= DNA unwinding measured by magnetic tweezers
{c}
{title2=$\Delta Tw=-\Delta Wr$}
At fixed <linking number>, local <DNA> unwinding causes compensating <writhe>. Its mechanical extension signal has opposite signs on positively and negatively supercoiled branches, distinguishing unwinding from simple shortening. The measured quantity is tether extension, converted through a torsional mechanical calibration, not direct optical separation of bases.<Promoter unwinding and promoter clearance by RNA polymerase: Detection by single-molecule DNA nanomanipulation by Andrey Revyakin and others>[Linking-number argument and opposite-sign extension signals]{ref}
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