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The recent news about a new supermassive virus being discovered got me thinking. What biological differences between viruses and cellular organisms have made viruses be deemed non-living?

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You might be interested by this [biology.stackexchange.com/questions/1842/… post as well. It concerns the origin of viruses. –  Remi.b Jul 29 '13 at 13:30

6 Answers 6

up vote 29 down vote accepted

If this is a topic that really interests you, I'd suggest searching for papers/reviews/opinions written by Didier Raoult. Raoult is one of the original discoverers of the massive Mimivirus and his work will lead you to some truly fascinating discussions that I couldn't hope to reproduce here.

The main argument for why viruses aren't living is basically what has been said already. Viruses are obligate parasites, and while plenty of parasites are indeed living what sets viruses apart is that they always rely on the host for the machinery with which to replicate. A parasitic worm may need the host to survive, using the host as a source for energy, but the worm produces and synthesizes its own proteins using its own ribosomes and associated complexes.

That's basically what it boils down to. No ribosomes? Not living. One advantage of this definition, for example, is that it is a positive selection (everyone alive has got ribosomes) and eliminates things like mitochondria that are sort of near the boundary of other definitions. There are examples on either side of something that breaks every other rule but not this one. A common one is metabolism and while that suffices for most cases some living parasites have lost metabolic activity, relying on their host for energy.

However (and this is the really interesting part) even the ribosome definition is a bit shaky, especially as viruses have been found encoding things like their own tRNAs. Here are a few points to think about:

  • Okay, so we've got ribosome encoding organisms (REOs), but why can't we define viruses as capsid encoding organisms (CEOs)?
  • Comparing viruses to living organisms such as a human is absurd, given the massive differences in complexity. A virus, really, is just a vehicle or genetic material, and should be more rightly compared to a sperm cell. Is a sperm cell alive, or is it a package for genetic material that is capable of life once it has infected/fertilized another cell?
  • The really large DNA viruses often create cytoplasmic features called virus factories. These look an awful lot like a nucleus. What is a nucleus anyway? Maybe it's just a very successful DNA virus.
  • Viruses can get viruses.

I'll wind down here, but suffice to say that while our current definition may have sufficed for a while, and still does, it is no longer so solid. In particular, there is a theory alluded to above that eukaryotic life itself actually formed because of viruses. I can expand on this if you like, but here are some great sources:

Boyer, M., Yutin, N., Pagnier, I., et al. 2009. Giant Marseillevirus highlights the role of amoebae as a melting pot in emergence of chimeric microorganisms. PNAS. 106(51):21848-21853 (http://dx.doi.org/10.1073/pnas.0911354106)

Claverie, JM. Viruses take center stage in cellular evolution. 2006. Genome Biology. 7:110. (http://dx.doi.org/10.1186/gb-2006-7-6-110)

Ogata, H., Ray, J., Toyoda, K., et al. 2011. Two new subfamilies of DNA mismatch repair proteins (MutS) specifically abundant in the marine environment. The ISME Journal. 5:1143-1151 (http://dx.doi.org/10.1038/ismej.2010.210)

Raoult, D. and Forterre, P. 2008. Redefining viruses: lessons from Mimivirus. Nature Reviews Microbiology. 6:315-319. (http://dx.doi.org/10.1038/nrmicro1858)

Scola, B., Desnues, C., Pagnier, I., et al. The virophage as a unique parasite of the giant mimivirus. 2008. Nature. 455:100-104 (http://dx.doi.org/10.1038/nature07218)

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One correction: by the suggested definition, mitochondria are alive since they have ribosomes. (See, e.g., Properties of human mitochondrial ribosomes). –  mgkrebbs Jul 28 '13 at 20:48
    
The rRNA in mitochondria is encoded there, but the ribosomal proteins are all nuclear. –  Amory Jul 29 '13 at 16:01

I agree with the answers already given, these are the reasons that viruses are not considered alive. I want to point out though that this isn't an area you find 100% agreement on; there is a decent subset of biologists who do consider viruses alive. I would say - completely on the basis of personal observation - that virologists themselves are the group most likely to claim that viruses are alive.

This paper and this article from the Scientific American have some coverage of the debate if you want to read more.

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Do you know of any specific literature pushing towards the definition of a virus as a living being? I would be curious to read that. –  nico Jul 27 '13 at 23:34
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@nico: I've updated my answer with a couple of links. –  Jack Aidley Jul 28 '13 at 11:59

There are quite some different definitions of being "alive", but a common one includes the need to have responsiveness, growth, metabolism, energy transformation, and reproduction (found from the Encyclopedia Britannica). Viruses depend on host cells to do all this, so seen alone as a virus outside a host cell, they are not alive.

There's another short, but to the point blog entry about this.

