On Mon, 15 Mar 2004 00:48:02 +0000 (UTC), dkomo 
wrote:

>Tim Tyler wrote:
>> 
>> dkomo  wrote or quoted:
>> 
>> [http://alife.co.uk/misc/universal_senescence/]
>> 
>> > Ok, I need some more time to read the essay out there, but in the
>> > meantime, since turnaround time on sbe is so long, let me put my next
>> > question regarding your idea of complex adaptive systems having
>> > inherent senesence: why don't bacteria senesence?  Aren't they complex
>> > adaptive systems (simple relative to others perhaps, but complex
>> > enough to be alive)?
>> 
>> The "simplicity" is significant in the case of bacteria.
 *Complex*
>> systems are the ones which have the most difficulty building self-repair
>> mechanisms - and the relative simplicity of bacteria means that they
>> have fewer parts to go wrong - and that environmental damage stands
>> a high chance of killing them outright - and a low chance of
>> merely damaging one of their components.
>> 
>> However, my answer to the question is to doubt the premise.  It is
>> commonly believed that many small micro-organisms don't senesce and
>> die - but the view is mistaken.  They /do/ senesce - albeit slowly.
>> 
>> Follow an individual bacterium through multiple cell divisions
>> (choosing at random when it divides) and after a while the bacterium
>> you are looking at will die.
>
>I'm not sure this makes sense.  When a bacterium fissions into two
>bacteria, how do you tell which one is the mother cell and which the
>daughter by observing the action from the outside with a microscope? 
>Remember that during mitosis the DNA is replicated, then divided
>equally between the two cells.  Properly, the mother cell is the one
>with the original DNA after mitosis.  I don't think you can easily
>determine this unless that DNA is marked in some way.
>
>If you say, are you kidding?  Just keep a careful eye on the original
>cell during division and you should be able to keep track of that cell
>during the entire mitosis.  Yes, but because of the way the DNA is
>replicated and partitioned between the two cells, what you thought was
>the original cell could receive the replicated DNA instead of the
>original DNA.  And it is in this replicated DNA that most mutations
>occur.
>
>Thus, it is useless to speak of a single bacterium's lifespan or
>whether it senesces or not.  This is not an observable.  You have to
>talk of the senescence of the bacterial colony as a whole.
> 

This is not the way that DNA works.  The replication is
semi-conservative.  That is, the two strands separate and each serves
as the template for the new second strand.  As a result, there is no
"original" DNA vs. the "replicated" DNA.  Each of the two new DNA
molecules contains an original strand and a replicated strand.  Nor is
it true that one strand contains the genes -- some are on one strand,
others are on the opposite strand. Mutations are equally likely in
either daughter.

Also, when a bacterium replicates, there is no telomere to shorten
since the DNA is ordinarily a circle and there are no ends.  Therefore
there is no DNA senescence.  Cell structures are in constant turnover
(there are no independent mitochondria or plastids or other complex
organelles).  So at the time of binary fission, the "life" of each
daughter cell is considered to start from zero; the clock is reset and
the cells are rejuvenated by fission.  Even eukaryotes that reproduce
asexually have the same process -- there is no telomere shortening to
produce cellular senescence.  For sexually reproducing organs, the
telomeres are reconstituted by passing through the gamete/zygote stage
which resets the aging clock.
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