Lecture 1

Question 1

What is a virus?

Answer 1

An ultramicroscopic (20 to 400 nm in diameter), metabolically inert and infectious agent, that replicates only within the cells of living hosts, mainly bacteria, plants, and animals (obligate parasite)

Question 2

What are the basic features of a virus structure?

Answer 2

DNA/RNA core, a protein coat and, in more complex types, a surrounding envelope

Question 3

Outline the basic route do viruses take to cause disease?

Answer 3

1. They are introduced to the population
2. They infect and spread (infect-replicate-infect)
3. They escape out body’s defenses
4. They cause disease

Question 4

What are the three main types on virus in terms of their abiltiy to evade host defenses?

Answer 4

1. The “city” virus - They change very quickly to escape becoming recognizable to the immune system playing a fitness cost game
2. The “I buy what I can afford” virus - They have been around long enough to have safe ways to escape from our immune system
3. The “infect whatever moves” virus - They are not particularly good at escaping our defenses, just highly infectious so they go to the next person before our responses kick in

Question 5

What are Koch’s postulates?

Answer 5

1. The organism must always be present, in every case of the disease.
2. The organism must be isolated from a host containing the disease and grown in pure culture.
3. Samples of the organism taken from pure culture must cause the same disease when inoculated into a healthy, susceptible animal in the laboratory.
4. The organism must be isolated from the inoculated animal and must be identified as the same original organism first isolated from the originally diseased host.

Question 6

Who is David Baltimore?

Answer 6

A nobel prize winning scientist who came up with the classification systems for viruses

Question 7

What are the Baltimore classes?

Answer 7

Class I: Double-stranded DNA viruses (e.g Herpesviruses: herpes simplex, chicken pox, EBV, Papillomaviruses: warts (cervical cancer) and Poxviruses: small pox)

Class II: Single-stranded DNA viruses (e.g. Anelloviridae, Circoviridae, and Parvoviridae (which infect vertebrates), the Geminiviridae and Nanoviridae (which infect plants), and the Microviridae (which infect prokaryotes))

Class III: Double-stranded RNA viruses (e.g. Reoviridae (reovirus, rotavirus) and Birnaviridae)

Class IV: Single-stranded RNA viruses - Positive-sense (e.g Picornaviruses: polio, rhinovirus (cold), hepatitis A, hepatitis C and Flaviviridae: Dengue, West Nile Virus)

Class V: Single-stranded RNA viruses - Negative-sense (e.g. Arenaviridae, Orthomyxoviridae (influenza), Paramyxoviridae, Bunyaviridae, Filoviridae (Ebola), and Rhabdoviridae (rabies))

Class VI: Positive-sense single stranded RNA viruses that replicate through a DNA intermediate (e.g. HIV and HTLV)

Class VII: Double-stranded DNA viruses that replicate through a single-stranded RNA intermediate (e.g. Hepatitis B)

Question 8

Whatis the common feature across all of the baltimore classes?

Answer 8

The genetic material of each class will code for mRNA that can be translated into proteins to make and release new viral particles

Question 9

What name applies to the extracellular phase of a viruses life-cycle?

Answer 9

The virion

Question 10

What size range can be sene in viral genomes?

Answer 10

Between 5000 and 23,000 base pairs

Question 11

What feature in common in the varying virion structures of different viruses?

Answer 11

A highly repetitve protective protein capsid surrounding the nucliec acid componenet

Question 12

Compare and contrast an enveloped and naked virus

Answer 12

Both enveloped and naked virueses have a nucelocapsid, composed of nucleid acid and a capsid made of capsomers, however, enveloped viruses also have an envelope surrounding the nucleocapsid.

Question 13

How is attachment to host cells mediated
?

Answer 13

Interactions between proteins on the virus surface and the host surface molecules (usually glycoproteins), which are called viral receptors.

Question 14

What are the two types of entry into host cells used by enveloped viruses?

