Mutagenesis and Background Neutron Radiation
MMutagenesis and Background Neutron Radiation
Augusto Gonz´alez
Instituto de Cibern´etica, Matem´atica y F´ısica, La Habana
We suggest a possible correlation between the ionization events caused by the background neutronradiation and the experimental data on mutations with damage in the DNA repair mechanism,coming from the Long Term Evolution Experiment in E. Coli populations.
PACS numbers: 61.80.Hg, 87.53.-j, 87.23.Kg
I. INTRODUCTION
In microelectronics, single failure events sporadicallyoccur which, in some areas, like plane and space naviga-tion, could have catastrophic consequences. Preliminaryestimations [1] and more recent experiments [2] indicatea correlation between these events and the BackgroundNeutron Radiation (BNR) [3]. The mechanism of failureis the collision of a neutron from the BNR with an atomicnucleus in the chip, leading to a shower of electrons andions that locally changes the conductivity and shortcutsthe device.In the present paper, we suggest the BNR as a causeof genetic “fails” in living cells, that is one of the possibleorigins of the so called spontaneous mutations. Cells ex-posed to the shower of electrons and ions, caused by thecollision of a neutron and a proton of water, could be ani-hilated or experience a permanent damage, in particular,a damage in the DNA. The frequency of such events issimilar to the rate of appearance of mutations with dam-age in the DNA repair mechanism [4], as measured inthe Long Term Evolution Experiment (LTEE), where E.Coli populations evolve under controlled conditions [5].
II. THE LTEE IN E. COLI POPULATIONS
The LTEE is an experiment conduced by Prof. R.Lenski and his group at the Michigan State University[5]. Each day, the bacteria undergo 6 - 7 generationsof binary evolution. In a year, around 3400 generationsoccur. This means that, since the experiment started in1988, it passed 60000 generations.In the experiment, 12 populations of bacteria, with acommon ancestor, independently evolve. Every day, 0.1ml of the bacterial culture is serially transferred to 9.9ml of a glucose solution, and mantained under controlledtemperature until the next day. The number of bacteriavaries approximately as shown in Fig. 1. That is, growsaccording to the law N t/t in the first 8h, until theglucose is depleted, and then reach a stationary state.In the last 16h there is no appreciable mortality. Thedependence 2 t/t is due to the way of reproduction, bycellular division.The experiment shows a set of very interesting results[4]. We shall stress only two of them. First, in a givenpopulation, the total number of single point mutations t [hours] N u m b e r o f b ac t e r i a FIG. 1. Daily evolution of the number of bacteria in a culturein the LTEE. in the DNA, after 20000 generations of evolution, is esti-mated as 3 × . That is, the rate of point mutuationsis: f SP M ≈ s − . (1)On the other hand, in 2 of the 12 cultures, after 2500- 3000 generations, mutations with a damaged DNA re-pair and edit mechanism appeared and became numer-ically dominant. A third line evolved the mutator phe-notype after 8500 generations, and a fourth after 15000generations. According to Prof. Lenski, the mechanismthrough which the mutator becomes numerically domi-nant is roughly the following. Once the mutation ap-pears, the rate of spontaneous mutations increases 100times, as compared with cells in which the DNA repairmechanism is not damaged. Thus, the mutator has ahigher chance to generate the next winner and becomedominant in a relatively short time scale, around 250generations.A second aspect, stressed by Prof. Lenski, is that mu-tations in which the DNA repair mechanism is damagedare “deleterious”, in the sense that a segment of the DNAis removed. a r X i v : . [ q - b i o . P E ] J un III. IONIZATION EVENTS CAUSED BY THEBNR
With regard to the BNR, we may assume that the cellslive in pure water. Indeed, water is the main componentof the solution, and the pH should be close to 7 in orderto preserve life [6]. In these conditions, the importantprocesses are the collisions between neutrons, from theBNR, and the Hydrogen nuclei (protons) of water. Theejected proton gives rise to a shower of ions and electronsthat is extended approximately 0.1 mm along the protontrajectory.In Fig. 2, we show the flux per unit energy of neutronsin the BNR [3], F , in units of Neutrons/(MeV s cm );the total cross section for neutron-proton dispersion [7], σ total , in units of 10 − cm ; and the product F σ total , inunits of 10 − s − MeV − , as functions of the energy ofincident neutrons. This last magnitude is proportionalto the probability that a neutron with a given energycollides with a proton in water. It can be noticed thatonly neutrons with energy lower than a few MeV have asignificant effect. -7 -6 -5 -4 -3 F σ t o t a l E [MeV] F σ t o t a l FIG. 2. Neutron flux per unit of energy in the BNR, F , totalcross section of the n+p collision, σ total , and the product F σ total , as functions of the energy of the incident neutron.
From these data, we may estimate the probability ofneutron-proton collisions:
P rob n + p = N p (cid:90) MeV d E F σ total = 8 × − s − , (2)where N p = 6 . × is the number of protons in 10 mlof water.In order to compute the energy transfer from the pro-ton to water, we use the data in Fig. 2 and the so-calledstopping power of protons in water, tabulated in Ref.8. By using a Monte Carlo algorithm, we arrive to theresults shown in Fig. 3. According to Ref. 8, energylosses are mainly due to the interaction of the protonwith the electrons in the medium, leading to the ioniza-tion of water molecules. The basic process of ionizationis: H O → e + H O + , which requires an energy of 12.62eV [9]. The ejected electron and H O + could lead to sec-ondary ionization processes. Dividing the y axis of Fig.3 by 10, we obtain a rough estimate of the number of ionsproduced in each 100 nm step of the proton motion, thatis around 300 ions at distances close to the n-p collisionpoint, and 30 ions when distance is of the order of 0.1mm. d [mm] ∆ E [ e V ] FIG. 3. Energy transfer from the proton to the medium, in100 nm steps, along the proton trajectory.
