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Viruses: Genisis and Classification

Giovanni Maga maga at vetbio.unizh.ch
Wed Jan 17 02:38:53 EST 1996

In article <DLA2zK.Cou at utcc.utoronto.ca>, AJ Boulay
<aj.boulay at utoronto.ca> wrote:

> I am interested in the genisis of viruses and thier classification as a 
> living organism in it's pre infection form.
> I understand that viruses are inert until they can combine thier DNA with 
> that of a living cell.
> My question's are:

Not all the viruses combine their genome with the one of the cell.
Moreover, not all the viruses store their genetic information as DNA, but
also as RNA. In order to be considered a virus it does not need to be
infecting a cell. The matter if it's a living organism has been a long
debate since their discovery. The answer relies upon what you call living.
Until ther will not be a unique and generally accepted definition of life
the problem of viruses as living organisms will remain uncertain, but it's
more a semantic problem that a substantial one.
> - are viruses still classified as viruses after infecting cells, or are 
> these cells simply classified as malignant? 

Sorry, but this is not clear. The virus is a discrete entity with
structural features (shape, size, structural proteins, DNA/RNA). These
features are used for the classification of the virus. The infected cell
is simply referred as an infected cell. The term malignant is used for
cells that undergo to neoplastic transformation and show an invasive
behaviour (malignant tumor vs. benign tumor). Sometimes this
transformation is the consequence of a viral infection (oncoviruses like
HPV or SV40), but this is rather an exception than the rule: in most of
the cases, productively infected cells (i.e. where the virus will
replicate) will simply break down and die. Thus not all the infected cells
are malignant and not all the malignant cells are infedcted.  

> - If infected cells are not classified as viruses, how is it that viruses 
> come to be? If they are inert they do not have metabolic processes or 
> division.

As I pointed out before, the cells are distinct from the viruses. The fact
that a virus before infection is *inert* does not mean that it doesn't
exist. The closer example I can find are parasites that are *inert* as
oocysts (for example) until they do not enter the appropriate host.
Another way for the virus to survive is latency: some viruses *store*
their DNA/RNA into appropriate host cells after infection. This can last
for all the life of the host. These cells are infected, but do not produce
viruses. The virus itself is inside the cell, but only as its DNA/RNA, so
it cannot be called virus (since the term usually refers to a complete
viral particle). Thus, as you can see, here is a situation opposite to the
one you figured out: the virus after infection is not anymore a complete
virus and the cell is infected but perfectly *healthy* and functional.
This is just to show you how the life cycle of viruses presents different
aspects that escape oversemplifications.

This is of course far less than a complete explanation, but  you can
answer to all your questions by looking at any microbiology/virology
textbook in your local library. Since your questions are basic ones, you
do not need even a specialistic book...the one used for biology courses at
high schools or for introductory courses in microbiology at the University
are more than enough. 

> Thanx,
> AJ

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