Exceptions of living things NOT composed of cells?

Scott Coutts scott.coutts at med.monash.edu.au
Tue Jan 27 23:24:33 EST 2004

Larry D. Farrell wrote:

> Mark wrote:
>>I guess a good distinction between the viruses and the intracellular
>>bacteria could be reached by considering the source of the replication -
>>viruses are replicated by using the host (by the host), while the
>>bacteria might be considered to be replicated by themselves, but within
>>the host. What do we all think about this?
> This is pretty much the point I was trying to make, but apparently did not do so
> clearly.  Anything that can be "considered to replicate by themselves" would have to
> have all of the basic mechanisms needed for replication.  Even those endosymbionts or
> obligate parasites that cannot, at present, be grown in the absence of their host
> organism do have all of the basic mechanisms but may be lacking the ability to make
> some specific nutrient/component that the host supplies (ATP, as indicated in another
> post, would be a perfect example of this).  On the other hand, viruses, viroids and
> prions clearly do not have those mechanisms and are completely dependent on the host
> to provide them.  In my opinion, that provides a pretty clear division between living
> and non-living.  However, one can still argue, as many do, that viruses, viroids and
> prions are examples of "some type" of life and it is had to refute that since the
> range of meanings for "some type" is pretty broad.  For me, having worked with
> bacterial viruses (phages) for 30 years, viruses are not living but they are damned
> clever parasites.

Yes, that is similar to my definition. I think an essential part of 
'life' is to have a metabolism. Viruses, prions, viroids etc dont have a 
metabolism - they become part of their host's metabolism. Rickettsias 
and other 'energy parasites' are still life, as far as I'm concerned, 
because they carry on their own, seperate metabolism, even though some 
of their nutrients are obtained from the host.


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