Viral properties

Question 1

5 key features of viruses & consequences of these features

Answer 1

1. Obligate intracellular parasites: so they must be infectious to endure in nature
2. Depend on biochemical machinery of host cell to replicate: viruses must use host processes to produce their components
3. Can not make energy nor produce nucleic acids/proteins independent of host: viruses must use host processes to produce their components
4. Replication is by self-assembly of individual components (nucleic acid, capsid, other proteins): Viral components must self-assemble
5. Genome is either DNA or RNA

Question 2

What is the structure of a virus?

Answer 2

1. Nucleocapsid: The genome (nucleic acid, RNA or DNA) contained in a protein capsule (lipid envelope)
2. Virion is the infective viral particle
   - For non-enveloped (naked) viruses, the nucleocapsid = virus particle = virion
   - For enveloped viruses, virion = nucleocapsid + envelope

Question 3

What is the virus’ capsid and envelope made up from?

Answer 3

1. Capsid = made up from proteins, each protein = capsomer

2. The envelope is made of lipids and spike proteins

Question 4

Naked (non-enveloped) vs enveloped viruses

Answer 4

1. Naked
   - More stable in the face of environmental stress
   - Spreads more easily (survives on surfaces)
   - Survives gut, poor water treatment etc.
2. Enveloped
   - Must stay wet to remain infectious
   - Very sensitive to detergents (due to membrane)
   - Spreads through large droplets
   - Does not need to kill cell to spread (buds)

Question 5

What are the 6 stages of the infectious (productive) cycle?

Answer 5

1. Attachment: of virus to host cell receptor
2. Penetration
3. Uncoating: to release its genetic material
4. Replication (gene expression): & proteins made
5. Assembly
6. Release

Question 6

What are the 4 effects of viral infection of a host cell?

Answer 6

1. Lytic infection: death of cell, release of virus particles and leads to lytic infection
2. Latent infection: If the genetic material or the virus is present and stays there. E.g a lung infection but you don’t have necessarily constant symptoms
3. Persistent infection: slow release of virus without cell death
4. Transformation of host cell into a tumour cell

Question 7

How can a virus transform a host cell into a cancer cell

Answer 7

1. Introduction of a viral oncogene

2. Insertion of viral DNA into host genome activates host cellular oncogene

Question 8

Why are viral infections prevalent, persistent and problematic?

Answer 8

• Few drugs available & drug resistance
• High mutagenic rates
• Emerging infectious diseases
• Re-emerging infectious diseases (polio)
• Public health issues/spread: clean water etc
• Latent infections and persistent infections