While many search for the proverbial fountain of youth, you might be wondering
why do we age in the first place? What is it about our bodies or cells
biologically that causes us to grow old? There is a variety of internal and
external factors such as diet, exercise or environmental stress which all
contribute to cell damage and repair and effect the rate of aging, But the
surprising truth is that apart from these, we actually have a biological
clock buried within our genetic makeup. And this clock can only run for so long,
in other words we are programmed to die. Your body is made up of trillions of cells
which are constantly going through cell division and every time they divide they
make a copy of their DNA as well. This DNA is tightly packed into structures
called chromosomes
of which humans have twenty three pairs. The problem is, DNA replication isn't
quite perfect and skips over the end of each chromosome.
To protect against important DNA information being cut out we have
something called telomeres on the end of chromosomes which are essentially
meaningless repeats of DNA that we can afford to lose. But everytime
our cells divide these telomeres become shorter and shorter until eventually
they've been entirely stripped away. At which point the cell no longer divides.
Some flat worms are able to endlessly regenerate their telomeres making them
effectively biologically immortal, but their lifespans do vary and they're
still susceptible to disease further suggesting that aging is a mix of
genetic and environmental factors.
But why don't our cells do this? Ultimately this replication limit
actually helps to prevent cancer which is the uncontrollable growth of cells
and evasion of cell death. The point at which a cell stops replicating is
known as cellular senescence.
In humans this replication limit is around fifty times. Once it is reached the cell
gradually begins to lose its function and die causing age-related
characteristics. This also helps to explain why life expectancy is a
strongly heritable trait from your parents, because you got your initial
telomere length from them.
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