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Hot Zone Question

Francisco Muril Zerbini fmzerbini at UCDAVIS.EDU
Thu Mar 23 14:17:11 EST 1995

On 22 Mar 1995, George McCabe wrote:

> 	What motivates "random" mutations in an organism like a
> virus?  Some of the talk here seems to attribute intelligence, even malice,
> to a virus.  I have always assumed that strong if infrequent events like 
> cosmic rays, disuptive contact with a foreign chemical agent, and the like...
> create new strains.  It is intriguing to think though that there exists
> some more tangible, perhaps deterministic motivator.  What for example,
> caused a new strain of Ebola, airborne and monkey specific to appear, if
> it didn't exist undetected before.
> George

Mutations are really random. The enzymes that replicate genetic material
(DNA or RNA) are not perfect, and might incorporate incorrect nucleotides,
generating a mutation in the progeny. This happens in all organisms,
humans included. However, most organisms (animals, plants, fungi and
bacteria) have "proof-reading" enzymes that scan the newly formed molecule
and correct misincorporated nucleotides. The actual rate of mutation will
then be much lower. Also, for mutations to be transmitted to the progeny
they must occur in germ cells, not in somatic cells.  However, most
viruses lack proof-reading enzymes and therefore all random mutations are
passed to the progeny. But you must keep in mind that the great majority 
of these mutations are lethal to the organism, and will kill it. Only if 
you have a large progeny (and I mean really large) and a high rate of 
mutation, there wil be enough favorable mutations, and the progeny that 
carries this favorable mutation might predominate after a few 
generations. This could perfectely happen with viruses, which produce 
huge numbers of copies in a short time without proof-reading. 

The main point is, viruses do not think "I'm gonna mutate now". It's all 
completely random. 

Murilo Zerbini
Dep. of Plant Pathology
University of California, Davis

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