The Cause of Aging

The Cause of AgingThe root cause of aging is very straightforward: we age because our cells age.

In 1961, Leonard Hayflick, a researcher at the Wistar Institute in Philadelphia, discovered that there was a limit to the number of times a human cell could divide.1 After about 70 divisions, a cell derived from embryonic tissue enters a stage where its ability to divide slows and eventually stops. This stage is called cellular senescence. Hayflick also observed that the number of times a cell could divide was governed by the age of the cells: cells from a twenty-year-old could divide more times than cells from a fifty-year-old, which in turn would divide more times than cells from a ninety-year-old.

Hayflick discovered that, in essence, there is a clock ticking inside every dividing cell of our body. Our aging process isn’t simply a consequence of accumulated damage: there is a specific property of our cells that limits how long we can live.

The nature of this property was proposed independently in the early 1970s by both Soviet and American scientists.2When a cell divides, the genetic material inside that cell needs to be copied. This process is called DNA replication. These scientists suggested that the limitation on cell division is rooted in the very nature of DNA replication. The enzymes that replicate a strand of DNA are unable to continue replicating all the way to the end, which causes the loss of some DNA.

As an analogy, think of a DNA as a long row of bricks, and of DNA replication as a bricklayer walking backwards on top of a brick wall laying a new layer on top of that row. When the end of the wall is reached, the bricklayer finds himself standing on top of the brick he’s supposed to replicate. Since he can’t put down a brick where his feet are, he steps back and falls off the wall – leaving the very end of the wall bare. As a result, the new copy of the wall is shorter.

Just like this brick wall was copied imperfectly, our DNA is unable to perfectly copy itself; when a strand is replicated, the new strand is shorter than the old strand.

If we lost portions of the information encoded in our DNA every time it replicated, human life would be impossible. Our cells couldn’t even divide enough times to allow us to be born. Fortunately, we are born with long, repetitive sequences of DNA at the end of each of our chromosomes, which later shorten during the normal DNA replication process.

These repetitive sequences are called “telomeres.”

Telomeres, like all DNA, are made up of units called nucleotides, arranged like beads on a string. The nucleotides in human telomeres are arranged in the repeating sequence TTAGGG (two thymine nucleotides, one adenine nucleotide, and three guanine nucleotides). This sequence is repeated hundreds of times in tandem in every telomere.

Each time our cells divide and our chromosomes replicate, our telomeres become shorter. When we are first conceived, the telomeres in our single-cell embryos are approximately 15,000 nucleotides long. Our cells divide rapidly in the womb, and by the time we are born, our telomeres have decreased in length to approximately 10,000 nucleotides. They shorten throughout our lifetime, and when they reach an average of about 5,000 nucleotides, our cells cannot divide any further, and we die of old age.

Leonard Hayflick had discovered that there was a clock ticking in every dividing cell of our body; telomere shortening explains what makes that clock tick.

The time remaining on this “telomere clock” can be measured from our blood cells. When such measurements are taken, a significant correlation is found between a person’s age and the number of “ticks” remaining on the person’s clock.


About The Author

Katherine Baltazar

I am a media reporter writing for the Hair, Beauty and Spa Industry. I've been writing and covering salons, beauty products and hair treatments for the pace 5 years.