From: ed at micro.uct.ac.za
To: una at phy.duke.edu
Subject: Re: are viruses alive?
Reply-to: ed at micro.uct.ac.za
Date: 27 Jul 92 10:28:48
I enclose my reply to Una as she saw fit to post mine! No acrimony intended, I
have been thinking on this subject for a long time.
Sorry for the delay, my wife and I have been busy producing our first
[in reply to query on definitions of virus and organisms by S Luria]
> It would be helpful if you would include page numbers when quoting from
p.4 [Luria et al, General Virology, 3rd Edn]
> I disagree, however, that a definition of organism is needed; a
> good definition of life certainly _is_ needed. I tend to agree with
> Harold Morowitz's definition of a living organism, and I'm looking
> forward to reading his new book, which I haven't had the priviledge of
> reading in manuscript.
Whaaat? Why does one need a definition of life? I'm a working
biologist and have been for 15 years, and I've never needed one -
definition of an organism suffices very well. Tell you what, I'll
give page-numbers if you won't quote from books you haven't read
> >People all too often think of life in terms of a"top-down" approach: going from
> >things with legs or leaves to less complex forms, until they reach viruses and
> >say: "hey, these things are just molecules, anyway!"
>> That's right, we're all just piles of molecules which exhibit a
> characteristic (but difficult to characterize) phenomena which we
> call "life" (unless we happen to be "dead"). I don't see what's
> wrong with such a train of thought.
What is wrong with the approach is that it leaves one convinced that
"we go from the living to the dead", and viruses happen to be the
dividing line for that way of thinking - give up on the live vs.
dead, and there is no problem with the concept of virus as organism.
> >"A material is living if, after isolation, it retains a specific configuration
> >that can be reintegrated into the cycle of genetic matter."
> >- Salvador Luria, 1953 (Virology, 1st Edn)
> >Prescient, wouldn't you say? And describes viruses as organisms
> >as life, succinctly and aptly. >
> Prescient of what? Salk had already shown by 1953, I believe,
> viral cultures could be fractionated into proteins and DNA, and thus
> "killed", but when the components were recombined, the viruses were
> able to re-encapsulate themselves and become infectious again. Since
> you've got the virology textbooks at hand, Ed, perhaps you could fill
> in the facts here.
Glad to. Jonas Salk was responsible for a poliovirus vaccine - not
for reconstitution experiments on viruses (poliovirus cannot be
successfully reconstituted). It was Heinz Fraenkel-Conrat and co-
workers, in 1955 and later, who showed that the virions or particles
of TMV (tobacco mosaic tobamovirus) could be broken down to
constituent RNA and protein, and reconstituted into infectious
particles in vitro. However, as the RNA is infectious alone, this
was not conclusive. It also cannot be done with all viruses (I would
> Luria's definition, above, reads to me like a sly
> attempt to say "viruses are alive" without coming out and saying so.
> This definition, as presented here, does not fit most living organisms,
> since if you fractionate them, then recombine the mess you've made,
> the result will not "be reintegrated into the cycle of genetic matter".
> Therefore, Luria's definition may apply to viruses, but it is generally
> invalid and thus inadequate.
It would be a good idea to get your facts straight before you launch
quite so confidently into such sweeping claims, or refutations of
carefully thought-out definitions, from outside a specialist field 8-)
And in any case, Luria is hardly attempting to slyly say "viruses are
alive" - he is doing what I have stated above, simply not worrying
about defining life as it is an intangible - and, if an organism
(which has the attributes we call "life", as long as it is not
"dead", or deep-frozen, or lyophilised, or deconstructed, or dormant)
is viable, its most obvious characteristic is that IT REPRODUCES
ACCORDING TO ITS KIND - and that is specified by its genome.
Luria's definition does fit all living organisms, as they all have
genomes composed of nucleic acid (DNA for all cellular life), which
is what defines the organism in terms of all the instructions for
structural components, and for how they are to be regulated. And it
is possible to reconstitute a cellular organism, by rendering a cell
(preferably large) enucleate (removing its nucleus, by micro-
manipulation), and putting in another, from another organism
(preferably closely related, to get a good cytoplasm-nucleus match).
The cell so reconstituted takes on the attributes of the cell type
supplying the nucleus - except for degenerate endosymbionts like
mitochondria, of course, which have their own genomes even if many of
their constituents are supplied from the nucleus. AND THE POINT IS,
THE CELLS A VIRUS INFECTS EFFECTIVELY BECOME THAT VIRUS - IT HAS ITS
WHOLE BEING IN A CELL AND FROM A CELL, BUT HAS ITS OWN EVOLUTIONARY
HISTORY. IT IS THE SAME WHICHEVER CELL TYPE IT INFECTS.
> >It is quite possible that many viruses evolved from cellular organisms (not
> >living orgs); it is equally possible that there are some viruses around now
> >which descend from bits of nucleic acid that may have been part of the very
> >first self-replicating (acellular) systems - some of the simpler (not
> >primitive) RNA viruses or parts thereof are very good candidates for direct
> >linear descendants of the ancestral RNA molorgs (molecular organisms).
>> Is it? Are they? Let's just say we don't know what viruses evolved from,
> but it sure would be nice to find out.
You're very quick on the draw - have you read much of the
speculation on the origin of viruses? It is becoming increasingly
clear that there is no single origin for viruses, and that while some
of them - notably the biggest, like the T-even phages and poxviruses
and herpesviruses - could very well be degenerate cells, the smallest
are definitely not, even if certain of their genome components derive
from cells. RNA viruses, for example, are the only extant examples
of RNA organisms - and RNA-based creatures are postulated to have been
THE ancestral organisms. The template dependent RNA-dependent RNA
polymerases of these viruses are central to their existence - and do
not have counterparts within cells. They are also extremely
distantly related to one another, prompting speculation that they may
be very ancient organisms indeed. So let's just say that we are
getting a good idea of where viruses came form, and we continue to
amass evidence for these ideas.
Quickness of intellect and an ability to be logical does not suffice
if you do not have all the facts. When in doubt, read the
| | |
| Ed Rybicki | Now you've got the hang of it |
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