Elasticity model of a supercoiled DNA molecule

Within a simple elastic theory, we study the elongation versus force characteristics of a supercoiled DNA molecule at thermal equilibrium in the regime of small supercoiling. The partition function is mapped to the path integral representation for a quantum charged particle in the field of a magnetic monopole with unquantized charge. We show that the theory is singular in the continuum limit and must be regularised at an intermediate length scale. We find good agreement with existing experimental data, and point out how to measure the twist rigidity accurately.