VIRUSES OVERVIEW
Viruses have killed and hurt us for thousands of years and they are not even alive!
For a better understanding of how viruses affect us, you need to understand the main characteristics of viruses.
Therefore, the learning objectives of this chapter are to understand:
What are viruses? / What do they look like? / How do they enter our bodies to make more copies of themselves? / How can we detect viruses?
What is a Virus?
A virus is a small, infectious, obligate intracellular parasite made of genetic material, either RNA or DNA, surrounded by a protective protein coat (Baron, 1996). Viruses have an extracellular form, called a virion, which can travel from host-to-host (Madigan et al., 2019).
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Viruses are very small, their size varying between 20-200 nm in diameter. In comparison, an average human cell is 10-30 µm in diameter, meaning that it is 100 to 1000 times larger than the viruses infecting it (Figure 1). The invention of the electron microscope allow scientists to visualise viruses in great detail (Louten, 2016).
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Being obligate intracellular parasites, viruses can reproduce and replicate only when the virion itself gains entry (i.e. infects) into a suitable growing host cell, using cell’s energy and machinery to make more virions (Louten, 2016).
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In comparison with all human, animal, plant or bacterial cells which have double-stranded DNA as their genetic material, viruses have genomes composed of either DNA or RNA which can be single-stranded or double-stranded (Louten, 2016).
* If you were a human cell, how small a virus would be?
Figure 1. Size comparison with eukaryotic and prokaryotic cells.
Key Question – Are Viruses Alive?
The question whether “viruses are alive” rose considerable debate over many years. Until recently, the answer to this question was often negative and viruses were not considered in discussions on the origin and definition of life (Forterre, 2010). However, nowadays it became clearer that a virus is an infectious organism with two phases:
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Virion (infectious particle) – non-living
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Infected cell (the virus producing virions) – alive (Brüssow, 2009)
Properties of Life
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Metabolism
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Homeostasis
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Heredity
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Growth and development
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Reproduction and death
(Rosslenbroich, 2016)
Viruses do NOT:
– Metabolise.
– Grow.
– Replicate by themselves.
– Demonstrate homeostasis.
(Reference needed).
Viruses do:
– Have a genome.
– Infect cells and use the cell to make more viruses.
– Cause disease in many organisms: they are pathogens.
(Reference needed).
Check Point
Can you answer the following questions?
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What is a virion?
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Why are viruses considered obligate intracellular pathogens?
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How does a virus differ from a cell?
Click here to see the answers.
The Structure of a Virus
The virion of any virus consists of two basic components:
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Nucleic acid (single- or double-stranded DNA or RNA)
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A protein coat, called the capsid.
The capsid is made up of multiple copies proteins, called capsomeres, each of which is encoded by a single viral gene. Together, the enclosed nucleic acid and the capsid form the nucleocapsid (Figure 2) (Lodish et al., 2000).
The capsid protects the viral genome from the activity of nucleases which can degrade the genetic material and also attaches the virion to specific receptors of the host cell during infection (Baron, 1996).
Figure 2. The Basic Structure of a Virus.
Check Point
Viral Life Cycle
To be added soon...
Detection of Viruses
To be added soon...
Next Chapters
Read more about these viruses that might interest you!
COVID-19
HIV
Hepatitis B
Influenza
...or about vaccines and how they work.
