Are viruses alive?

David Sourdive sourdive at pasteur.fr
Mon Jul 20 04:58:55 EST 1992


una at phy.duke.edu (Una Smith) writes:


>eanv20 at castle.ed.ac.uk (John Woods) writes:

>>   P.S. Are viruses alive or dead?  Just a thought.

>drw at banach.mit.edu (Dale R. Worley) writes:

>>The absolute, eternal dogma (for the last decade or so) is "Life is
>>self-replicating information."  Thus, it's the genetic information
>>inside you that is the part that is *really* alive.  And viruses are
>>alive, although they're dormant most of the time.

>Viruses are _not_ self-replicating.  They are (we think) evolved
>from self-replicating, living cells.  In the course of evolution
>viruses have discarded all but a very few genes, and now depend
>on the host cells which harbor them to replicate the viral DNA
>and build the viral coat proteins.

>This is at best a very indirect form of self-replication, and I
>do not think it is problematic for theories of life, just as I
>don't think parasitic plants which have lost the ability to
>photosynthesize are a problem for general theories or definitions
>of plant life (as discussed recently in bionet.molbio.evolution).
>Viruses are alive in that they evolved from living organisms...


Replication, like all biological functions is achieved in a given
environment including given substrates. Whether it is direct or
indirect is very subjective. In fact, a virus needs cellular components
to replicate like we need food to grow old enough to have babies,
or like flowers need bees to "mate". Some autotrophic organisms
only need mineral substrates to grow and replicate, but they still
need something ! In that sense, a mitochondrion or even a
propagating intron, completely unable to exist without eukaryotic
cells, are just as alive as us. (We would probably also stop
existing without other eukaryotic organisms.)

The critical word in the dogma is "self".
                             - It can bee understood
restrictively, that is "with no external element", in which case you
have to draw, at some point, an arbitrary line in order to be able
to say that we are alive. You will probably need to define the "degree
of autonomy" recquired to be living thing for example.
                             - It can also be understood
less restrictively, that is "it is needed for its own replication", in which
case anything that replicates even with the help of external elements
(such as food, or bees in my examples) is alive.

The latter way to see things is probably less arbitrary, and caries less
implicit assumptions. Yet, if accepted, it leads to the frightening
conclusion that, in the example of propagating introns or plasmids, we
can generate life in a tube !



 But then again, haven't we been dreaming of this for centuries ....



David Sourdive
Institut Pasteur
Departement des Biotechnologies

sourdive at pasteur.fr



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