DNA synthesis is commonly accepted to occur as a nucleophilic reaction catalyzed by Zn2+, Ca2+ and Mg2+ ions. The substitution of these ions with nonmagnetic nuclei by ions with magnetic nuclei was shown to produce a huge isotope effect: magnetic ions suppress DNA synthesis by 3–5 times with respect to nonmagnetic ones. This observation unambiguously evidences that the DNA synthesis occurs by radical pair mechanism, which is well known in chemistry and implies pair wise generation of radicals by electron transfer between reaction partners. Magnetic field dependence of the DNA synthesis convincingly proves radical pair mechanism, which manifests itself even in the polymerase chain reaction. This mechanism, being on the energy scale by order of magnitude cheaper that the nucleophilic one, is switched on, when at least two ions enter into the catalytic site. It coexists with nucleophilic mechanism almost on a par; their competition is controlled by concentration of ions. Radical pair mechanism is induced by both sorts of ions, magnetic and nonmagnetic; the only difference is that it functions by 3-5 times more efficiently with magnetic ions. Nuclear magnetic ions 25Mg2+, 43Ca2+, and 67Zn2+, decreasing catalytic activity of polymerase by 2-3 times, even more strongly, by 30-50 times, increase mortality of cancer cells. These ions may be considered as the cheap, easily available, safe (no influence on the immune, signaling and other protein systems), powerful, and universal anti-cancer means for selective killing cancer cells of any types.