The WLC model treats a polymer as a continuously flexible rod with bending persistence length Lp. Thermal fluctuations create an entropic restoring force that resists extension.
WLC force-extension: f·Lp/(kT) = ¼(1−x/L)⁻² − ¼ + x/L. Marko-Siggia (1995). Lp ≈ 50 nm for dsDNA, ≈ 1 nm for ssDNA. Optical tweezers experiments confirm the WLC prediction to within 10%. The left panel shows sampled chain conformations; the right shows the theoretical and simulated force-extension curve.