Medically Relevant Viruses

Question 1

What are nucleation sites ?

Answer 1

Nucleation sites are where viruses attach to

Question 2

What is the latency period for HIV ?

Answer 2

10 years

Question 3

What is the difference between a naked and enveloped virion ?

Answer 3

Both contain a nucleic acid surrounded by a capsid but enveloped virions then have an envelope
The envelope has a phospholipid bilayer and viral proteins

Question 4

What are the viral replication rules ?

Answer 4

• viruses must induce a living host cell to synthesize essential components to make new virions
• dead host cells cannot replicate viruses because viral replication requires energy
• in bacterial cells, the viral nucleic acid enters the host cell but the capsid stays outside
• in plant and animal cells, the entire virion enters the host cell

Question 5

What is the replication cycle of bacteriophages ?

Answer 5

1) the virion attaches to the host cell
2) the DNA enters the host and the capsid remains on the outside
3) minutes after infection, the genome and proteins replicate and translate and this is known as the eclipse phase
4) the synthesised viral nucleic acids become packaged inside their capsids and this is known as the maturation phase
5) the mature virions are then released from the host by lysis or budding off

Question 6

What is the latent period ?

Answer 6

Eclipse phase + maturation phase = latent period
It is the time between infection and disease

Question 7

What are the rules for adsorption and penetration of animal viruses ?

Answer 7

• viruses can only infect a host cell if it can interact with a specific receptor on its cell membrane
• the presence of receptors determines the limits of the virus host range
• animal viruses enter by receptor mediated endocytosis or membrane fusion

Question 8

What is tissue tropism ?

Answer 8

The ability of a virus to replicate in a particular cell/tissue

Question 9

What is the Baltimore classification system ?

Answer 9

It is the system in which viruses are categorised and classified according to their genome

Question 10

Give some examples of class I animal viruses

Answer 10

• Adenovirus (common cold)
• herpesvirus

Question 11

What is the mechanism of viral infection and the viral life cycle of the common cold ?

Answer 11

1) there is a special interaction between the viral fibre protein and the host cell membrane
2) receptor mediated endocytosis engulfs the virus in an endosome
3) the endosome fuses with a lysosome and the acidic pH causes the endosome to rupture which releases the virus into the cytoplasm
4) the virus migrates to the nucleus and is uncoated
5) a viral DNA cellular histone complex forms
6) host RNA polymerase transcribes viral genes into mRNA
7) the viral genome encodes DNA polymerase for viral DNA replication

Question 12

What is the mechanism of viral infection and the viral life cycle of the flu ?

Answer 12

1) the influenza virus binds to spikes on the cell membrane
- these spikes also bind sialic acid residues
2) it is engulfed by endocytosis
3) the acidic environment causes membrane fusion and uncoating
4) the viral RNA and RNA polymerase enter the nucleus
5) a viral RNA replicase enzyme synthesises new strands of RNA
6) these strands are translated or copied into new genomes
7) nucleocapsids are assembled
8) the envelope proteins incorporate into the cell membrane and the nucleocapsids bud out

Question 13

What is the mechanism of viral infection and the viral life cycle of HIV ?

Answer 13

1) HIV binds to the receptor - coreceptor complex of a helper T cell
2) the viral envelope and cell membrane fuse
3) the nucleocapsid enters the cytosine of the helper T cell
4) the reverse transcriptase enzyme makes viral DNA from the viral RNA
5) the viral proteins are assembled into new viruses and these bud off from the cell membrane

Question 14

Give some examples of class IV viruses

Answer 14

• coronavirus
• rubella

Question 15

Give some examples of class V viruses

Answer 15

• Influenza
• Ebola