You, I, and almost every human in the world have forty-six chromosomes in each cell of our bodies (the unfortunate few who do not are victims of a number of physical and mental defects, and suffer shortened life spans). A chromosome is a long, twisted strand of DNA, and together these forty-six strands define genetically a human being. All other multi-celled animals and plants have their own particular chromosomes, varying in number from species to species and uniquely defining those species.

During the 1930s and 1940s, Barbara McClintock and Hermann Muller, each of whom later won the Nobel Prize for separate and unrelated work in genetics, noted and studied an important feature of chromosomes. They observed that each one terminates in a stretch of DNA material of variable length called a telomere. The function and structure of the telomeres was unknown at the time, but there was evidence to support the idea that they prevented chromosomes from fusing together, end to end, which would be a disaster from the point of view of cell reproduction.

Today we know that the genetic information in chromosomes is totally contained in the sequence of four particular molecules that occur again and again along the DNA string. These four are adenine (A), cytosine (C), thymine (T), and guanine (G). More than that, we know telomeres are made up of a certain sequence, which in the case of humans and all vertebrates is T-T-A-G-G-G, repeated up to a couple of thousand times. Other organisms have their own repeat patterns; for instance T-T-A-G-G-C for roundworms, and C-C-C-C-A-A for yeast. Of course, this tells us nothing about the functions of a telomere, though it might seem curious and redundant that the same simple sequence should repeat over and over.

Light began to dawn in the 1960s, when Leonard Hayflick found that human cells will only divide and replicate themselves a limited number of times - usually, between thirty-five and fifty. This is known as the "Hayflick limit." About the same time, Francis Crick and others had noted the "end problem" of chromosome reproduction. The problem is, when a chromosome replicates it can't do so all the way to its very end. A small portion is lost.

Put these two facts together and another possible role appears for the telomere. If all that is lost during cell reproduction is a few T-T-A-G-G-G sequences, and these sequences are genetically unimportant, we have the reason why all DNA strands end in telomeres. They are partially dispensable. At the same time, we see why our cells won't make copies of themselves indefinitely often. After a while their replication will have used up all the telomere and will start eating into the portion of the DNA strand that contains the real genetic information.

There was one problem with such an argument. If the telomeres are being consumed at every cell replication, why aren't babies "born old," with worn-down telomeres? There could be only one logical explanation. Something else had to be able to build telomeres back to their full length during the reproductive process. In 1984, Elizabeth Blackburn and Carol Greider discovered the enzyme, now known as telomerase, which does the trick.

And now we come to an intriguing possibility. Suppose that we could find a way to make telomerase work in the cells of a mature animal. Then the Hayflick limit would no longer apply, cells could replicate themselves indefinitely, and perhaps the aging process itself would be avoided. The prospect of longer life, even of hugely longer life, seemed to be on the horizon.

It was not until the 1990s that a problem came along to match the promise of rebuilt telomeres. That's when it was discovered that although telomerase is not present and active in normal cells, it is found in ninety percent of human cancer cells. No one knows how these cancer cells are able to turn on the gene for producing telomerase, but there is an ominous implication: increase telomerase production and lengthen your life; but at the same time run an increased risk of developing cancer.

Of course, the presence of telomerase in cancer cells can be looked upon either as a problem or an opportunity. If we could turn off throughout the body the gene that causes telomerase production, we would end the uncontrolled replication, which permits the growth of a cancer. We would also lose the possibility of life extension through telomerase production; however, maybe telomerase could be re-introduced to the body, once the cancer had been destroyed.

We and all other living organisms on earth are the end products of almost four billion years of evolution. As a result, organisms like us are exquisitely tuned to balance the value of a long life by cell replication, against the risk of developing cancer that could end that life. The amount of telomerase in our bodies, and the length of our telomeres, is just enough to permit that balance.

However, longevity of the individual is not one of Nature's goals; rather, the objective is to live until the age of reproduction is finished, after which survival for most organisms has no value. In some insects, the body of the mother actually serves as a food source for the offspring.

We now have to recognize how "unnatural" people are. We desire our individual lives to extend far beyond the age at which we are able to reproduce. In order for human maximum life span to increase, we will have to change the "natural" balance in our bodies, as that balance is represented by the length of our telomeres and our production of telomerase. We must find a new fine-tuning, one that maximizes not the number of our surviving offspring, but our individual life expectancy.

This is a difficult and a dangerous goal. When we stray away from the optimum produced by billions of years of evolution, we are in unknown territory. There may be possible side effects that we can't even guess at. Will we humans, over the next century, move into that risk-filled region of the unknown?

Absolutely. The dream of long life (I won't say immortality, because accidental death will always be with us) is very old. When they ask for volunteers in telomerase life-extension experiments, I will be first in line - unless you find a way to beat me to it.

"Borderlands of Science" is syndicated by: