Using transverse magnetic tweezers, we studied the dynamics of DNA compaction induced by hexaammine cobalt chloride under constant forces. Discontinuous DNA compaction events were found for forces ranging from 0.5 to 1.7 pN, with approximately 270 nm DNA adsorbed in each compaction event. Forces larger than 6 pN were found able to unravel the toroid in a similar intermittent stepwise manner. The observations indicate that the folding/unfolding events are transitions between two metastable structural states which are separated by a tension-dependent energy barrier. Analysis of the waiting time revealed that the degree of the package ordering of DNA in a toroid depends on the compaction kinetics.
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