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A neat answer, but I'm not satisfied. There are lots of parasites (and symbiotes) that rely on the host for biological processes, so I'm not sure if this covers such a niche point of taxonomy. I also disagree with that blog post about "passive" chemical reactions. In that case we are all none living as all our cells just passively carry out reactions with one another. –  Good Gravy Jul 26 '13 at 6:49
    
But the parasites, symbionts and multicellular organisms do reproduce and act (not only re-act). As I said, there are different definitions of life. But most of them agree, that to be called alive an organism has to at least fit one of the criteria (some definitions also include more criteria to choose from, for example when I was at school, movement was in the books.). Viruses fit none, but parasites do, as far as I am aware. And as far as science is aware. "Life" is a critical point, it is hard to define both scientifically and philosophically. –  skymninge Jul 26 '13 at 7:30
    
@GoodGravy, most reactions in living organisms are active, in that they are specifically regulated by enzymes. Reactions related to virus replication are not, or they take advantage of the enzymes of the host cell. –  Brandon Invergo Jul 26 '13 at 10:17
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@BrandonInvergo, that's not really true. Many viruses encode proteins that regulate viral replication. In fact that is a major part of the replication cycle for many viruses, determining the early/late or lytic/lysogenic activity during infection. Many other viral proteins are enzymes that specifically cleave or modify both viral and host proteins. Viruses often play a very active role in their own replication. –  Amory Jul 27 '13 at 0:18
    
Interesting. I stand corrected! –  Brandon Invergo Jul 27 '13 at 8:58

It is only a question of definition. You can set the boundaries between living things and not living things anywhere.

Some philosophers have argued that using a clear boundary between living and non-living things is not such a good solution. In nature there would rather be a continuum from a stone to a bacteria.

It is true that in thinking of viruses such as Lausannevirus or Marseillevirus we might be willing to integrate them in the category of living things. These viruses are giant, and even can be parasitized by other viruses.

Viruses are made of proteins and contain nucleic acids (RNA or DNA). If you consider that they are alive, what would you say about viroids? A viroid is just an nucleic acid that is able to infect a host and cause the replication of itself. What about a prion? A prion is a protein that, roughly speaking, has the same consequences as that of a viroid.

I think (one should check the literature, I might be mistaken) that there is a species of parasitoid wasp that produce out of its own genomes, viruses that reduce the host immune system in order to make the caterpillar a suitable habitat for the egg. Is this virus alive? Isn't it just a toxin of the wasp?

I guess one reason of considering viruses as non-living is that we do not know how to branch them in the tree of life ! Some might argue by the way that viruses would not at all form a monophyletic group.

There are several people tackling the question of "what is alive". Unfortunately the best book I know on the subject comes from the French literature; it is called "Comment définir la vie ?" by Bersini and Reisse. In this field, the most popular authors are Varella and Maturana. Again, if I'm not mistaken, the definitions of life are quite different from philosophers, from people having interest in the origin of life, and from people seeking a definition suitable for extra-terrestrial life.

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There is a nice discussions along these lines also at the very beginning of the very nice essay by Jacques Monod "Le hasard et la nécessité", where he speaks about distinguishing "natural" (living) objects from human artefacts. –  nico Jul 27 '13 at 23:33

Despite great answers from Amory and Remi.b, I want to emphasize this: there is continual debate about the definition of life because "life" is not something that exists in the real world.

People seek a definition of life that satisfies an intuitive notion of what alive should mean. They feel that, say, intracellular parasites should be considered alive, but (say) only if they have an enclosing membrane, like Rickettsia, and not if they are just a virus, or just an RNA molecule like a viroid.

While people have a built-in intuition that, for example, reliably categorizes a tiger as alive and a rock as not alive, that intuition can't be precisely bounded by a definition such that everyone is satisfied by the boundary. There are arrangements of matter in the physical world that fall outside the clear region of the intuitive concept of life, and this leads to continual, unresolvable, argument on what a precise definition of life should be, and what it should include.

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In addition to the good answers given here, I would like to propose a more intuitive argument against viruses being alive.

Viruses are, at one point of their "life", simply a piece of DNA (or RNA). Would you consider a piece of DNA to be alive? If so, then are transposons alive? Are chromosomes alive? How about synthesizing a piece of DNA - is that creating life? The answer will probably be "no" for most people.

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One could argue that that is true for any cell... –  nico Jul 26 '13 at 23:50
    
@nico sorry I didn't understand - how is a cell a piece of DNA/RNA? –  Bitwise Jul 27 '13 at 0:22
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@nico: I disagree: a cell is never simply a piece of DNA, it always has membranes, organelles, proteins, etc. The division of mother->daughter is not the creation of a novel cell de novo from DNA but rather a process of growth followed by division. Even DNA replication is semiconservative - both copies are one part original, one part newly synthesized so neither can really claim to be more original than the other. –  Jack Aidley Jul 27 '13 at 9:09
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@nico: You said that a cell is a some point just DNA. This is false; it is never just DNA. –  Jack Aidley Jul 27 '13 at 18:10
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@JackAidley: and why would that be? It really depends on definitions, which are not given here. When the DNA replicates is the newly formed DNA already part of the new cell? Or it is part of the mother cell? Is the new cell "coming to life" only after division? You cannot just skip through those (and many other) points when defining whether a virus is alive. Otherwise you are just saying that a virus is not alive because it is not alive. Not very useful definition... –  nico Jul 27 '13 at 18:52

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