Chemical reactions can be isotope-sensitive and alter the isotope composition in the products concerning that in the initial compounds. Besides that, the rate and, probably, the pathway of certain reactions can depend on the nuclear spin of the atoms participating in the reaction. The sensitivity to the atoms' spin implies the sensitivity of the "spin-sensitive" reactions to the magnetic field as it is, regardless if this field is an external one or originated from the spin of some atoms. Here we demonstrate the effect of the external magnetic field on the enzymatic DNA synthesis together with the effect of the magnetic magnesium ions. The rate of DNA synthesis monotonously decreased with the external magnetic field intensity in presence of the non-magnetic magnesium ions. On the contrary, in the presence of the spin-bearing ions, the dependence of the rate on the magnetic field intensity was nonmonotonous and possess a distinct minimum at 800-1000 G. We also suggest a scheme explaining the observed effects.