ecco

I know little about biology so bear with me here. I have heard that the cell is the smallest living entity. I also think that a virus can cause infection by getting into the structure of a cell and change things. I don’t think that uncontaminated rock dust can cause infections. So where exactly does a virus fit into the scheme of things? Is it considered “living” or is it considered inanimate? How is the line drawn?

liquid

The simple fact is that we don't have a good definition of life.

We have all sorts of definitions, from listing characteristics (i.e. replication, growth etc), to cybernetic definitions (A set of positive feedback systems under a negative feedback control).

None of these is universally supported, and it implies a grey area between life and non-life. (Note the similar parallels with the grey area between species.) This is perfectly evidenced by organisms such as viruses, which do not metabolise, yet are able to hijack the metabolism of a cell to replicate. Other examples are prions, which are even simpler proteins than viruses, that live as semi-autonomous agents in our body. Rogue prions are what cause CJD.

So are viruses considered alive? Most scientists would say yes, but acknowledge that they do not have many of the characteristics that prokaryotic (read 'simple')cells have. Then again, prokaryotic cells don't have many of the functions that eukaryotic cells do (read 'complicated' - they are essentially cells formed from prokaryotes that merged in their evolutionary history, and they make up our cells and those of all mammals).

As a final note, in case you are a creationist or somesuch, this is one of our strongest evidences for non-life being able to arise from life. Pasteur was wrong, and there is a grey continuum between the two states.

tronvillain

Would most scientists say that viruses are alive? If they would then it would appear that they have reduced life to replication, but they don't appear to do this in other cases. After all, a cell is considered alive, but a chromosome is not. As for prions, they are proteins which exist in a conformation capable of catalyzing the change of similar proteins to the prion conformation - it's not even clear that this should be called replication, let alone life.

liquid, I will assume you meant to say "this is one of our strongest evdidences for life being able to arise from non-life" but I don't quite see how.

[ January 25, 2002: Message edited by: tronvillain ]</p>

ecco

Liquid,

Thanks for the reply. I’m not a creationist by any means and the gray line between non-life and life certainly seems indicative for the concept that life can form without any “divine intervention”

ecco

Oolon Colluphid

I'd say viruses are alive. They share two crucial features with everything else we consider alive: replication, of course, but also, adaptive complexity. They are, in Dawkins's term, designoid objects. They are too complex in a certain way to have come about by chance, and their design is intimately linked to their survival and replication.

Just my tuppence, mind you... but I don't recall anyone listing adaptive complexity in the usual 'replication, respiration, excretion, irritability' etc list. Thoughts?

TTFN, Oolon

Lannergard

Did a search on Google for "Definition of life" and got almost as many definitions as hits... Here is one from <a href="http://web.mit.edu/12.000/www/finalpresentation/science/life.html" target="_blank">http://web.mit.edu/12.000/www/finalpresentation/science/life.html</a> that is pretty close to my own intuitive definition:

1. Shows evidence of growth and replication;
2. Shows evidence of purposeful energy transfer;
3. Responds to stimuli;
4. Acts in such a way as to ensure self-preservation;
5. Is significantly different from the surrounding environment.

Viruses would be excluded since they lack the two first characteristics.

Viti

Viruses do replicate though don't they?

Lannergard

Replicate, yes, but no growth.

MortalWombat

If replication were the sole definition of life, then plasmids and transposons (which share many characteristics with viruses) would be considered to be alive too.

Ape

I find it interesting to note that viruses, while they may or may not be considered living cause researchers looking for cures more trouble than bacteria which are living.


Darn things keep changing on us