Are Viri (viruses?) Alive?

Since I study a strange class of virus like elements that pervade the genomes of just about all lifeforms, I thought I would throw in my two cents. I also think that making a clear distinction between life and non-life in the case of viruses is a tricky. From population genetics to evolutionary strategy, there is very little (if anything) to distinguish a virus from any other "living" replicating organsism. They are under the same constraints and are influenced by the same evolutionary parameters as any other parasite. That they require a host cell to live is not such a fixed trait of non-life as we require food (i.e. external factors that we do not produce to live) and their are other non-viral obligate parasites. The viruses evolutionary strategy is to minimize its own content and maximize its usage of the environment i.e. extremely streamlined. Even at that, some viruses such as HIV for example are monsterously complex. There is also some theoretical work suggesting that retrotransposons (related to retroviruses) are the basis of life i.e. they are simple replicators that diversified and produced genes with other functions not virally related. So proto-life, simple-life, streamlined life..whatever, it still seems they meet the criteria.But I agree with sidelined as well, defining life is probably as difficult a concept as defining a species.

actually a virus is more like a bacteria than a prion. You produce the prion protein, it is not an exogenous agent. In addition, a prion infection does not lead to an increase in the number of prion molecules. The infecting prion causes the host prions to take on the misfolded form. Thus, prions are not replicators but modifiers. Also, prions are exculsively endogenous i.e. produced from a vertically transmitted gene, and the misfolded ones are more like a poison contained in food, than an exogenous infectious agent.A virus contains a genome with enough information to replicate itself in a cell and enough information to get itself back out of the cell when necessary including modifiying and controlling host cell functions to serve that end. These are hardly minimal functions. Saying that they do not have "enough" to qualify as alive to me is too arbitrary. Then only plants are alive because they can photosynthesize and we are absolutely dependent on their photosynthesis to survive.

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Nonetheless I see the distinction as arbitrary. A virus can only replicate in a specific chemical environment - the cell. A prion can only replicate in a specific chemical environment - one that includes the nominal protien. A bacterium can replicate in any envrionment in which it can maintain life chemistry.

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You can only replicate in a specific environment yet you are alive. And the specific environement on which the bacterium depends is equivalent to the environment on which the virus depends in many ways.

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"Modifers?" Prions modify protiens into exact copies of themselves. Living things "modify" materials into copies of themselves. Seems like the terms aren't much of a distinction to me.

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Viruses have no metabolism. Prions have no metabolism. Bacteria have metabolism.

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There is a distinction. You do not replicate i.e. reproduce by modifying your wife to become a copy of you. You produce a novel offspring. Prions do not. There is no net gain of prions. Only a net loss of normally folded prions. Prions are a part of normal metabolism so they do not have metabolism. The bacterial mutB gene does not have metabolism either because it is part of a lifeform. Without input in the form of nutrients, bacteria have no metabolism. Viruses, without a cell host, have no metabolism.I am not sure if defining life would benefit by looking at one system and then comparing it to those that lack that system. We lack things bacteria have in terms of metabolism and function but we consider both ourselves and bacteria to be life.

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Viruses don't respond to changes in their environment. Prions don't respond to changes in their environment. Bacteria respond to changes in their environment.

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Viruses certainly do respond to changes in the environment. They go into a latent proviral phase by integrating into the genome (some viruses not all). If the cell is damaged or distressed, the viruses respond by going into a lytic phase ultimately destroying the cell and releasing many copies of themselves to infect new targets. Viruses have all sorts of ways of responding to the cells signals and changes in cellular environment. As before, I don't think prions are alive either so no, they don't respond to the environment.

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If I might ask - how would you tell the difference between a "living" virus and a dead one?

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There are many examples of dead viruses. They are called HERVs or human endogenous retroviruses (though not all are dead..you know, biology can never be simple ). They are unable to produce any viral proteins or replicate even in the appropriate environment.I don't really have such a strong position or feeling regarding life vs. non-life. However, given that viruses have population genetics, evolve, and do so in a host independent way I find them clearly distinct from prions which lack these qualities. Even if they are much simpler than bacteria.

A recent entry is 28 Days Later...a fun flick to watch but you really have to suspend you disbelief with the virus itself.