The invasion of nuclear magnetic isotopes of magnesium, calcium, and zinc in enzymatic catalysis discovered new facets of gene chemistry related to magnetism. Numerous experimental observations convincingly demonstrate that the genes are sensitive to the magnetic fields, both permanent and oscillating. The focus of this paper is placed on the molecular mechanism of the magnetic effects as a means for elucidating magneto-chemistry of genes, for understanding and using magnetic effects in medicine. The loading of polymerases with 25Mg2+, 43Ca2+, and 67Zn2+ ions carrying magnetic nuclei instead of 24Mg2+, 40Ca2+, and 64Zn2+ ions with nonmagnetic nuclei disclosed a huge isotope effect: the former ions suppress DNA synthesis by 3-5 times with respect to the latter ions. The effect certifies new, enzymatic radical pair mechanism (ERPM), which includes electron transfer from the growing DNA chain to the catalyzing ion. This mechanism is induced by compression of the catalytic site in the DNA polymerases; the compression removes water molecules from the site and partly dehydrates catalyzing ions, switching on electron transfer and ERPM. The key processes of gene functioning - DNA synthesis, DNA damage, and DNA repair - are shown to be magnetically controlled and mechanism of the control is physically substantiated. A new anti-cancer strategy is suggested based on the using of the nuclear magnetic ions of magnesium, calcium, and zinc as a powerful and universal means to selectively kill only cancer cells; they are supposed to be highly promising for medical applications.