The nature of spin communication between DNA nucleotide pairs is discussed. The results are based on CIU (2 x 10(6) configurations, 6-311G** basis set) quantum chemistry computations at a constant temperature T = 310 K of complementary nucleotide pairs, guanosine-cytidine (G-C) and adenosine-thymidine (A-T) monophosphates, assembled into DNA fragments of different length. Calculations reveal alternation of low energy triplet-singlet (T-S) potential energy surfaces (PESs), assigned to individual nucleotides. In a narrow energy interval these PESs approach, showing repulsion and uncommon crossings. Complementary nucleotide pairing, a result of Watson-Crick hydrogen bonding, produces a global minimum in total energy, coming from the unique crossing between two singlet by nature PESs strictly around 310 K. Interaction between non-complementary nucleotides reveals no minima and points rather to system destabilization. Computations show that regularly organized DNA is a structure of similarly oriented spins along each of its two chains, so that the resultant spin of the whole structure is equal to zero. Disordering in spin structure produces coherent effects, appearing in spin flipping, which serves the basis for constructing DNA-based quantum computing. (c) 2007 Elsevier B.V. All rights reserved.