Is it reasonable to label viruses, prions, and the like as "bordercases of life?"

[Mike Mantala]

Some references (textbooks, journals, and modules) refer to viruses and prions as "bordercases of life." Is the label justified or reasonable?

[Alison Hutt]

For all the reasons that are no doubt discussed in biology classes around the world annually, viruses some life characteristics and not others. Putting them is a borderline category seems logical.

This is the first I have learned of prions; they don't even have genetic material, yet they "reproduce" more copies of themselves by causing other proteins to morph. Is that really reproduction? If I walk into a classroom and had a ray gun that would take other people and turn them into copies of me, am I reproducing myself? Not really. I am not creating new life, I am just changing existing life into a form that resembles me. I don't know that prions should be considered life-like at all. 

Both these examples, though, generate great discussion regarding the nature of life...

[Marta Toran]

I agree with Alison that viruses should be considered "borderline" living because they are acellular and do not carry out most of the characteristics of living things. The one thing that unites all living things is nucleic acid (which even the viruses and viroids contain), but prions don't even have that. They are ust abnormal forms of harmless proteins that are able to transform/ destroy other proteins and cause disease (like Mad Cow). I don't see how they could be considered living by any definition, otherwise proteins and other molecules should also be considered living things. It is interesting because the more we know, the more important it is to revise the definitions we've always held true, like what a "living" thing is. Along the same lines,  I always enjoy discussing with my class not only what is living and non living, but when something stops being alive. With all the modern medical technology, you can really keep someone "alive" with their body hardly doing any work (and resuscitate too of course).

So, is it okay that scientists are split in considering viruses and prions living, is it that subjective, should we keep to the traditional definition of life (as being cellular, reproducing, transforming energy etc) or is it time for the definition be revised?

[Mike Mantala]

Quite indeed... There are not a few instances in Biology where terminologies are reduced to catchy phrases and in the process, lose a bit of the concept's precision. For example, High School Biology - at least in the Philippines - mentions of the lysosome as a "suicide bag", the mitochondrion as a "power house", and the like... I'm not sure though if the term, "border-cases" - as used in describing viruses, viroids, and prions - amounted to the same instance of losing precision for the concepts of "living" and "nonliving."

[Cheryl Hollinger]

This is an interesting question. I had the opportunity to meet and talk to 2 virus researchers and when I asked them if viruses were considered "living things" they could not agree. One immediately said no and the other said, "They are alive when they are alive and they are not when they are not." It was weird to hear them talk about "species" of viruses instead of strains. I personally don't feel that viruses are living things and describe them as "obligate intracellular parasites" to my students like one of the researchers did. I'm curious what others might have to say on this topic.

[Mike Mantala]

There's this interesting essay by Daniel E. Koshland Jr. titled,"The Seven Pillars of Life." He recognizes in his essay that "although everyone knows what life is there is no simple definition of life."