# Are cosmic muons causing mutations or even influence evolutionary rate?

As there are experiments studying influence of cosmic rays on organisms, http://www.ncbi.nlm.nih.gov/pubmed/11541768

I ask my self, if there any influence to DNA from atmospheric muons on the Earth surface as well?

The flux is well known, http://arxiv.org/abs/hep-ph/9803488 but, what is the energy needed to change the DNA? Do cosmics actually have higher probability to change DNA despite of cellular reparation mechanisms? If one takes greater timescales into account, could muons have been one of the causes of biological evolution?

Edit: Broader context is to approximate, if fluctuations of high energy primary cosmic rays (considering galactic flux, not the sun activity) are correlated to biological evolution. Apart from muons, other direct feedback from cosmic rays to living organisms is the $C-14$ production and incorporation.

Specific question is to approximate, how little is the volume in a cell (whether it is DNA it self or RNA mechanisms modifying DNA), how often does it effectively hit by a secondary cosmic, how high is the probability to hit the cell, when it's most susceptible for mutation.

I think, rate of successful genetic mutation can be calculated as $f_c= (\tau_1 + \tau_2)\cdot f_1 \cdot f_2$, where $f_1$ and $\tau_1$ would be the cosmics rate and duration of passage, and $f_2$ and $\tau_2$ the sensitivity "rate" or probability and time window for a DNA change

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@troyner: The rate of biological mutation has no simple formula, it is determined internally, by which mutation rate determines your descendent's best success, and the mutations are coherent modifications of DNA according to a plan encoded by RNA, they are not random. Further, when in non-coding RNA, the notion of "mutation" is difficult to quantitate--- does a single 100 bp insertion count as one mutation or a hundred? It's intelligent design, just like Behe says. Except the intelligent designer is RNA. – Ron Maimon May 17 '12 at 16:45
I agree, that mutation is difficult to quantize, but it is the best approach to start with a first order approximation, isn't it? muons are interesting in academical sense, but the calculation is extensible to other mutation sources. – IljaBek May 18 '12 at 0:06

Of course.

Essentially any ionizing radiation can diddle DNA. Altering the DNA in most bodily cells may kill that cell or cause it to misbehave in various ways (including cancer). Or it may do nothing at all (for instance, if it lands on a unused portion of the strand, or turns off one of several copies of a particular genetic switch).

Any ionizing radiation that reaches the reproductive cells can cause mutations in offspring.

That isn't "evolutionary pressure" (which arises from differential reproductive success) but it is the "random variation" part of the process of evolution. That is to say, it is one of the engines of eveolution, but plays no part in the steering.

If you live in an industrialized nation and get the usual complement of medical and dental screens then the cosmic ray background makes up roughly (very roughly) half or your on-going ionizing radiation dose, so you would expect cosmic rays to be responsible for roughly half of the radiation induced mutations (there are other mechanisms so this does not add up to half of all mutations). The next biggest contributor is naturally occurring radio-isotopes ( Potassium-40 and various Uranium and Thorium daughters).

Go to the question of "more or less probable" to do damange is a little trickier, but to first order muons are less likely to change things than other ionizing radiation because they deposit less energy per unit areal-mass-density of track length (the usual unit of energy loss is $\frac{\mathrm{MeV}}{\frac{\mathrm{g}}{\mathrm{cm}^2}}$ where the denominator should be thought or as (mass density) * (track length) or $(\frac{\mathrm{g}}{\mathrm{cm}^3} \mathrm{cm})$ (yeah, particle physicists have stuck with cgs for this because for minimum ionizing particles the answer is 2 in these units)).

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I went for the muons because though, as you correctly said, they deposit less energy, their absorption length is much higher than for betas or alphas, and they are most abundant at sea level. Muon can traverse a human from head to toe, unlike electrons, so they should come cross more DNA molecules - and perhaps a DNA "bit-flip" does not need much energy to occur – IljaBek May 16 '12 at 22:27
Chemical reaction take place at the few to a few dozen eV scale. Any ionizing radiation has processes which deposit that much and more in on place. – dmckee May 16 '12 at 22:48
What do you mean by "unused portion of the strand" ? – user144542 Aug 15 '14 at 18:11
@user144542 Large parts of any given chromosome are not actually used for by genes, or are part of gene but are permanently switched off by the epigenome. Damage to those parts does not express itself (or perhaps does not express itself any time soon). – dmckee Aug 15 '14 at 18:15
Those parts do not code for proteins, but they are functional, they contain instructions to switch genes off and on: medicalnewstoday.com/articles/250006.php and other parts are also translated to non-coding RNAs: rdmag.com/news/2014/07/junk-dna-not-worthless-once-thought – user144542 Aug 15 '14 at 20:00

