Exceptions of living things NOT composed of cells?

Larry D. Farrell farrlarr at isu.edu
Tue Jan 27 10:37:29 EST 2004

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?
> mark

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.

Larry D. Farrell, Ph.D.
Professor of Microbiology
Idaho State University

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