definition of life

suter at VAX.MPIZ-KOELN.MPG.d400.de suter at VAX.MPIZ-KOELN.MPG.d400.de
Wed Feb 16 09:02:34 EST 1994


marshall wrote:

+>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....

i 'tried' to give the answer to this point:

+>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.

that is: there is no continuous information flow 'towards eternity'. basically,
bodycells are only there to serve the lifeform in which they exist. the
mere presence of nucleic acids does not mean something is a lifeform
(an eppendorf with DNA), but all lifeforms contain nucleic acids (a harley is
thus no lifeform). naturally, the presence of nucleic acid has
essential consequences (replication etc.).

+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 it is used all the time. what is a chair ? an object to sit on. does this
make a garbage can a chair ? clearly not, i would say. therefore, the human
mind has to use some abstract, constantly changing picture of a chair, 
something with legs (3,4,5 ?) of a certain size with a certain purpose.
one has to take one (or more) items which describes the objects characteristics
in the best possible way. 
saying that all lifeforms must contain nucleic acids is from that viewpoint 
a legitimate thing to do.

+>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..".

i wouldn't include carbon, because mineral oil also contains
carbon. why should i exchange a very stringent criterium for a rather 
loose, general one. why should i include characteristics of 'a bed'
in my definition of 'a chair' ???
 
+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 talked about this before.

+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.

why do you want to make your life so hard ? all of a sudden people
(perhaps not you, but you hint at it a little bit) want to include 
computer virusses as genuine lifeforms, with rather weak arguments.
sure they replicate. but if i throw a stone against the wall, and
it shatters in thousands of pieces it also replicates ? and perhaps
there are lifeforms in the universe, that do not require nucleic acids, 
but alas, i have never seen one, and neither did you (fair guess?).
as a 'working definition' i still think that the presence of
nucleic acids (as the matrix to guide its own replication and the
biochemical processes necessary for survival) is valid.
computer viruses are -at the most- 'lifelike', interesting to study,
or to use to study life itself, but not the real thing, imho...
 
cheers,
clemens
===============================================================================
Clemens Suter-Crazzolara, PhD
Max-Planck-Institut fuer Zuechtungsforschung
Abteilung Genetische Grundlagen der Zuechtungsforschung
Carl-von-Linne Weg 10,        D-50829 Koeln, Germany
Tel.xx49.221.5062-221    Fax.-213      e-mail: suter at vax.mpiz-koeln.mpg.d400.de
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