= Solution
In a tensile <atomic force microscopy> measurement, a <DNA> tether connects the <cantilever> tip and a surface. Moving the support stretches the <molecule>, and calibrated <cantilever> deflection supplies the tensile <force>; extension must be corrected for <cantilever> deflection and any linkers. At low <force>, extension mainly straightens an initially fluctuating coil, giving approximately linear <entropic elasticity>. At higher <force> the available conformations become increasingly aligned, so additional extension costs progressively greater <force>. The readable source curve then continues beyond the nominal inextensible-model contour length, which signals a failure of that simple model or of the stated parameter interpretation, rather than negative elasticity.
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