The Counting Problem at the Ends of Your Chromosomes

A small flaw in how cells copy their DNA forces a workaround at the ends of every chromosome — and that workaround may be tangled up with why we age, and with why we do not all die of cancer.

There is a small flaw in the way our cells copy their DNA, and the body’s workaround for it may be tangled up with why we age. The flaw is mechanical. Our chromosomes are linear, like lengths of string, and the molecular machine that copies DNA cannot quite finish the job at the very tip. Each time a cell divides and duplicates its chromosomes, a little bit of the end is left uncopied and lost. Copy the string enough times and you start eating into the words written near the end. If those ends held important genes, this would be a disaster, with vital information trimmed away a little more at every division.

Evolution’s answer is to put a buffer there. The ends of our chromosomes are capped with telomeres, long stretches of a short sequence repeated over and over, in humans the six letters TTAGGG written thousands of times. Telomeres carry no genes. They are deliberately meaningless, sacrificial padding whose whole purpose is to be the part that gets shortened so that the genes further in stay safe. They also tuck the chromosome ends away so the cell does not mistake them for broken DNA and try to repair them by fusing chromosomes together, which would be catastrophic. The telomere is both a buffer and a cap.

But a buffer that shrinks is a clock. Every division trims the telomeres a little more, and they cannot shrink forever. When they get critically short, the cell notices, and it stops dividing, entering a permanent retirement called senescence, or else it self-destructs. This is the molecular basis of a phenomenon discovered before anyone understood it. In 1961 Leonard Hayflick found that normal human cells grown in a dish do not divide indefinitely. They divide a certain number of times, roughly forty to sixty, and then they stop, every time. People had assumed cells were intrinsically immortal and only died from bad culture conditions. Hayflick showed they have a built-in division limit, now called the Hayflick limit, and the shortening telomere turned out to be the counter ticking it down.

So you might think the cure for aging is obvious: keep the telomeres long. And there is, in fact, an enzyme that does exactly that. It is called telomerase, and it carries its own little template of RNA that it uses to rebuild the telomere sequence, extending the ends back out and resetting the clock. Elizabeth Blackburn and Carol Greider identified it in 1985, work that, with Jack Szostak’s, earned a Nobel Prize in 2009. Telomerase is the reason some cells do not run down. Germ cells, the ones that make eggs and sperm, keep their telomerase switched on, which is why the lineage from parent to child does not shorten away to nothing across generations. Certain stem cells run it too, at lower levels.

Here is the catch that keeps this from being a simple anti-aging story, and it is a serious one. Most of the ordinary cells of your body keep telomerase switched off, and that is not an oversight. It is a defense against cancer. A cancer cell is, among other things, a cell that has escaped the division limit and multiplies without end. To do that, it almost always has to solve the same telomere problem, and the overwhelming majority of cancers solve it by switching telomerase back on. The shortening telomere, in other words, is a brake. It limits how many times a rogue cell can divide before it hits the wall and stops, which caps how large a tumor can grow from a single misbehaving cell. Switch telomerase on everywhere to fight aging, and you may be handing every incipient tumor the very tool it needs to become immortal.

This tension is why the relationship between telomeres and aging has resisted the tidy headline. It is true that telomeres shorten with age, that people with certain rare telomere-maintenance diseases show signs of premature aging, and that very short telomeres are associated with cellular senescence. It does not follow that longer telomeres simply mean longer, healthier lives. The body appears to be balancing two risks against each other: the slow decline that comes as cells exhaust their divisions, and the fast catastrophe of a cell that never stops dividing. Telomere length is one of the dials it uses to manage that trade-off, and turning the dial in either direction has costs.

What I find quietly remarkable is that so much of this traces back to a clumsy detail of chemistry, the fact that a particular enzyme cannot copy the last few letters at the end of a strand. The whole apparatus of telomeres and telomerase, the Hayflick limit, a major part of how cells guard against cancer, and perhaps a piece of why bodies wear out, all sit downstream of that one shortfall. Life did not fix the flaw. It built an elaborate accommodation around it, and then learned to read the slowly shortening buffer as a clock and a brake at once.

Telomere length is one dial the body uses to balance two risks — the slow exhaustion of dividing cells against the runaway division of a cancer — and turning it in either direction has a cost.