On Sat, 1 Apr 1995, Oliver Bogler wrote:
> I agree with Andy - some cells in a lineage must be immortal. This line of
> thought tends to blur the distinction between organisms and species (for
> me at least). If you consider the DNA in you, it is a copy of the DNA that
> was in the very first human, albeit not an accurate one. It has been
> carried from that human to you by an unbroken chain of cells. Of course
> most of those cells are dead now, but the chain is unbroken, and so the
Hardly a satisfactory treatment of ageing. By this thought process,
humans (and everything else alive today) are "immortal" in that they are
all copies, albeit imperfect, of predecessors...in an unbroken chain. By
this same token, we are ALL the imperfect copy of the first true living
organism which, by extension, is "immortal" with an age of some 4.5
billion years and counting. Hell, why all the fuss about any kind of
life extension?
The DNA and proteins that first made me no longer exist, as they were
the precise strands of DNA and extant proteins within the zygote that led
to me. Those original cells and their contents have long ceased to exist
leaving me with many imperfect copies of that first progenitor. I have
cells dying all the time, some of which are replaced, many that are not,
none of which are immortal. Even stem cells cannot be said to be, in and
of themselves, immortal (I really hate that word...NOTHING can be
immortal because even matter has a finite existence). The stem cells
(plural) that lead to the blood presently coursing through my veins need
by no means to still exist tomorrow. They have divided into other stem
cell copies, some of which die...perhaps including those that carried the
original parental DNA. These cells have led to differentiated cells,
which die even as some of the stem cells die. What PARTICULAR cell in
anyone's body lives indefinitely? Tag any stem cell, perhaps with a
fluorescent tag or radioactive P, C, or H and then tell us whether THAT
PARTICULAR cell remains for 70 or 80 years, or even a significant
fraction thereof. I do not believe you would ever discover such a cell.
Stem cells do not have to be immortal, only constantly replicated. The
progenitors to generation A stem cells need not remain to be the
progenitors of generation F. The original, parental DNA from any stem
cell gets diluted out after only a handful of cellular generations, and
can even be entirely replaced by mismatch repair, excision repair,
recombination, and cellular death. The strands aren't the energizer bunny.
Patrick
