Are Viruses Alive?

Table of Contents

Introduction

What are viruses?

What does it mean to be alive?

Why is there controversy over viruses being alive?

Argument 1: viruses are not alive because they cannot self-organise or self-maintain.

Argument 2: viruses are not alive because they don’t replicate or evolve without the aid of cells.

Argument 3: viruses are just as alive as bacterial endospores

Argument 4: viruses use the same biological materials as all organisms

Argument 5: how can viruses be alive when plasmids aren’t?

Argument 6: viruses have had a role in evolution.

Conclusion

References

 Word count: 1,925.

What are viruses?

There is an ongoing debate on whether viruses are alive. There is no straightforward answer to this question and there are many arguments for and against the idea of viruses being alive. This essay aims to explore these arguments. Firstly, we must consider what a virus is. A virus is a small particle that is capable of infecting a cell and potentially causing a disease. all viruses contain genetic material, either DNA or RNA, enclosed in a

protein coat (see figure 1). Viruses are unable to reproduce without the help of a host cell; however, they are capable of reproducing within the host cell by making use of the cellular processes inside the host cell (Zakaryan, 2019).

Figure 1: structure of a virus. Source: https://6636.stem.org.uk/pathogens4.html

 

 

What does it mean to be alive?

What does it mean to be alive? This is a philosophical question as well as a biological one. There are several definitions of life and being alive. From a philosophical perspective, being conscious of yourself and others around you may be a possible definition of being alive, whereas the biological definition of alive would include the capability of reproduction and the ability to internally repair oneself without external help. This essay will focus on the biological definition of being alive to simplify the arguments and maintain definite clarity throughout.

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Why is there controversy over viruses being alive?

It has been argued that viruses are not living organisms because they are not composed of cells, which goes against the idea that all living organisms are made up of one or more cells. So, without cells, can viruses be truly alive?. In addition, viruses cannot reproduce without a host cell, and when they do, they use energy from the cell rather than using their own metabolic energy. However, on the other hand, some scientists insist that viruses are alive because although they cannot reproduce on their own, they have the ability to reproduce inside host organisms, which other living organisms must also do in order to reproduce I.e. parasites. Moreover, viruses do have organisation in their structure and contain nucleic acid, which are exclusive traits of living organisms.

As viruses lack any form of energy and carbon metabolism, they are not alive according to this definition (Moreira and Lopez-Garcia, 2009). This contrasts the majority of organisms on the earth that would be considered alive. Bacteria for example, are able to produce their own energy through respiration, using glucose as a substrate, which is one reason they are considered alive. this same cellular process occurs in humans, which allows us to make comparisons between organisms that are considered living. Viruses, however, lack the ability to create their own energy, and instead rely on their host organism’s metabolism to reproduce (see figure 2).

Figure 2: Table comparing viral and cellular traits. Source: https://edisciplinas.usp.br/pluginfile.php/2871445/mod_resource/content/2/2009_Ten%20reasons%20to%20exclude%20viruses%20from%20the%20tree%20of%20life.pdf

Viruses neither replicate nor evolve, they are evolved by cells. Even if some viruses encode their own polymerases, some of them error prone, their expression and function require the cell machinery so that in practice, viruses are evolved by cells – no cells, no viral evolution. (Moreira and Lopez-Garcia, 2009). This shows another important distinction between viruses and other organisms which are classified as living. It can be argued that to be alive, something must follow the natural course of evolution without needing another organism, which viruses cannot do. In addition, Moreira and Lopez-Garcia highlight the fact that viruses are unable to reproduce without the aid of cells. This is because they lack the necessary organelles that are crucial to cell reproduction, such as ribosomes. Because of this, viruses must hijack a host cell and use the cell’s machinery to replicate their DNA (see figure 3). This is an important distinction between viruses and other organisms as it shows us that without living organisms, viruses would simply be free floating chemicals void of any function or ability to carry out any processes we usually attribute to living organisms.

Figure 3: Viral reproduction involving a host cell. Source: https://courses.lumenlearning.com/boundless-microbiology/chapter/viral-replication/

 

If viruses are not alive, what about parasitic bacteria and spores? To exacerbate the difference between viruses and cellular organisms, the authors focused on the ‘virion’ state of minimal viruses (such as RNA viruses) compared with ‘free living’ bacteria in a metabolically active state. This is not a valid comparison. Virions should be compared with bacterial spores that are metabolically inactive. (Claverie and Ogata, 2009). Here, Claverie and Ogata criticise the statements made by Moreira and Lopez-Garcia. They argue that viruses cannot be compared to metabolically active free-living bacteria but can more easily be compared to bacterial endospores (see figure 4). Bacterial endospores develop in bacteria when conditions are unfavourable, and during this period, the bacteria does not reproduce, and metabolic activity is shut down. Scientists would generally agree that these bacterial endospores are alive, and because they are so similar to viruses, it can be argued that viruses are also alive under the same definition.

