Double-stranded DNA is not a rigid rod nor a fully random tangle — it behaves like a semi-flexible polymer described by the worm-like chain (WLC) model. The chain's direction "forgets" its original orientation gradually as you move along its contour; the length scale over which that memory decays is the persistence length (about 50nm, or ~150 base pairs, for B-form DNA in physiological salt). Over distances much shorter than the persistence length the molecule looks like a stiff rod; over distances much longer, it looks like a random coil.
F·Lp/kT = 1/4(1−x/L)⁻² − 1/4 + x/L, the standard force–extension relation for the WLC model.Single-molecule stretching experiments on lambda-phage DNA by Bustamante, Smith and colleagues in the 1990s used exactly this force–extension relation to measure DNA's persistence length directly, launching the field of single-molecule biophysics.
A single DNA molecule is anchored at one end and pulled from the other, rendered as a twisted double-helix strand whose bending statistics follow the worm-like chain model of polymer physics.
Persistence length sets how quickly the strand's direction randomizes along its contour; applied tension biases each segment toward the pulling axis, matching the Marko–Siggia force–extension law used to interpret real optical-tweezers experiments.
Drag persistence length from floppy to stiff, pull harder with the tension slider, and change the contour length to see the chain shift between a random coil and a taut rod. Toggle thermal wiggling to freeze a single conformation.
B-form DNA has a persistence length of roughly 50nm — about 150 base pairs — which is why short DNA fragments look like stiff rods while genome-scale DNA folds into a floppy coil.