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kottke.org posts about cells

The Hayflick Limit

posted by Tim Carmody   Jun 28, 2019

Cells Dividing.jpg

Biology is one field I don’t know supremely well, having had a couple of college courses and then mostly just public television documentaries. So it’s always cool to get a new concept or two to play with, like the Hayflick Limit.

What is it?

In normal, replicating cells, all the important genetic code in a cell’s nucleus is protected by telomeres—sections of non-coding DNA on the ends of chromosomes. (Elizabeth Blackburn, who won a Nobel for her work on telomeres, compares them to the caps on the end of shoelaces that keep them from fraying.) Every time cells divide, telomeres shorten ever so slightly; the white blood cells in newborn humans have telomeres that consist of about 8,000 base pairs, which falls to around 1,500 in the elderly.

The Hayflick limit is thought to occur when telomeres are gone and cell division would be risky, because without their protection, loss of genetic information would occur. When cells no longer replicate, they’re considered “senescent”: they carry on most of their normal cellular activity and eventually die. The Hayflick limit is one cause, but external stress, like an infection, physical trauma, or UV radiation can hasten cell death, according to Jan van Deursen, a cancer biologist at the Mayo Clinic in Rochester, Minnesota.

Senescent cells are a culprit in aging, but they also have benefits. They give off proteins that can recruit immune cells, which can promote wound healing, and they’re one of our body’s defenses against cancer. One of the reasons cancer occurs is when cells switch on a gene that allows them to rebuild their telomeres—kind of like speeding through a stop sign. That’s why the only immortal human cells are cancer cells.

So the Hayflick Limit, if it could be waived, could theoretically prevent or delay aging. But as it is, it’s a built-in cap on how many replications a body’s cells and tissues can undergo, so it guarantees our mortality.

Here’s what Hayflick himself had to say about it:

To slow, or even arrest, the aging process in humans is fraught with serious problems in the relationships of humans to each other and to all of our institutions. By allowing asocial people, tyrants, dictators, mass murderers, and people who cause wars to have their longevity increased should be undesirable. Yet, that would be one outcome of being able to tamper with the aging process.

I guess at a minimum, as bad as everyone is, at least they (as an individual) are not around to be bad forever? I’ll take that silver lining.