On 5/16/2014 5:17 PM, Russell Standish wrote:
On Fri, May 16, 2014 at 11:02:35AM -0700, meekerdb wrote:
On 5/16/2014 12:55 AM, Bruno Marchal wrote:
It turns out the carbon atoms in the DNA of neural cells is remarkable
long lived, as chronicled via the radiation spike due to atmospheric
nuclear weapons testing in 50s & 60s. I don't have a cite on hand,
but the result is that your neuronal DNA is on average about two years
younger than your own age. For most other cell types, the average age
is around 7 years, or something like that.
That looks like the age of the cell, but all piece of DNA are
changed many times,
Do you mean replaced by a copy as part of cell metabolism (which I
think happens on cell division? Or do you mean each DNA molecule
suffers random changes during the life of a cell - due to radiation,
etc.
so the age of a DNA does not seem to me to be necessarily the age
of the atoms making the structure.
Of course the carbon atoms were produce in a super nova and are likely millions
of years old.
What is being measured is the age of the DNA molecule, not that of the
individual atoms. DNA is made up of carbon atoms, and the ratio of
Carbon 14 to Carbon 12 will reflect the environmental ratio of those
two isotopes at the time the DNA was constructed. In particular,
during the period of atmospheric atomic testing, there was a spike in
carbon 14, and this is visible in the DNA extracted from neurons of
organisms born during that period, indicating that the DNA atoms
haven't been replaced during the organism's lifetime.
Yeah, I understood that. I wondered about you statement that on average a DNA molecule in
your neurons was only two years younger than your age and a DNA in other parts of your
body was only seven years younger. I can understand the first as saying essentially all
your neurons are in place by age 4. But to say that say almost all your body cells were
in place by age 14 and haven't been replaced seems like a stretch. Blood and skin cells
certainly have much higher turn over rate than that.
brain is the place where the metabolic activity is the highest, so
I am not sure our neurons are so stable at the constitutive level.
The common theory is that long term memories are encoded by growth
and change in neuronal axons and synaptic connections, which would
take metabolic activity. But it wouldn't require changes in neuron
cell DNA.
Yes - of more interest is whether or not the cytoskeleton's material
structure is preserved, particularly in the axons, as this is
presumably where memories are stored.
I mainly offer this research tidbit as a counter to those who claim
that every atom in our bodies has been replaced over the course of our
lifetimes. Well, it just aint so. The situation is far more
subtle. It's an open question as to whether the atoms that are
preserved during a lifetime are important for an individual's
identity, to be sure, but you can't just dismiss the question by
saying that all atoms are recycled, hence identity cannot be tied to
matter.
Exactly.
Brent
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