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Oct. 2, 2006 — What is the issue
regarding whether viruses are alive or dead? Why
do some scientists believe a virus is alive and
some believe it's dead?
—Anonymous
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Response
by Joana Loureiro
Graduate student in Whitehead Member Hidde
Ploegh's lab |
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Viruses are infectious particles that consist of a
DNA or an RNA molecule (the viral genome) packaged in
a protein capsid, a protective coat that allows their
transfer from one cell to another. Although it is in
our nature as humans to try to classify things in order
to make sense of the world around us, viruses may exhibit
characteristics of both living and non-living creatures
depending on the definition of life used. According
to Schwann and Schleiden’s “cell theory”
dating back to 1839, all living creatures are made of
“individual units of life” called cells
– small membrane-bounded compartments filled with
a concentrated aqueous solution of chemicals. The simplest
life forms are unicellular organisms; higher organisms,
such as ourselves, are like cellular cities in which
specialized functions are performed by different groups
of cells linked by intricate communication systems.
Under this definition, viruses are acellular particles
and thus are definitely not alive. If one’s definition
of life is a more evolutionary one, with an organism
being defined as “the unit element of a continuous
lineage with an individual evolutionary history”
(Luria et al., 1978), then viruses are definitely alive.
When one examines the criteria for a more functional
and generally accepted definition of living organisms:
the ability to reproduce, obtain and use energy, respond
to the environment, grow, develop, and die, we find
viruses are respectably somewhere in the middle. Obviously,
one cannot say that viruses grow, develop or die. Viruses
cannot replicate on their own, but they can do so rather
efficiently inside a host cell, whose genetic mechanisms
the virus subverts for its own reproduction. Viruses
do not have their own metabolism, but they can hijack
cellular metabolic pathways to their advantage, often
times redirecting all of the virus-infected cell energy
to virus production. In addition to having the ability
to affect their hosts’ behavior rather profoundly
(as anyone experiencing a bad case of flu will tell
you), viruses can respond to their environment by means
of rapid genetic mutations as is well documented for
HIV, for example.
Alas, although this question has been argued extensively,
the answer will remain elusive. Viruses may just have
to be left on the border between very simple biological
entities and very sophisticated molecular machines.
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