, 220-450 nm long and 140-260 wide
Surface membrane displayed surface tubules or suface filiments
(Smallpox/mouse pox)

Question 19

In what types of capsids are virus envelopes present?

Answer 19

Few viruses with icosahedral capsid
All viruses with helical capsid
Complex capsid

Question 20

What are the differences between a enveloped vs unenveloped virus release?

Answer 20

Enveloped: acquire envelope as they buf through the host cell membrane, viral envelope contains host cell lipid bilayer as well as viral proteins, viral proteins contain receptor needed for virus entry

Unenveloped: Naked viruses are released by lysis of the infected cell, viral receptors are present on the capsid surface

Question 21

What is the biological properties of enveloped viruses?

Answer 21

> More fragile that viruses with just a capsid
More easily destroyed by
- Detergents
- Disinfectants
- Outside environment
If the envelope is destroyed then the virus is not infectious
Destroys the receptor needed for entry

Question 22

What are the differences in biological properties of enveloped and unenveloped viruses? 6 marks

Answer 22

Undeveloped:
Component: protein
Properties: Environmentally stable to: Temperature, pH, proteases, detergents, drying
Consequences: Resitant to detergent, can drdy out and retain infectivity are spread easily, can survive adverse conditions in the gut, lyse cells to release =, therefore has to kill the cell, usually cause acute infections

Enveloped:
Components: Lipids, proteins, glycoproteins
Properties: Environmentally labile destroyed by: Acid, detergents, drying, heat
Consequences: Easilydestroyed by detergent, must stay wet, not easily spread (large droplets), Cannot survive in the GIT, released by budding, does not need to kill th ecell to spread, can cause persistent infections

Question 23

What is the function of capsid protein?

Answer 23

Ø Structural component of the virus capsid
Ø Protect the viral nucleic acid and deliver the viral nucleic acid to the cell
Ø Capsids of naked viruses
- Contain receptors that attach to the host membrane to allow entry
Contain sites that will induce an Ab response

Question 24

What ?are virus envelope proteins?

Answer 24

Ø Contain receptor that allows the virus to attach and then enter the host cell
Ø Are targets of the humoral and cellular immune response—> Ab will recognise these surfaces exposed viral proteins
Interact with capsid during virus assembly

Question 25

What is the matrix proteins?

Answer 25

Ø Found in enveloped viruses
Ø Forms a layer on inside of virus envelope
Ø Play an important role in virus assembly as the interact with viral core and envelopes
Ø Not all viruses have this
Found in influenza, retrovirus, paramyxoviruses, rhabdovirus

Question 26

What are non-structural viral proteins?

Answer 26

Not structural components of the virus particles
Made in the virus-infected cell following infection
often enzymes involved in viral replication: proteases, helicases, polymerase(can be structural), protein primers for nucleic acid replication
Can be proteins that help the virus avoid the immune system
Target of the host cellular response

Question 27

Some naked viruses can be transmitted inside vesicles so sometimes they need a envelope. WHY?

Answer 27

Ø Some RNA viruses can be transmitted as virus cluster inside vesicles
Ø Rotavirus and norovirus are transmitted as cluster
Ø Vesicles remain intact and they pass through the GI tract to the intestines