Viruses, prions and viroids cannot be classified as living since show characteristics of inanimate things when they are outside a living organism. At the same time, they cannot be classified as non-living since they demonstrate various characteristics of living things once inside the cells of a living host. According to Jain (2009), viruses are neither living nor non-living since when they are not in a living organism, they do not replicate but once they invade a living organism, they start replicating. They act as parasites and to be specific, they are obligate parasites since their survival and reproduction is dependent on a living host. They can therefore be classified to be in-between living and non-living. Viruses are mere particles of nucleoproteins and they have inanimate characteristics hence they do not strictly qualify as living organisms. At the same time, the fact that they are made of molecules of life such as proteins and nucleic acids makes them be excluded from the absolute class of non-living organisms (Jain, 2010).
Viruses are merely composed of nucleic acid (RNA or DNA) coated in a protein. They have no capability of reproducing on their own, a characteristic of living things hence they have to gain entry to a living host cell for them to reproduce. In this way, viruses fail to fall clearly into the class of living organisms. On the other hand, viral replication occurs once the virus is inside the living host cell and the fact that they undergo reproduction in a living host cell make them appear to be more of living than non-living organisms(Gupta, 2007). It is this lack of clear boundary between living and non-living cells that make viruses to be classified as being in the boundary of living and non-living organisms.
The fact that viruses do not have the basic components of a living organism i.e. cell membrane, cell wall and protoplast; they have no functional autonomy, they do not experience growth in terms of size and they have no respiration process that distances them from living organisms. On the other hand, viruses have characteristics that make them appear as living organisms (Kango, 2010). For instance, viruses have a genetic component with the ability to replicate, they multiply once they are in a host, they demonstrate response/sensitivity to stimuli e.g. temperature and radiations, they can mutate but they do not carry out metabolism (Various, 2007) and they demonstrate host specificity (Gupta, 2007). This mixture of characteristics, living and inanimate characteristics, make viruses hard to exclude as either living or non-living and instead it becomes appropriate to classify them as being in the boundary of living and non-living.
Prions and viroids are simply molecules of life since they are either proteins or nucleic acids (Jain, 2010). It is impossible to classify viruses, prions and viroids as either living or non-living since they have characteristics that are almost similar to those of viruses. Viroids are infectious agents made up of nucleic acid RNA and they do not have a protein coat. The nucleic acid of the viroids gets copied into the nucleus of the host cell and not any other part of the host cell. These particles are not truly living or non-living as they also behave like viruses i.e., they replicate in the host cells only (Kango, 2010). Prions are infections particles made up of proteins only with no nucleic acid (Faquet & International Committee on Taxonomy of Viruses, 2005). They show resistance to heat making them appear as non-living but the fact that they display sensitivity to proteases gives them a characteristic of living organisms. This lack of a clear boundary between living and non-living organisms makes prions also not be classified as either living or non-living and rather fall in the boundary (Gupta, 2007).
From the above discussion, it is evident that viruses, prions and viroids are difficult to classify as either living or non-living. Instead, they appear to be in the boundary of living and non-living. This is mainly because they demonstrate characteristics of both living and non-living organisms depending on the availability or absence of a living host.
Fauquet, C. A. & International Committee on Taxonomy of Viruses (2005). Virus taxonomy: classification and nomenclature of viruses: eighth report of the International Committee on Taxonomy of Viruses (2nd edition). San Diego, CA: Elsevier Academic Press.
Gupta, P.K. (2007). Genetics classical to modern. New Delhi: Rastogi Publications.
Jain, M. (2009, February). Zoology: Virus. Competition Science Vision, 11(132), 1531-1647.
Jain, S. P. (2010). Textbook of botany diversity of microbes and cryptogams (Fourth revised edition). New Delhi: Rastogi Publications.
Kango, N. (2010). Textbook of microbiology. New Delhi: I.K. International House Pvt. Ltd.
Various. (2007). Biological diversity and classification. West Sussex: Global Media.