Viruses; Genisis and Taxonomy

Ed Rybicki ED at MOLBIOL.UCT.AC.ZA
Fri Jan 19 10:32:20 EST 1996


> The virus is a free entity until it infects a host.  I was correcting a 
> professor last week when she called viruses "organisms" but I failed to 
> come up with a suitable term to refer to them by.  Calling them organisms 
> is just for lack of better terminology.  When a virus has infected an 
> organism and is replicating, it can be considered alive and is classified 
> as an organism.

You should not have corrected the professor.  This professor has 
taught since 1981 - and will continue to do so - that:

"The concept of a virus as an organism challenges the way we define life: viruses do not respire, nor
do they display irritability; they do not move and nor do they grow, however, they do most certainly
reproduce, and may adapt to new hosts. By older, more zoologically and botanically biased criteria,
then, viruses are not living. However, this sort of argument results from a "top down" sort of
definition, which has been modified over years to take account of smaller and smaller things (with
fewer and fewer legs, or leaves), until it has met the ultimate "molechisms" or "organules" - that is to
say, viruses - and has proved inadequate.

If one defines life from the bottom up - that is, from the simplest forms capable of displaying the
most essential attributes of a living thing - one very quickly realises that the only real criterion for life
is:

The ability to replicate


and that only systems that contain nucleic acids - in the natural world, at least - are capable of this
phenomenon. This sort of reasoning has led to a new definition of organisms: 



  "An organism is the unit element of a continuous lineage
            with an individual evolutionary history."

The ability to replicate


and that only systems that contain nucleic acids - in the natural world, at least - are capable of this
phenomenon. This sort of reasoning has led to a new definition of organisms: 



  "An organism is the unit element of a continuous lineage
            with an individual evolutionary history."



The key words here are UNIT ELEMENT, and INDIVIDUAL: the thing that you see, now, as
an organism is merely the current slice in a continuous lineage; the individual evolutionary history
denotes the independence of the organism over time. Thus, mitochondria and chloroplasts
and nuclei and chromosomes are not organisms, in that together they constitute a continuous
lineage, but separately have no possibility of survival, despite their independence before they entered
initially symbiotic, and then dependent associations. The concept of replication is contained within
the concepts of a continuous lineage, and an evolutionary history. 

Thus, given this sort of lateral thinking, viruses become quite respectable as organisms: 

     they most definitely replicate, 
     their evolution can (within limits) be traced quite effectively, and 
     they are independent in terms of not being limited to a single organism as host, or even
     necessarily to a single species, genus or phylum of host. "

(available at http://www.uct.ac.za/microbiology/virwhat.html)


> Viruses replicate using the machinery in the host organism.  All the 
> components of the progeny are assembled by the host cell (ie. the DNA/RNA 
> and amino acids).  They cheat their way through a life cycle in a way.

Rubbish.  If poxviruses did any more they would be cellular
lifeforms.  If rickettsias did any less...would they be poxviruses? 
Many of your essential amino acids, vitamins, etc., are made by
other organisms, and if you did not have them, you would die. Does
this make you a parasite, or any less of an organism?  Don't be so
dogmatic (and in the end, does it MATTER?).

                     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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