While bending and twist can be treated as independent degrees of freedom for linear DNAmolecules, the loop closure constraint introduces a coupling between these variables in circularDNA. We performed Monte Carlo simulations of wormlike rods with both bending and twist rigidityin order to study the coupling between the writhe and twist distributions for various DNA lengths.We find that for sufficiently short DNA, the writhe distribution differs from that of a model withbending energy only. We show that the factorization approximation introduced by previousresearchers coincides, within numerical accuracy, with our simulation results, and conclude that the closure constraint is fully accounted for by the White–Fuller relation. Experimental tests of ourresults for short DNA plasmids are proposed.
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