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Bacteria and Viruses?

Arlin Stoltzfus arlin at is.dal.ca
Sun Sep 15 09:28:45 EST 1996


Arthur Chandler wrote:
> 
>  A question from a non-biologist (who has been reading Stephen Gould's
> new book, *Full House*). Gould asserts (page 170):
> 
>   "...bacteria lie right next to the left wall of minimal conceivable
> complexity. Life therefore began with a bacterial mode."
> 
>  My questions:
> 
>  1) Are viruses more or less complex than bacteria?

Complexity is a notorious concept in biology.  For what its 
worth,  many biologists tend to think in terms of numbers of
genes.  From the small set of bacteria that have been characterized 
(a few archaebacteria and eubacteria), it seems that the mean 
gene number is perhaps 2000-3500, with the low end being about
600 and the high end being unknown.  Bacteriophages and other
episomes that inhabit bacteria typically have only a handful
of genes, though some have more (e.g., lambda has about 50 and 
T4 has maybe 150).  I don't know much about eukaryotic viruses,
but there are some with more than a hundred genes (Epstein-Barr)
and others with only a few (HIV).  So, by the gene-number 
criterion, even the most 'complex' viruses and phages seem to be
less complex than the 'simplest' bacteria.  

>  2) Are viruses alive?

Another sticky question.  Traditionally, 'life' is often 
defined based on a list of properties like reproduction (it
makes more of itself), irritability (e.g., it responds to stimuli,
e.g., poking, shining a light, applying a chemical irritant)
or homeostasis (it tends to actively maintain its status 
with regard to temperature, pH, etc).  Viruses can do all of 
these things-- IF they are in the context of their host cells.  

This non-independence of viruses is the sticking point for many
biologists-- because of their dependency on host cells, viruses 
are often not considered to be alive.  In this view, extant
earthly life is associated with *cells*: even for viruses, 
their life-like properties come about by virtue of their 
association with cellular organisms.  Other biologists use
"life" to include organic viruses and (sometimes) even computer 
viruses.

However, even if one accepts that extant earthly life is 
cellular, this does not mean i) that life *must* be associated
with cells as complex as bacterial ones, nor ii) that life 
(anywhere) must be cellular, nor iii) that earthly life
began with cells.  So it is not clear what Gould is saying. Perhaps 
you could clarify what Gould means by saying that life "began 
with a bacterial mode".  

Arlin
-- 
Arlin Stoltzfus
Department of Biochemistry
Dalhousie University
Halifax, Nova Scotia B3H 4H7 CANADA
(email) arlin at is.dal.ca 
(phone) 902-494-3569 
(fax) 902-494-1355



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