Are viruses alive?

Ewen Cartwright - PPATH cartwrte
Tue Nov 7 13:06:50 EST 1995


mickj at outside.com (Mick) wrote:
>A recent disucussion into the nature of life and, specifically, viruses
>made me wonder if a virus is alive. Since a virus can lie dormant for
>extremely long periods of time and then begin replicating itself in the
>presence of a host, is it, indeed, in "control"? Is it replicating itself
>or is the host replicating it? Is the fact that a virus may be carried by
>one species with no deleterious effects because that carrier hasn't the 
>proper raw materials to allow or instigate replication? Also, in a species
>where the virus is harmful or even fatal, does it stop because the virus
>has run out of "fuel" or, if the host is in control, because it no longer
>has the ability to continue supplying either energy or raw materials?
>
>I have no background in biology so I apologize if my questions are
>ridiculous.

Parasites usually rely on their host organism to provide a form of sustenance
and often a safe place to incubate their offspring. Viruses are parasites taken
to their logical extreme -- they reproduce (or 'replicate') within the host
cell using the host cell's machinery, and so it is the host cell, and not the
virus, that produces virus offspring -- all the virus needs to do is to supply
the host with the necessary information (genes) to make new viruses (this
usually includes a gene to make an enzyme to replicate the viral genome, and
another to make the viral coat protein; there are usually three or four more
that produce proteins to do various other tasks, depending on the virus
species). As the host is doing all the work, the virus itself needs no energy.
Most viruses that are harmful tend to be so because they hijack the cellular
machinery to such an extent that often the sheer number of viruses produced
destroys the cell.

Viruses are often carried by one species (the 'vector') which they do not harm.
The vector then introduces the virus to the host species. An example of this is
the way that aphids carry potyviruses. Depending on the species of virus, virus
particles (virions) attach themselves to part of the aphid's feeding apparatus
or gut. When the aphid feeds on the sap of a plant, the virus is injected into
the plant sap vessels as a consequence. There can be many reasons why a virus
is only able to replicate in the plant and not the aphid; usually it comes down
to one of the virus' components not being compatible with that of a potential
host's. For instance, the virus may lack a protein which allows it to be taken
into an aphid cell. Viruses can be extremely specific to one host or another
for this reason. However, some viruses are able to replicate in both their
vector and their host (the so-called 'propagative' viruses). Usually these will
only replicate to a limited extent within the vector cells, perhaps because
they are not fully compatible with them. If they killed large numbers of vector
cells, the vector's ability to transmit the virus to its host would be
impaired, and in effect the vector itself would become a host. Usually there
are significant evolutionary hurdles preventing this from happening.

As for whether viruses are alive or not -- I suppose that in most cases you
could say that they are forms of life (mainly because of their possession of
DNA/RNA, and other chemical similarities to living cells) that aren't exactly
'alive', in that they themselves make no effort to remain alive. It just so
happens that their numbers continue to multiply entirely through their host's
efforts. Their ancestors may well have been more complex forms that possessed
their own replicative machinery, which was lost over time because for one
reason or another using the host's machinery was more efficient. In my
(limited) experience, most definitions of 'life' that I've come across have all
been rather loose at best...

I hope this helped...

-ejc




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