Virus Morphology and Growth

Question 1

Describe what a virus’s host range is, and how a virus can be transmitted.

Answer 1

Where a virus is found can be described as its ‘host range’.
Importantly, some viruses can move between hosts.

Viruses can be found in reservoirs or amplifying hosts. These are organisms in which the virus exists, but doesn’t necessarily cause disease.

Viruses are transmitted via vectors, such as flies and ticks. These vectors can either infect humans or incident/dead-end hosts. They’re called dead-end because the virus cannot spread to humans from them.

Between humans, we have horizontal (from mother to child) transmission and vertical (from human to human via contact/ air, etc.) transmission.

Question 2

In what tissues and organs can viruses be found?

Answer 2

Viruses can be found in multiple tissues and organs of their host. The tissues that viruses can be found in are referred to as their ‘tissue tropism’.

Through the process of evolution, viruses evolve to replicate in tissues that favour transmission between hosts (eg. replicate in lungs, transmitted in air) and allow them to evade the immune system of the host (eg. replicate in cells of immune system, preventing immune function).

Question 3

How do viruses cause disease?

Answer 3

Viruses can cause a range of diseases, either directly (as a product of their host range) or indirectly (by inhibiting cell function).

The disease can be acute (severe and sudden onset) or chronic (long-developing).

Question 4

List some key differences between bacteria, prions and viruses.

Answer 4

BACTERIA:

• contain nucleic acid (DNA) covered in protein
• have a cell wall (composed of sugar molecules)
• can replicate outside of the cell

PRIONS:

• are proteins
• do not contain nucleic acid
• replicate inside the cell

VIRUSES:

• are nucleic acid (DNA or RNA) covered in proteins
• have no cell wall, may or may not have a lipid coat
• mus replicate inside the cell

Question 5

What are the essential structural features of a virus?

Answer 5

• must contain nucleic acid (DNA or RNA)
• covered in a protein coat (as a capsomere or capsid)

Different viruses have different structures, but retain similar organisation.
Virus structure can dictate host range and tissue tropism.

Question 6

Describe the essential features of virus replication.

Answer 6

VIRUS WITH DNA:
A virus will enter the cell and uncoat its DNA. The genome then transcribes this DNA to produce new capsid proteins. At the same time, the viral DNA is being replicated.
There is then the self-assembly of new virus particles and their exit from the cell, either by budding off or lysis of the host cell.
(Depending on the independent strategies of the virus, the genome will go to specific compartments in the cell to replicate. This compartment will dictate what the virus can access. Viruses must replicate inside these to use these specific cell factors.)

VIRUS WITH RNA:
The essential features of the replication are quite similar.
In this scenario, the capsid makes it into the cell. the capsid can recognise proteins in the cell, so it can direct the virus to its designated compartment.
Once again, there is the production of the viral genome proteins for the new capsids, and the replication of the viral genome.
Then, there is the self-assembly of the new viruses and their exit from the cell.