Definition of Life - VIRUSES LIVE OK!!!!

Ed Rybicki ed at micro.uct.ac.za
Wed Feb 9 17:44:23 EST 1994


In article <9402031302.AA24039 at net.bio.net> LYONSW at UCONNVM.bitnet (James) writes:

>Douglas Adams    <adamsd.cerf.net> writes:

>>I have to give a talk next week, for which it would be useful for me to
>>know a couple of things:

>We can typically think of an object as "alive" or "with life" if it is
>capable of ALL of the following AT SOME POINT IN ITS EXISTENCE:
>- Growth
>- Metabolism
>- Reproduction

>Two excpetions to these criteria might be viruses and viroids, which are
>not capable or grwoth or metabolism at all - in fact, some biologists
>prefer to think of them as non-living molcules of RNA wrapped up in
>a protein coat.  

And they would be WRONG....because all definitions of life have come down from 
zoologists and/or botanists (who have progressed from the days of "it walks, 
so it is alive", or "it is green and sucks water, so...", but not much!), it 
means that they are heavily biased towards definitions of CELLULAR life forms. 
 Salvador Luria et al have a good answer to that: in General Virology, 3rd 
Edn, they define an organism as "..a unit element of a continuous lineage with 
an independent evolutionary history".  This neatly dodges the question of 
whether things are "alive" or not, because one can say that the attribute of 
organisms is that they are alive.  However, for those of a cellular bent who 
remain unmoved by paradigm shifts, they also say that: "A material is living 
if, afeter isolation, it retains a specific configuration that can be 
integrated into the cycle of genetic matter".  This covers viruses, and 
bacterial genomes, and also chromosomes - after all, one can transform 
enucleated cells with nuclei extracted from other cells, neither of which are 
viable by themselves, and the properties of the newly reconstituted cell are 
largely determined by the new nucleus.

A "bottom up" approach to the definition of life (or what constitutes a living 
entity) is most educational:  all sorts of preconceptions have to go out of 
the window, because one is left essentially (in our evolutionary biosphere) 
with "If it replicates itself, it is alive" - and IT is always nucleic 
acid-based (so far).

Which leads to the Rybicki definition of terrestrial life:

"That phenomenon associated with the propensity of nucleic acids to replicate 
themselves". 

Enjoy...!
  ____________________________________________________________________
 | Ed Rybicki, PhD             |    "Now you've got the hang of it    |
 | (ed at micro.uct.ac.za)        | There's nothing you can't do with it |
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