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Plasmids as life?

Ed Rybicki ED at MOLBIOL.UCT.AC.ZA
Mon Jan 22 04:52:47 EST 1996


> Hi.  I just read your one of your postings in bionet.virology, but I haven't
> worked out how to post a reply on this arcane Unix mail system, so this 
> e-mail will have to suffice for now.

Post to virology at net.bio.net

> In your posting, you claimed that viruses deserved to be classified as 
> organims as they had the ability to replicate, an evolutionary history 
> (part and parcel of being replicable) and also had a degree of 
> independence of hosts.
> 
> Does this not also mean that the humble plasmid also deserves to be 
> called an organism??  Plasmids can be replicated, (and thus may 
> accumulate mutations and hence evolve), and some plasmids are also able 
> to infect multiple species.

Plasmids most often do not mediate their own replication (?) and are 
almost never potentially pathogenic - meaning they are less 
independent of their hosts than a virus, more a commensal than a 
pathogen.  However, they are part of a continuum of 
nucleic-acid-based replicons starting with chromoosmal DNA and ending 
with viruses.  So where do you draw the line....

> Under the right conditions, plasmids can exist independently outside of a 
> cell as well.  Will you extend your definition of an organism now to a 
> plasmid (remembering that all a plasmid is is nucleic acid)???

Plasmids can only exist independently of a cell as purified DNA...not 
part of their natural life cycle, is it?

> Please correct me if you think I am wrong.  I just think that under your 
> line of logic, a plasmid may be also classified as an organism, and I 
> don't really think a length of nucleic acid deserves that title.

If you really think about it, the definition of an organism fairly
arbitrarily draws a line across a continuum of forms, from the
humble chromosome through the plasmid to the virusoid, the satellite
nucleic acid, the satellite virus, the viroid, then the virus: it is 
the definition itself, and the insistence on it by humans, that is 
the problem.  Nucleic acids know what they are and what they code 
for; if it is in a dependent system of associations called a cellular 
genome, is that any more reason for it to be called an organism or 
part thereof than if it is a naked piece of RNA which codes only for 
its own structure (and incidentally, its own replication via host 
cell RNA pol II)?

And Salvador Luria et al. are responsible for the definitions of 
organisms, etc., not me: taken from General Virology Edn 2 (full 
citation at the Web site).

Nice debating with you B-)


                     Ed Rybicki, PhD  
      Dept Microbiology     |  ed at molbiol.uct.ac.za   
   University of Cape Town  | phone: x27-21-650-3265
   Private Bag, Rondebosch  |  fax: x27-21-650 4023
      7700, South Africa    |   
    WWW URL: http://www.uct.ac.za/microbiology/ed.html      
                                        
"And then one day you find, ten years have got behind you"



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