Answer 14

1) Membrane Fusion: Proteins of the virion attach to viral receptors and the membranes will fuse. The nucleoplasmid is then released into the cytoplasm and the viral envelope remains part of the plasma membrane. The nucleic acid seperates from nthe capsid coat protein.
2) Endocytosis:Virion is adsorbed into the host cell; as the plasma membrane forms a vesicle around the virion. Enevelope of the virion fuses with the plasma membrane and the nucleoplasmid is released from the vesicle into the cytoplasm. The nucleic acid is seperated from the capsid protein.

Question 15

How is viral genome of a class I virus replicated?

Give an example class I virus

Answer 15

1) Viral DNA is transported to the host nucleus
2) Host RNA polymerase converts viral DNA to viral mRNA in nucleus, then transported to cytoplasm
3) Viral RNA’s translated to new viral proteins, transported back to nucleus
4) Viral DNA replicated by host DNA ploymerase and viral factors
5) Reassembly of viral genome and proteins, followed by release

Herpesvirus is an example of a class I virus

Question 16

How is the viral genome of a classIV virus replicated?

Give an example class IV virus

Answer 16

1) Viral RNA translated to protein by cytoplasmic ribosomes
2) Viral RNA genome is replicated by the viral synthetase
3) Viral proteins form the capsid and encapsulate the replictaed RNA genomes forming new virions
4) The new virions bud from the infected cell

Hepatitis C is a class IV virus

Question 17

How is the viral genome of a Class V virus replicated?

Give an example class V virus

Answer 17

1) The virion is taken up by endocytosis
2) RNA negative strand is converted to mRNA using viral RNA polymerase
3) mRNA is translocated in cytoplasm to produce viral proteins

Infulenza is a class V virus

Question 18

How is the viral genome of a class VI virus replicated?

Give an example class VI virus

Answer 18

1) The virion enters the host cell via binding and fusion of viral envelope proteins to receptors
2) The virion is uncoated
3) Viral reverse transcriptase converts one RNA strand into ssDNA and then into dsRNA. The dsDNA then enters the nucleus
4) The viral DNA is integrated into host genome by viral integrase. The provirus is formed
5) Viral DNA is transcribed into viral mRNA and progeny RNA
6) HIV polyproteins are synthesised and processed by viral proteases to generate mature functional proteins
7) Viral ssRNA is encapsulated
8) The virion buddes and is released to infect new cells

HIV is a class VI virus

Question 19

How is the viral genome of a class VII virus replicated?

Give an example class VII virus

Answer 19

1) VIrion binds to the cell membrane
2) Virion fuses with cell membrane and the core enters the cytoplasm
3) Double stranded genome is completed by enzymes within the core
4) DNA enters the nucleus
5) DNA is transcibed to genomic and mRNA
6) RNA enters cytoplasm and the viral proteins are translated
7) Core assembles around + sense genomic RNA
8) + sense RNA copied to - sense DNA
9) - sense DNA partly copied to + sense DNA
10) Core associates with antigen in cell membrane
11) Virus buds through cell membrane and virion is released

Hepatitis B is an exmaple of a class VII virus

Question 20

What are the four consequences of a viral infection?

Answer 20

1) Transformation into a tumour cell
2) Death of cell and release of virus (lytic infection)
3) Slow release of virions without cell death (latent infection)
4) Virus present but not cauing harm to cell, but may later emerge in lytic action (latent infection)

Question 21

What are the three types of viral infection?

Answer 21

1) Lytic infection: Virus disappears after disease ends
2) Latent infection: After initial infection, virus replicates in non-infectious state. Virus is then activated to produce new disease symptoms
3) Persistent infection: After initial infection, with or without disease symptoms, the virus is released from host with no symptoms

Question 22

What makes a virus a virus?

Answer 22

The ability to disintegrate and reconstitute itself

Question 23

What does a plaque forming unit assay measure?

Answer 23

Infectious Virions

Question 24

What does an ELISA measure?

Answer 24

Antigen Levels

Question 25

What does serology measure?

Answer 25

Antibody Levels

Question 26

What are electorn microscope used for?

Answer 26

Visualising virions

Question 27

What does PCR achieve?

Answer 27

Molecular amplification and can be used to measure genomic copies