DNA as a Mechanical Object
DNA is not just an information carrier — it is a physical polymer subject to stretching, bending, and twisting forces. In the cell, DNA is constantly pulled by molecular motors, bent around histones, and unwound by helicases. Single-molecule experiments have revealed that DNA's mechanical response follows the worm-like chain model with remarkable precision, connecting polymer physics to molecular biology.
The Worm-Like Chain Model
The WLC model describes a semi-flexible polymer using two parameters: contour length (Lc, the total stretched-out length) and persistence length (Lp, the bending stiffness). At low forces, the molecule resists extension entropically — pulling it straight reduces the number of available conformations. At high forces, the backbone itself begins to stretch elastically. The Marko-Siggia interpolation formula captures both regimes in a single elegant expression.
Force-Extension Experiments
Optical tweezers revolutionized DNA mechanics by enabling piconewton-precision force measurements on single molecules. The resulting force-extension curves show a characteristic nonlinear shape: gradual extension at low forces, rapid stiffening near full extension, and a dramatic overstretching plateau around 65 pN. This simulation reproduces these features, letting you explore how persistence length and temperature affect the mechanical response.
Biological Implications
DNA mechanics matters for gene regulation, chromosome organization, and viral DNA packaging. Nucleosomes bend DNA sharply, requiring ~50 kT of energy per wrap. Bacteriophages pack their genomes to near-crystalline density, generating internal pressures of ~60 atmospheres. Understanding DNA as a mechanical polymer connects physics to the most fundamental processes in biology.