  A dynamical model is presented for chiral change in DNA molecules. The model
is an extension of the conventional elastic model which incorporates the
structure of base pairs and uses a spinor representation for the DNA
configuration together with a gauge principle. Motivated by a recent experiment
reporting chiral transitions between right-handed B-DNA and left-handed Z-DNA
[M. Lee, et. al., Proc. Natl. Acad. Sci. (USA) 107, 4985 (2010)], we analyze
the free energy for the particular case of linear DNA with an externally
applied torque. The model shows that there exists, at low temperature, a rapid
structural change depending on the torque exerted on the DNA, which causes
switching in B and Z domain sizes. This can explain the frequent switches of
DNA extension observed in experiments.
