In article <m0s1aix-0004PHC at uctmail.uct.ac.za>, ed at molbiol.uct.ac.za wrote:
>> But seeing as you brought it up, there can be no real question as to
> the motive of DNA/RNA: its purpose in life, in that a molecule can
> be said to have a life or have a purpose, is to REPLICATE ITSELF...I can
think of no better exemplar
> of this than the humble viroid, which is simply a piece of RNA which
> relies on inherent tertiary structure to get itself replicated by a
> cellular RNA polymerase. Ted Diener, are you listening/reading?
This idea brings up an interesting point with respect to the "are viruses
living" post as well. The entire metaphor of viruses as living creatures
in the sense that they have a "purpose" may confuse the issue more than
clarify it. As you noted, rocks have a propensity to form pebbles, but
certainly this is not considered the "purpose" of rocks, and rocks which
easily turn to gravel aren't considered to be especially efficient in
achieving their purpose. Likewise, a piece of nucleic acid, or even a
viral particle may replicate under a very specific set of conditions, but
this is simply a result of being in the right place at the right time.
The confusion arises from the ambiguity of cause and effect in the "viral
purpose" metaphor. DNA will be transcribed in a cell whether it is
delivered naturally by a viral particle or artificially by transfection.
This process is somewhat akin to the propensity of wood to burn in a
fire. If the result of transcription of this piece of DNA is the
production of more similar pieces of DNA, and if the piece of DNA happens
to code for a set of proteins which can form a capsid to protect the DNA
in the outside world, then that DNA will likely be propagated elsewhere
(ie. in another host). The whole metaphor, however, is one of outside
forces resulting in propagation of a piece of DNA, not a piece of DNA
whose purpose is to subvert the cell in order to replicate itself. The
cause is the existence of the specialized environment of the cell. The
effect is propagation of the virus.
David P. Frazier