Viruses as Organisms

[Joao Vasconcelos Costa]

Bruce Phillips wrote:
> 
>         It seems to me that the ability to distinguish viruses from plasmids,
> i.e. whether both or neither are organisms, is a bit thornier than I had
> heretofore considered.  Both plasmids and viruses are capable of self-
> duplication and their distinction is further muddled by the fact that,
> positive-sense viruses (except retroviruses) contain nucleic acids that are
> infectious.  Well, you might say, viruses exhibit a life cycle by (passively)
> transferring themselves from one cell or host to another.  But plasmids
> clearly do that as well, albeit presumably with lesser efficiency.  Remember
> RTFs- one of the early concepts associated with plasmids.  And the fact is
> that both plasmids and viruses depend on pre-existing cellular processes
> in order to replicate.  It may be an academic discussion, we all can
> distinguish plasmids from viruses, but providing a definition of viruses
> to students that distinguishes them from plasmids, should one care to do
> that, is more difficult than I had formerly conceived.

I think that coding for proteins that assemble in particles is a
major distinctive difference. It has an important evolutionary
meaning, since it enables viruses to survive out of cells, while
plasmids can only be transferred from cell to cell.
However, I must admit that this distinction is not clearcut unless
we don't consider viroids. Besides, there are transitions, like
the yeast retrotransposon Ty, which codes for a gag analog that
may be assembled in intracellular particles.

Joao
 
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Dr. JOAO V. COSTA                        mailto:jcosta at gulbenkian.pt
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