The recent issue of Scientific American (August, 1991), which has a good
article on "Telomeres". Telomeres are the DNA sequences at the end of the double
helix. In humans, this sequence is TTAGGG repeated some large number of times. It
is on the end of ALL human chromosomes, and on the end of the chromosomes of ALL
species studied (appr 100 vertebrate species that have not had a common ancestor
for over 400 million years).
It has also been noted that cellular DNA replication often "truncates" the DNA
molecule. My reference is the article itself, and a report in "Foresight Update No. 10",
from the Foresight Institute, which briefly profiles the work of Harley, et al. at
McMaster University and Cold Spring Harbor Laboratory (Nature 345:458-460, 31May90)
Organisms that do not age have "telomerase" enzymes that "repair" the truncation.
Higher organisms have these enzymes as well, but they are only active in the production
of reproductive cells.
The Foresight Institute article gives an approximation of 180 cell divisions
in human somatic cells before the telomeres are exhausted. The speculation, of course,
is that this may be the cause of cellular death due to ageing.