This is a biology answer, not a physics answer, but you must not look to direct modifications of DNA by cosmic rays as a source of mutations. This is a ridiculous idea, and it is basically unsupported, and I am certain that it is completely wrong (although among biologists, I might be in the minority).

Generally, the cell has error correcting features, and mutation is not simple cleaving of DNA and rejoining with a point error. This type of mutation is rare. The idea that this is the origin of mutation comes from the 1950s, when it was noticed that stressing organisms with ionizing radiation both breaks DNA and leads to consistent increase in the mutation rate.

But there is absolutely no reason to believe the mechanism is direct stress of DNA, the biology is vastly more complicated. One can get germ-line mutation by exposing flowers to heat, by placing them next to a flame (Darwin noticed this and reports it in the Origin). One can also get mutations by various stress mechanisms in different ways that are not affecting the DNA in a cleaving sort of way.

The notion that mutations are primitive undirected random events belongs to the prehistory of biology, before 2001, and predates the recognition that there is a big brain of RNA in the eukaryotic cell, directing everything that is going on. I am not sure how many biologists believe this, but it was proposed by John Mattick in 2001, and as far as I am concerned, it is certainly correct.

The RNA brain is as important to the cell as your brain is important to you. It directs alternative splicing, gene shutting on/off (by sending out micro-RNA's), transcription and translation, and it also just does thinking, by computing with gigabytes of data.

Given this (although it is possibly still a minority view), the only correct mechanism to attribute germ-line mutations to is to active RNA editing of the DNA in germ-line, both in response to complicated RNA messaging, and in response to heavy gigabyte-sized (or, in an egg, terabyte sized) internal RNA computation, and through the direction of crossing-over. The RNA is sensitive to radiation in both getting cleaved and getting bent, but it is also temperature sensitive, and generally as fungible as you would expect of RAM. The DNA is kept stable and is corrected, and is as immutable (except for direct editing) as you would expect of ROM.

The result of the RNA editing is major modifications in non-coding regions, and these control everything we think of as complicated biology, including embryogenesis, neurogenesis, neuron activity, everything sophisticated. The proteins are as complicated as muscle and bone, doing mechanical and chemical things, but no significant computation. The mutations in genes are mostly meaningless random-drift spot-mutations that are neutral as far as selection is concerned (protien coding genes have not been the major determinant of evolution at least since we diverged from something slightly more complex than a worm).

Given this point of view, the background of ionizing radiation is no more significant for mutation generation than the thermal background of random motion--- it is one more source of error that the computational mechanism of RNA evolves to correct. If you have a high radiation environment, organisms will adapt to it by adding more error correction (there are such organisms around), and if you have a low radiation environment, you will still mutate at the same rate, since the mutations are introduced from inside, they are not external to the system.

The above is what I believe, but I haven't been following the literature to see how mainstream this view has become in the last few years. I am sure it is more mainstream now than in 2003, when it was considered lunatic crazy.

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I see your point about the in-depth process of a mutation being more complex, than my image of it. Though assuming the direct findings about radiation influence in orbit and from radioactive contamination - would cosmics not have a less frequent rather than no effect on mutation, whether it is direct influence on DNA or on sensitive translation processes involving RNA? – IljaBek May 17 '12 at 10:20
@troyaner: The problem is that it isn't a simple phenomenon--- if DNA is cleaved, it gets rejoined and repaired. If you make too much damage, the cell kills itself. It's not possible to find a causal link from molecular damage to effect without knowing all the details of the RNA brain, and right now, most biologists don't even believe it exists! This is the pathetic state of biology. This is changing a little, but too slowly. Theorists discovered it, not experimentalists, and as far as I can tell, it's the first real prediction of theoretical biology. There are many more. – Ron Maimon May 17 '12 at 16:52