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 affect 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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