Electron transfer (ET) in biological molecules, such as peptides and proteins, consists of electrons moving between well-defined localized states (donors to acceptors) through a tunneling process. Here, we present an analytical model for ET by tunneling in DNA in the presence of spin-orbit (SO) interaction to produce a strong spin asymmetry with the intrinsic atomic SO strength in the meV range. We obtain a Hamiltonian consistent with charge transport through π orbitals on the DNA bases and derive the behavior of ET as a function of the injection state momentum, the spin-orbit coupling, and barrier length and strength. Both tunneling energies, deep below the barrier and close to the barrier height, are considered. A highly consistent scenario arises where two concomitant mechanisms for spin selection arises; spin interference and differential spin amplitude decay. High spin filtering can take place at the cost of reduced amplitude transmission assuming realistic values for the SO coupling. The spin filtering scenario is completed by addressing the spin-dependent torque under the barrier with a consistent conserved definition for the spin current.