= Solution
An inextensible <freely jointed chain> of $N$ links of <Kuhn length> $b$ has contour length $L_c=Nb$ and cannot be extended beyond it at any finite <force>. There is an explicit numerical contradiction in the printed parameters:
$$
\boxed{186\times7\,\mathrm{nm}=1302\,\mathrm{nm}=1.302\,\mu\mathrm m,\quad\text{not }26\,\mu\mathrm m.}
$$
The discrepancy is present in the original page, not introduced by digitization. To obtain $26\,\mu\mathrm m$ with $186$ links requires $b\simeq140\,\mathrm{nm}$; to obtain it with $b=7\,\mathrm{nm}$ requires $N\simeq3714$. No single inextensible FJC can satisfy all three stated numbers. The graph's much larger extension is an immediate counterexample to treating the literal $N,b$ as an accurate global fit.
Separately, accepting the graph's nominal $26\,\mu\mathrm m$ contour as an effective low-<force> model, its continuation to $32.7\,\mu\mathrm m$ is about $26\%$ beyond that contour. An inextensible <ideal chain> contains orientational <entropy> only; it excludes stretching of segments, energetic compliance of bonds, changes in helix structure or <base pairing>, and heterogeneous attachments. An <extensible freely jointed chain> or a semiflexible-chain model with appropriate energetic terms can describe some departures. Calibration or contour-length assignment can also be wrong. The graph alone does not identify which mechanism contributes, and its displayed moderate <forces> do not justify assigning a specific high-<force> <DNA> structural transition. Arithmetic inconsistency in $Nb$ is not explained away by these physical mechanisms.
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