What are viruses?
Viruses are pseudo-living parasitic particles (virions), generally 100x smaller than bacteria, that have a core of RNA or DNA, protein shell (capsid), and occasionally a fatty outer layer. They can infect all forms of life — animals, plants, protists, fungi, bacteria, and archaea — but cannot thrive or reproduce without first infecting a host. Once inside a host, viruses reproduce by hijacking parts of the cellular machinery and forcing the production of thousands of identical virus copies.
As we have witnessed with the new coronavirus, some viruses can overwhelm and kill their host. Some can persist within a host cell, dormant, until they are triggered to reactivate. While others, called retroviruses, use a special enzyme to incorporate their genes into the host's DNA (ie. HIV).
Viruses can spread in many ways:
- from a disease-bearing organism (vector)
- from mother to child at birth
- by consuming contaminated food or water
- sexual contact
- open wounds
Person-to-person transmission is the most common mechanism of spread.
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Viruses can be good, too.
Some viruses can have mutually beneficial (symbiotic) relationships with their host (Roossinck, 2011). A fascinating example of this is the involvement of retroviruses in the evolution of mammalian placentas!
Viruses come in all shapes & sizes.
The genetic material found in the core of a virus can be DNA or RNA, but never both. Viral DNA or RNA can be single or double stranded.
Viruses are named and grouped by scientists based on morphology and genetics using the International Committee on Taxonomy of Viruses (ICTV) and Baltimore classification systems.
In this system, viruses are hierarchically classified using distinct genetic and biological properties (ie. capsid shape, genome sequence).
The ICTV hierarchy is organized from the widest level to the most specific and viruses are named by order, family, subfamily, genus, and species (King et al., 2011).
Devised in 1971 by David Baltimore, the Nobel Prize-winning biologist, the Baltimore system classifies viruses based on genetic material of the virus (RNA/DNA), strandedness (single/double), method of replication (mRNA transcription), and sense (+/-).
Viral classes of the Baltimore system:
I: dsDNA viruses (ie. Herpes, Chickenpox, Smallpox)
II: ssDNA viruses (ie. Parvoviruses)
III: dsRNA viruses (ie. Rotaviruses)
IV: (+)ssRNA viruses (ie. Rhinovirus, Hepatitis A/C, Coronaviruses)
V: (-)ssRNA viruses (ie. Influenza viruses, Rabies, Ebola)
VI: ssRNA-RT viruses (ie. HIV)
VII: dsDNA-RT viruses (ie. Hepatitis B) *this group is a recent addition to the scheme!
All viruses require mRNA molecules to instruct a host's cellular machinery to make copies of the virus. In this system, viruses are grouped according to how they duplicate and express their genetic information (Baltimore, 1971).
To learn more, watch the video below to get an introduction on the Baltimore system (and viruses) from legend himself!
Baltimore, D. (1971). Expression of animal virus genomes. Bacteriological Reviews, 35(3), 235-241.
King, A.M.Q., Adams, M.J., et al. (2011). Virus taxonomy: Ninth report of the International Committee on Taxonomy of Viruses. Elsevier Science & Technology Books.
Roossinck, M. J. (2011). The good viruses: Viral mutualistic symbioses. Nature Reviews Microbiology, 9(2), 99-108.
Featured image: Elena Mozhvilo