Asked by: Simon Brown, Glasgow
Bones do repair themselves to some extent. But they can't regenerate or replace themselves fully for the same reason that we can't grow ourselves a new lung or an extra eye.
Although the DNA to build a complete copy of the entire body is present in every cell with a nucleus, not all of that DNA is active. Most of our cells become specialised during embryonic development so that they only divide to produce cells appropriate to their location in the body.
This is an essential mechanism that keeps us all looking roughly the right shape, despite the fact that our cells are continually dividing and dying off. There seems to be an inverse relationship between how complex an organism is and how well it is able to regenerate after an injury. So newts can regrow a leg but humans can't.
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