Classification of Viruses

A virus is a microscopic intracellular organism consisting of either a DNA or RNA genome enveloped by a protective shield called a capsid. It lives inside a cell of another organism, the host. The process of classifying viruses involves the naming and placing of virus into groups called taxonomic units. In the past, viruses were classified on the basis of the part of body or organ system infected. In modern times, they are grouped on the basis of chemical structure, chemical composition, shape, size and method of multiplication. The common basis for classification include: herpes viruses, adenoviruses, Hepadnavirusses, papilloma virus, coxsackie viruses, Corona viruses, Arboviruses, Reoviruses, picornaviruses, Togaviruses, Flaviviruses, Arenaviruses, Retroviruses, Bornaviruses, Orthomyxoviruses, Bunyaviruses, Rhabdoviruses, Paramyxoviruses, Filoviruses, Viroids, Prions. Some of the diseases caused by viruses include: AIDS, flu, smallpox, herpes, measles, mumps and Ebola (Crowley, 2012).

All living things can be categorized into one of the six kingdoms: Bacteria, Fungi, Plantae, Animalia, Archaea and Protista. The question whether viruses are living or non-living is still subject to an ongoing debate. Without a host they are inert; with a host they suddenly spring into action. They do not share many of the characteristics of living organisms. Viruses cannot metabolize and excrete wastes. A virus can only reproduce itself is inside an organism’s cell. This is because it is not composed of cells; it consists solely of a nucleic matter. Without the cell constituents, it lacks the ability to reproduce. They do not show cellular organization, growth or homeostasis. Due to this, viruses are generally categorized as non-living organisms, hence, cannot be categorized into any of the six kingdoms.

The question is where do viruses belong to? In his book “Aliens among Us”, Carl Zimmer raises the argument that viruses could be aliens. The theory of DNA based life evolving from RNA based life was also reviewed. The common belief, according to Steven Benner, being that DNA triumphed over RNA due to its superiority. Benner believed that billions of years ago, organisms were RNA based, over time they evolved to the double-stranded DNA. To support his claim, Benner suggests that there could still be RNA based organisms living on earth today. This is yet to be proved defending this to the inability of today’s biological tools to detect RNA-based life, which Davies disagrees (Zimmer, 2007).

The International Committee on Taxonomy of Viruses (ICTV) has since 1966 provided a unified taxonomy system. The system consists of ranked taxon as follows from highest to lowest: Order, Family, Subfamily, Genus, and Species. In a report in 2011, the ICTV recorded more than 30,000 viruses grouped into 6 orders, 87 families, 19 subfamilies, 349 genera and 2284 species. The orders include: Herpesvirales, Picomavirales, Caudovirales, Mononegavirales, Tymovirales and Nidovirales.

Another system that classifies viruses according to their chemical composition (DNA or RNA) and mode of replication is the Baltimore Classification, established in the 1970s. In this system, viruses are classified into seven groups. They include: dsDNA viruses, ssDNA viruses, dsRNA viruses, (+) ssRNA viruses, (-) ssRNA viruses, ssRNA-RT viruses and dsDNA-RT viruses.

Conclusion

Almost all viruses cause diseases to humans, plants and animals. It is worth noting that viruses also provide benefits in the development of life on earth. The method of characterizing virus is changing rapidly subject to debates and proposals. In conclusion, viruses do not contribute to any kingdom therefore do not belong to any of the six kingdoms of life.

References

Crowley, L. (2012). An Introduction to Human Disease: Pathology and Pathophysiology Correlations. Jones & Bartlett Publishers.

Zimmer, C. (2007). Aliens Among Us. Discover Magazine, Vol. 28, Issue 7.