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dave-the-scientist t1_jboxhrl wrote

Reply to comment by Nymaz in I just learned that the known shortest DNA in an “organism” is about 1700 base pairs in a certain virus. Is there a minimum amount of “code” required for an organism (or virus) to function in any capacity? by mcbergstedt

It really depends who you ask. Some consider viruses to be alive, others do not.

True, a virus can't function unless it's inside a cell with access to those nutrients/cofactors, and existing proteins and other components. But a bacteria also can't function unless it's in an environment with appropriate nutrients/cofactors and existing proteins and other components. The only difference is that those proteins etc already exist when the bacteria is born (divides into daughter cells), while a virus has to go and find them. Does that difference define life?

But then, if you consider a virus to be alive, what about self-replicating plasmids? They're really not much different from a virus. What about transposable elements in a genome? Are they alive too? They're not much different than a virus in latent phase.

The definition of life is something that sounds like it should be easy, and high school textbooks do give a precise definition. But the reality is a lot more complicated and murky than it seems.

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Dachannien t1_jbozrot wrote

For that matter, what about a prion? Does nothing when not in the presence of similar amino acid chains that are in a vulnerable conformation, but makes more of itself when those resources are available.

Then again, prions don't breathe and eat and grow, and that is how we know they're not alive.

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dave-the-scientist t1_jbpdrcx wrote

Right? The concept of "life" is surprisingly tricky. But I personally would not consider prions to be alive.

I will say though, prions definitely "eat", when they destroy the normal form of the protein. They "grow" by increasing their population, much like bacteria / viruses. Breathing is not a requirement for life.

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mdielmann t1_jbq3wst wrote

By these definitions for eat and grow, a fire is alive. I'd consider prions no more alive than fire, while still being as dangerous as fire.

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dave-the-scientist t1_jbq760p wrote

Behold, a man!

But yes, examples like fire (nice idea on that one btw) are why "life" is weirdly tricky to define.

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mdielmann t1_jbq8clj wrote

I get the edges are very blurry when defining what something is or isn't in biology, but I wouldn't equate destroy or alter with consume, or grow with reproduce, either.

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dave-the-scientist t1_jbqteik wrote

When you hear "consume" or "metabolize", it doesn't just mean destroying or altering a molecule. The important bit is that energy is removed from the molecule and used by the "organism" in question. Fire definitely does count for that particular one, as the reaction to burn something is almost exactly what we do in our bodies. We burn our food, just much more slowly. A prion though, does not meet that criteria. It does alter a molecule (the non-dangerous form of the prion protein), and energy is removed (the dangerous form of the prion is at a slightly lower energy level, I believe), but the original prion doesn't do anything with that energy. It is unchanged.

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Elladan71 t1_jbrnlfg wrote

I think you're on the right track, here. A virus needs a cell to function. A bacterium needs a nutrient-rich environment to function. An animal needs gravity oxygen, water, and food. Plants and animals are *environments for other kinds of life, so it's no wonder that the question is difficult to answer. When asked in the other direction, you're confronted with whether the Earth itself is alive.

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