Exceptions of living things NOT composed of cells?

Scott Coutts scott.coutts at med.monash.edu.au
Mon Jan 26 21:33:36 EST 2004


Larry D. Farrell wrote:

> Des O'Connor wrote:
> 
> 
>>"Mark" <nospam at nspam.com> wrote in message
>>news:6JdRb.520$X2.32765 at news.tufts.edu...
>>
>>>Larry D. Farrell wrote:
>>>
>>>>Scott Coutts wrote:
>>>>
>>>>
>>>>
>>>>>Tom Williams wrote:
>>>>>
>>>>>
>>>>>>I'm an undergraduate student who's curious about
>>>>>
>>>>>>something mentioned in an anatomy lecture:
>>>>>
>>>>>>My professor said that it was possible to have
>>>>>
>>>>>>life without a cell, or cells.
>>>>>
>>>>>>How can this be??? Can somebody please explain
>>>>>
>>>>>>how, or even if this is true?
>>>>>
>>>>>Hi Tom,
>>>>>
>>>>>It depends on how he defines 'life'. This always instigates a big
>>>>>discussion on what 'life' actually means.
>>>>>
>>>>>He's probably talking about anything able to replicate, so then he's
>>>>>including viruses. Viruses are able to replicate, but as far as I'm
>>>>>concerned, this is not 'life'... I dont regard viruses as 'living'. If
>>>>>you're interested, you might like to look up viriods and prions -
>>>>>they're able to replicate, but they're smaller than viruses... They're
>>>>>single molecules!
>>>>>
>>>>>Scott.
>>>>
>>>>
>>>>However, none of these are capable of replicating independtly of other
>>>>organisms, which is pretty much a requirement for "life" as it is
>>>>generally defined.  (Note, of course, that most eukaryotes need another
>>>>organism in the form of a mate to initiate the process of replication
>>
>>but,
>>
>>>>once begun, the process does not depend specifically on the other
>>
>>organism
>>
>>>>providing on-going functions for the process to continue to completion.)
>>>>Viruses and viroids are essentially inert unless placed into (or at
>>
>>least
>>
>>>>in very close proximity to) host cells that provide the essential
>>>>functions and materials needed for replication, while prions produce
>>>>copies of themselves only if provided with properly folded copies of the
>>>>prion protein, usually provided by living host cells.
>>>>
>>>>--
>>>>Larry D. Farrell, Ph.D.
>>>>Professor of Microbiology
>>>>Idaho State University
>>>>
>>>>
>>>
>>>Of course, we start to get into muddy water with this kind of definition
>>>when we consider endosymbiotic bacteria such as Buchnera, which I think
>>>(I might be wrong) is an obligate symbiont - can't live independently of
>>>the host (aphids in this case). I sometimes wonder if this is really all
>>>that different to a virus - both seem to grow and reproduce, but only
>>>given a cellular environment to do it in. Someone was suggested to me
>>>that the cellular environment that is so essential to these creatures
>>>might be analogous to the terrestrial environment that we depend upon -
>>>once outside of it we are not all that good at reproducing or surviving.
>>>Not too sure about that myself though. Thoughts?
>>>
>>>mark
>>>
>>
>>Hi Tom
>>
>>This really is a wonderfull topic for a budding Biologist to  put some
>>serious effort into the study of. Id
>>encourage a lot or reading and thinking around this question as you will the
>>encounter many concepts
>>relating to  the majority or core biological areas. An endless feast of
>>science no less. I seem to remeber the
>>topic has been addressed several times  in Scientific American and New
>>Scientist.
>>
>>One interesting approach might be to look at the topic from an ecological
>>point of view rather than pure a biochemcial/structural/molecular
>>viewpoint.
>>
>>At one time I personally considered the  cell  was ''Natures's Wheel''  and
>>anything not bounded by a functional membrane
>>did not qualify as life , these days Im not so certain. I think we would all
>>be interested to know what conclusion you come to
>>after diving into this area.
>>
>>Best Des
>>
>>I wish you luck and joy on your graduate course.
> 
> 
> Mark's post hasn't showed up yet on my server so I am piggybacking on Des's post
> to respond to Mark.
> 
> With the "obligate symbionts," the issue is simply that we have not yet defined
> an artifical medium/environment that will allow them to grow without their
> hosts.  However, there is almost certainly such a set of conditions that would
> allow the to grow independently.  For viruses and viroids, we may actually get
> to something of the same thing one day, when we can construct an artificial
> cell, but the bottom line is that the virus/viroid still cannot replicate
> without the help of some sort of host, whether natural or artificial.
> 
> I do agree with Des that this is an excellent discussion topic, and there are
> folks that come down on both sides of the "living" argument.  My position is
> pretty obvious above.
> 

Well, we can certainly do cell-free transcription and translation on a 
routine basis now, so I think the viroid replication in vitro would not 
be a problem!

Generally, one of the requirements for my definition of life is 
metabolism. So obligate intracellular organisms still have their own 
metabolism.

Scott.






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