Figure 4: Labelled diagram of a bacterial endospore. Source:  content/uploads/2017/05/Bacterial-Endospore-Structure.jpg

Viruses use the same macromolecules (proteins and nucleic acids) as cellular organisms for the reproduction and expression of genetic information. This indicates that viruses and cells fit into the same historical process that we call “life”. (Forterre, 2016). As shown in Figure 5, viruses are composed of a protein envelope, enclosing nucleic acid, either DNA or RNA. All other living organisms on earth contain DNA/RNA and proteins. DNA is a biological molecule which codes for proteins, which are then used for biological processes. Because viruses use these same macromolecules, they can be put into the same broad category as all other organisms on earth. If viruses weren’t alive, then surely, they would not contain the same macromolecules as all other organisms on earth. But furthermore, they use these macromolecules in the same way that other organisms do. Their DNA/RNA is transcribed to mRNA, which is then translated into proteins, which are used to re assemble new viral particles, also known as reproduction. Many scientists argue that this alone is reason enough to classify viruses as living organisms. If viruses aren’t alive, then they are at least on the boundary of living.

Figure 5: comparison of a virus to a cellular organism. Source: http://www.abc.net.au/science/articles/2011/03/02/3151861.htm

 

The only difference between the smallest virus and the smallest plasmid is the presence of a capsid gene in the viral genome but not in the plasmid (Krupovic & Bamford, 2010). Plasmids are usually considered to be non-living because they are pure chemicals (macromolecules). However, if we consider that viruses are living but plasmids are not, one should conclude that a single gene encoding a capsid protein is sufficient to confer the living status to a biological entity! Should we conclude that plasmids are finally living, when their genes are expressed, and their genomes replicated in a living cell? (Forterre, 2016). The genomes of plasmids and viruses are related, and a plasmid can be turned into a virus if it gains a gene which allows it to produce a capsid. As mentioned above, the only difference between the smallest virus and the smallest plasmid is the gene encoding capsid production. Plasmids are usually considered as non-living as they are simply chemicals. If we class plasmids as non-living and viruses as living, then there is simply one gene that encodes the ability to produce a capsid that gives viruses living status. This seems radical and unrealistic; however, it shows the complexity of classifying things as living or non-living.

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 When looking at individual organelles, such as mitochondria or chloroplasts, many would agree that these alone are non-living, but part of a complex biological system that, when all the pieces are together, is a living organism. Mitochondrion were once free living ‘alive’ bacteria, but now that they are a part of a more complex biological system, they are no longer seen as alive. This model can be applied to viruses. Just because viruses contain the same macromolecules as other living organisms and have the ability to do some living processes, it does not mean that they are alive in the same way that plasmids are not alive.

Of course, evolutionary biologists do not deny that viruses have had some role in evolution. But by viewing viruses as inanimate, these investigators place them in the same category

of influences as, say, climate change. Viruses directly exchange genetic information with living organisms—that is, within the web of life itself. A possible surprise to most physicians, and perhaps to most evolutionary biologists as well, is that most known viruses are persistent and innocuous, not pathogenic. They take up residence in cells, where they may remain dormant for long periods or take advantage of the cells’ replication apparatus to reproduce at a slow and steady rate. These viruses have developed many clever ways to avoid detection by the host immune system—essentially every step in the immune process can be altered or controlled by various genes found in one virus or another. (Villarreal, 2005). It is clear that viruses have played an important role in evolution. They cannot be viewed as simple non-alive particles when so much of our evolutionary history is linked to viruses and their ability to share genetic information with cellular organisms.

In conclusion, many features of viruses go against the broad definitions of life, such as being able to self-organise or self-maintain. In their article, Moreira and Lopez-Garcia highlight the fact that viruses are unable to produce their own metabolic energy through respiration. They also state that viruses do not have the cell machinery required to reproduce without a host cell, which is also a fundamental property of life. This may suggest that viruses cannot be classified as fully living organisms as their (lack of) metabolism and reproduction contrasts almost all living organisms on earth.

 However, on the other hand, viruses can be closely compared to some other organisms that are very much alive, such as bacterial spores. Because there are so many similarities between viruses and other living organisms, there must be some life to viruses. In addition, viruses share the same macromolecules of almost all living organisms; DNA/RNA and proteins, which again shows some close similarities between viruses and living organisms. many scientists agree that viruses have had a role in evolutionary biology and that they play a part in the history of almost all organisms.

 It is a difficult task to clearly define what it means to be alive. It seems that viruses have all the constituent parts required to be classified as ‘alive’ but when looking at the bigger picture, viruses lack the characteristics that are observed in all living organisms. there is no clear answer to whether viruses are alive, and each answer to the question would differ depending on the definition one uses of being alive.

References

Claverie, J. and Ogata, H., 2009. Ten good reasons not to exclude giruses from the evolutionary picture. Nature reviews microbiology, 7, pp.306.

Forterre, P., 2016. To be or not to be alive: How recent discoveries challenge the traditional definitions of viruses and life. Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences, 59, pp.100-108.

Krupovic, M. and Bamford, D., 2010. Order to the viral universe. Journal of virology, 84(24), pp. 12476-12479.

Moreira, D. and Lopez-Garcia, P., 2009. Ten reasons to exclude viruses from the tree of life.  Nature reviews microbiology, 7, pp.306-309.

Villarreal, L., 2005. Are viruses alive? scientific American, 291, pp. 100-105.

Zakaryan, H., 2019. Porcine viruses: from pathogenesis to strategies for control. Armenia: Canister Academic Press.