Inevitability of spiral-shape in DNA

Cyto-fluid dynamic theory, which clarifies the inevitability of the size ratio of purine and pyrimidine in the base-pairs, is extended to reveal the inevitability of the spiral-shape in DNA, i.e., the double-strand of bases-pairs. First, we will define the cluster consisting of a nitrogenous base and water molecules as continuum flexible column. Then, the differential equation describing deformation motions of the columns having spiral-shapes is derived based on the momentum conservation law. The stability theory, which is based on the concept of the quasistability weaker than the neutral stability, clarifies the inevitability of the spiral shape of B-DNA. It is stressed that the other mode corresponding to A-DNA or C-DNA is also predicted by the present approach.