Another way to look at the why we die is to consider how long a cell line
can last. Mammal cells in culture seem to last only a certain number of
generations - an exception is tranformed (cancerous) cells, such as HeLa
cells which appear to be immortal. Its interesting that plants differ
here, in that plant meristem cells can continue to regerate entire
indoividuals for very long periods of time, and can regenerate an entire
organism in favourable culture conditions.
It has long been known that telomeres (the specialsed end bits of
chromosomes) shorten through the lifetime of vertebrate tissue, apart
from cencer cells in culture. So that our chromosomes just wear out, from
the ends in.
Recently it has been found that telomerase, the enzyme that makes
telomeric DNA, is expressed in immortal cells but not in normal cells. I
imagine that telomerase is expressed at some stage in the development of
a vertebrate, so that each new generation has its alotted lifetime of
chromosome. If telomerase could be switched on at will, or better still
swtiched on whenever telomeres get too short, then our chromosomes might
keep better, and longer.