definition of life

Larry Marshall lmarshal at pnfi.forestry.ca
Fri Feb 11 12:39:34 EST 1994


In <2jgdg9$il at mserv1.dl.ac.uk> suter at VAX.MPIZ-KOELN.MPG.d400.de writes:

>sure they contain DNA, but so do many of my eppendorfs. however, these
>cells use this dna for maintanance only, whereas a true lifeform uses 
>it also to pass on to offspring, enabling evolutionary continuity.

The problem with definitions of life is that they are dependent upon
the definitions of so many other things.  You say that cells don't
use DNA to produce offspring.  What is cell division?  How is the 
information that results in 2 skin cells being created from one stored
if not in the nucleic acid sequences?  In other words, what are
"offspring" if they are not created by the mitotic process?  Do
we need meiosis?  Is that the point you are trying to make?  Do
we need "reproduction".  And so it goes...life definitions become
an ongoing problem of defining the terms used to define something
which is used to define....

>your body cells may be alive (they breath
>and feed and move and so on, even the dna is transcribed and mRNA is
>translated) but they are no independent lifeforms, because they have
>lost the ability to create offspring.

I believe this was the point I was trying to make when suggesting that
defining life in terms of the biochemical units required for its
existence, as we know it, is not very viable. 

>but still, there is no 'continuous' lifeform that doesn't contain a
>nucleic acid, is there ?

Nor one that doesn't contain carbon.  To the limits of our knowledge
we could come up with a pretty long list of "all lifeforms have..".
We would be no further along in our quest for a definition for life.
You've centered on nucleic acids presumably either because they 
carry the information upon which life occurs or the fact that you 
believe that the ability to replicate that information is required.
I'd be more willing to buy those concepts, which are not tied to the
simple presence of guanine but they are much broader concepts than
simply biochemical composition.


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