IV. BNR EFFECTS ON THE LTEE
As mentioned above, n+p dispersion events in the glu-cose solution take place every 125 s. We already knowthat the shower of ions and electrons, created by the pro-ton, is more intense in the first 0.1 mm along the protontrajectory. The bacteria touched by this ion shower couldbe destroyed or experience a permanent damage, espe-cially in their DNA, which can be later inherited by thedescendants. We shall stress that, for the DNA changesto be transmitted, the ionization event should take placein the first 8h of daily evolution, according to Fig. 1.Otherwise, there is practically no cellular division in theday it occurred, and the probability to pass to the nextday is only 1/100.The mean number of bacteria in the first 8h is:¯ N = N (cid:90) d t t/t = 21 . N , (3)where N = 5 × bacteria, and 2 h/t = 100.Each bacterium occupies a mean volume of around 10cm / (21 . N ), that is, a cube with sides 45 µ m long.In the first 0.1 mm=100 µ m of the ion shower, only 2such cubes could be allocated. The probability that theshower touches a bacterium is, thus:2 Shower VolumeCube Volume = 2 l × µm (45 µm ) , (4)where l is the lateral dimension of the ion shower. l could be estimated from the Debye screening length ofpure water: λ D = (cid:18) k B T (cid:15)(cid:15) nq (cid:19) / , (5)where k b denotes the Boltzman constant, (cid:15) ≈
80 is therelative dielectric constant of water [10], q = 1 is thecharge of the ions H + y OH − in water, and n their con-centration: n = 10 − × (3 × molecules / cm )= 3 × ions / cm . (6)Taking all these numbers together, we get λ D =500 nm = 0 . µm . And putting l = λ D in Eq. (4), we geta probability of 2 . × − . Notice that l is a magnitudeof the same order of the E. Coli dimensions, thus the ionshower may cause strong effects on a bacterium.We may compute the rate in which bacteria from a sin-gle population are touched by the BNR ionization events: f BNR ≈ (2 . × − ) × (8 × − s − ) ≈ × − s − . (7)This number is very small, as compared with f SP M , Eq.(1). However, it is consistent with the frequency of dele-terious mutations, with damage in the DNA repair mech-anism, mentioned in section II. Indeed, in ∆ t ∼ ∼ ∼ × s, the BNR had a di-rect incidence on f BNR × ∆ t ∼
60 bacteria. Some ofthem could have experienced damages in the DNA repairmechanism. The 100 times increase in the mutation ratecould have given this subpopulation, after 100 - 600 gen-erations, the possibility to generate beneficial mutationsthat would be fixed, allowing them to become numeri-cally dominant. The fact that only 4 of 12 populations evolved in this way could be related to the probability ∼ / τ ≈ (cid:15)(cid:15) σ , (8)where σ = 5 . × − Coul / (V s m) is the conductivityof pure water [11]. That is, τ ≈ − s.A second estimate for the duration comes from the di-fussion constants of ions in water [12], D ≈ µm /s .Taking λ D = 0 . µm as a characteristic dimension, re-sults in: τ ≈ λ D D ≈ . × − s. (9)In both cases, the times are of the order of 10 − s. Takinginto account that, at ambient temperatures, the typicalspeeds of bacterial motion are around 2 mm/s, only bac-teria in contact with the ion shower, or very close to it,will be affected.The fact that mutations with damage in the DNA re-pair mechanism are deleterious [4] is also consistent withthe nature of BNR ionization processes. Indeed, the elec-tron and ion shower is highly energetic and may producesuch damages in the DNA, especially in the first stepsafter the n+p collision.We shall compare the concentration of produced ionswith the concentration of spontaneous ions in water, Eq.(6). In each of the first 100 nm steps, the ejected protoncreates around 300 ions. The induced concentration is,thus: n ind = 3000 . × . × µ m = 1 . × ions / cm , (10)that is, 4 times higher than n given in Ec. (6). The pres-ence of ions in such high concentrations is also a strongmutagenic factor. V. CONCLUDING REMARKS
In the present paper, we indicate a possible correlationbetween BNR ionization events and the LTEE observedrates of deleterious mutations with damages in the DNArepair and edit mechanism. In this way, we are indicatingthe probable origin of a class of “spontaneous” mutations.The experimental confirmation of this possible correla-tion is plausible: restart the experiment by using fossils,and shield some of the evolving populations against theBNR. The shielded cultures should exhibit much lowerrates for deleterious mutations with damages in the DNArepair mechanism. In around 1 - 2 years (2500 - 5000 gen-erations), changes in mutation rates should be manifest.On the other hand, a comment by Prof. Lenski [4]that some cancer cells also exhibit damages in the DNArepair mechanism, motivates us to rise the hypothesisabout the BNR as one os the processes triggering can-cer. Other events, like inhalation of radioactive Radoncontained in air through breathing, are recognized car-cinogens [13]. Defficient feeding, infectious proceeses, etccould be considered as conditions creating an evolutivepressure over the expossed cells, similar to the limitedamount of glucose in the LTEE. Under these conditions,the BNR induced deleterious mutations, with damages inthe DNA repair and edit mechanism, and the subsequent rise in the rate of spontaneous mutations, could allow themutators to generate well adapted individuals that couldbecome numerically dominant. In order to check thishypothesis, a controlled experiment in animals could bedesigned, for example in mouses, which are widely usedas models of cancer in humans [14].
ACKNOWLEDGMENTS