Virus, Viroid, Virusoid, Prion etc...Life or Nonlife..What say you?

Question

These subcellular "organisms" (?) fascinate me because they challenge current scientific ideas about what is life.

My personal view: Science as yet has no clear understanding of what "life" is. Merely giving a shopping list of what "most" living things have (replication,metabolism, cell walls etc.) doesn't "define" life but merely describes "most" of it..(not the same thing).

When I say "What say you?" I specifically mean: "What are your own personal thoughts ..Have you ever considered proposing a new "definition" of life etc...Original thought , please. My goal is stimulating thoughtful exchange of ideas..please!!

thanks electropath, yes thats the kind of "answer" I was hoping for !

I agree that most likely "life" is part of a continuum..rather than there being a clear border between "life" and "nonlife"...The implications of that intrigue me.

Personally I feel that the presence or absence of a protein coat ought not make any real qualitative difference..and so I'd consider virus and virusoid equally alive..which also intrigues me since that gets dangerously close to labelling a single molecule as "alive"

selene thanks for your detailed answer. you seem to know quite a bit on the topic. I am not a biologist but want to learn more, can you suggest a few books?

Answer 1

I agree with you that it´s a complex question. I don´t think that there is a convincing criterion to define life. Apparently organic molecules tend to a great degree of complexity than inorganic ones. Prions are just proteins, I don´t think you can consider them as living organisms. Their multiplication is determined by genes of complex (and alive) organisms. Under certain circumstances the activation of these genes induces malignant multiplication of prions originating a pathology. Viroids and virions are probably in the frontier of life. Viruses are able to make copies of themselves, but they are not autonomous. They need to use host resources to make copies of their own genomes. In spite of their simplicity, they have an amazing range of strategies to elude host´s defenses and replicate... so... are they alive? Moreover, there is an interesting phenomenon called coevolution: viruses tend to evolve as their host do so. As consequence of it, they tend to adapt better to the host, causing less damage to it (they tend to establish persistent infections or look for vector species which are not affected by the infection, rather than cause acute infections that kill the host and themselves). To conclude, I would consider life as an assay along evolution to create organisms that are able to replicate and transmit genetic information to other organisms. Probably viroids and viruses are the first assays of this tendency to perpetuate genetics. And genes are in the last term organic molecules with a high degree of complexity than tend to perpetuate themselves.

Answer 2

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Answer 3

I agree as well that there's a grey area. Maybe the problem here is our own human instinct to use binary logic (either "yes" or "no" with no in-betweens) upon things in the universe which don't abide by that logic.


Prions - defintely not. I believe that life must meet a certain level of organization. A simple protein is not even organized on more than one level, even if it can serve as a template for making more of itself from available amino acids. It wouldn't qualify.

I would also exclude a viroid as merely a rogue string of genetic material that life can produce more of when life happens to take it up into itself.

Just because something persists in being produced by life doesn't make it alive. Books would meet that definition, especially a Bible or related book that makes its reader want to spread the book.

Viruses are more organized, having genetic material in a protein coat but not much else. If we had to classify viruses, I'd call them almost alive, for reasons not very different from the reason I gave for a viroid.

Answer 4

No one ever said the viruses et al are definitely not living things. The debate is still open. Viruses and prions et al are just not as organized as "classic" life forms.
By organized life forms, scientists mean ones with cells.

Answer 5

They all appear to meet the general accepted definition of life to a point, even that they reproduce though that is not in the same way that true living organisms do. Keep in mind though that there are always exceptions (some skink species require female only to reproduce and they are higher life forms).

Answer 6

You are quite correct. The transition from non-life to life is a tricky one, and I have yet to hear a definitive list of attributes that capture the definition of life in a way that does not seem too inclusive, or seems to exclude somethings that might be alive.

Answer 7

Good question. My view is very similar to yours. Viruses seem to me to be sort of a "missing link" between life and not life. Proteins seem to be a tool life uses as a building block.

The other thing is, humans invented the word "life" and I guess we can define and redefine it any way we want as science progresses.

Answer 8

No metabolism = not alive. These things do not themselves synthesize the macromolecules which they comprise.