The real truth is likely to be much more complicated than this paper
discusses due to the difficulties of accounting for the locations of and
relative damage caused by both radioactive species.
The data:
12C generation:n + 14N ® 14/6C + p The neutrons coming from interactions of cosmic rays in the upper atmosphere. [Interesting from the perspective of nearby supernovas since that will "bump" the production of 14C and make things look younger than they would typically be dated as.]
12C breakdown:
14/6C ®b- (an electron w/ 0.15658 MeV) + 14/7N (stable)
Abundance: 1*10-10% = ~10-12
Half life: 5715 or 5730 years (different sources).In so far as the N causes interesting bond rearrangements in the DNA, Spike may be onto something.
40K breakdown:
40/19K ®b- (e- w/ 1.32 MeV) + 40/20Ca (stable)
Natural abundance: 0.0117% = ~10-4
Half life: 1.26*109 years.
Looking at atom count in the DNA bases themselves:
C count is: A (5); C(4); G(5), T(5); [U(4)] -- Average: 4.75Seems interesting that we already have a lot of N, though this could simply be due to the fact that we started from a protein soup.
N count is: A (5); C(3); G(5); T(2); [U(2)] -- Averge: 3.75
In addition you have 1 ribose per base in the backbone with 5 more C's.
Now the biological effect of C®N in a DNA base is going to vary depending on the atom. It is the N's and O's that are involved in cross-strand hydrogen bonding, so this will not impact on that. If you change a C®N in a methyl group (CH3), you probably end up with NH2, which probably isn't going to effect much. If you change a C that has no attached H's, then you might break one of the C-N rings in the bases which would definately be messy.
So looking at a duplex genome size of 6 billion bases it looks like you have (4.75+5)*6*109 = 58.5*109 C atoms. If we take the Freitas paper1 figure of 14C as 1*10-12, then that gives us a count of ~0.06 14C atoms in the genome of each cell. Ideally, this figure needs to be corrected for the human G:C/A:T ratio, because the C count for G:C is 9, while for A:T it is 10.
However, the DNA backbone and bases in your non-dividing cells *does not* turn over very much (damage & repair occur at some moderately low fixed rate. [We could probably look at the Ames estimates for oxidative "hits"/day as a start on this, but that would require a trip to the library since there is some controversy on getting those assays to be accurate.] My guess is that Spike's suggestion will not work because unless the DNA recycling rate is very high, the only way to replace the 12C atoms with 14C atoms will be to wait for most of them to decay (which will be tens of thousands of years). The other alternative is nuclear abortion and replacement with nanobots (as I suggested at Extro4 based on the capabilities outlined in Nanomedicine). Since the nanobots can weigh the bases to atomic accuracy, they can replace the DNA with pure, unadulterated 12C DNA.
When considering the relative danger of radition, it is important to
consider the "relative biological effectiveness" which according to Van
Nostrand's Scientific Encyclopedia is as follows:
| Type | RBE |
|---|---|
| X- & g- rays | 1 |
| b particles (e-) | 1 |
| a particles (He++) |
20
|
| Fast neutrons |
10
|
| Slow neutrons |
5
|
| Cosmic rays (heavy ions) |
???
|
Radiation is measured in roentgens (R) which equals an energy value of 83.8 ergs/g of air or ~93.8 ergs/g in body tissue. The REM values (roentgen equivalent man) are derived from the RBE * R.
So, now how do 14C and 40K stack up in overall radiation dose? Well, 40K releases more energy and is more abundant than 14C, but from Nanomedicine, Chp 3.1, you have 365x as many C atoms as K atoms in your body. So comparing 40K with 14C in the body it looks like:
However, my comments on other radiation sources seem to have some merit. Radon is nasty stuff, various isotopes have half lives of seconds to minutes and mostly decay through b particles. Iodine and Cesium fall seem to follow similar patterns. So even though their abundances are lower, the RBE of a particles and the much more rapid decay rates make them good candidates for causing more of the real damage.
In theory we could take the energy of the beta, alpha, etc. particles, from the various radioactive isotopes in the body and compute the REM values delivered for all of the atoms in the body. But this sounds like about a weekend of work so we will have to wait until one of us gets really ambitious.
On Wed, 20 Oct 1999, Spike Jones wrote:
> > On Tue, 19 Oct 1999, Spike Jones wrote:
> > ...
> > > greenhouse, in which we grow food which is free of carbon 14.
If one
> > > eats only food from this greenhouse, one should be able to reduce
> > > substantially the amount of carbon 14 in ones system. Right?
spike
> > Robert J. Bradbury wrote: I believe you're approach would work.
> > However I think that most of our internal radiation exposure comes
from 40K,
> > not 14C. ... So while it is an interesting idea, I think
we would have to sit down
> > with our caculators and figure out how much this really buys us.
Robert
> I chose controlling 14C because carbon is in the DNA. If
a carbon atom
> transforms into a nitrogen, it wrecks that strand. Seems like the
potassium
> decays would be relatively harmless.
A very good point. The decay of a carbon atom *in* the DNA could have
a
much greater effect than a 40K atom nearby releasing a large
amount of ionizing
radiation (since most of the ionization effect is absorbed by the water
molecules).
However if the 40K abundance or "ionizing potential" can
can produce more
mutations than the average 14C abundance, then it is the
more dangerous element.
> Now that I recall, someone did some calcs on this about a year ago,
> but I do not remember how they figured it would not be a problem.
spike
It depends entirely on the abundance of 14C in the DNA vs.
the abundance
of ionization effects from solvated 40K (or other radioactive
isotopes) compared
with their half lives and ionizing potentials (and distances from "critical"
targets).
This isn't a simple equation.
However, my general impression is that in the "real world" the carcinogenesis
effects are something like:
So I suspect this balances out in terms of the doses and "radioactive efficacy" of the atoms one is typically exposed to. But to really discuss it we need hard numbers.
Robert