Understanding the elasticity of sub-micron DNA molecules is important because many crucial biological structures and processes occur on this length scale. We measured the force-extension relationships of four short ds-DNA molecules, which are 1298 bp, 662 bp, 390 bp, and 247 bp long, with axial optical tweezers. Using a modified worm-like chain (WLC) model for the extended DNA molecule that incorporates excluded-volume entropic effects from the coverslip and microsphere, we obtained effective persistence lengths for each of these molecules.
The fitted values for the persistence length decrease with the contour length of the DNA, which is qualitatively consistent with observations by Seol et al. on longer, micron- and sub-micron sized constructs (Seol, 2007). On the other hand, for long DNA molecules the effective persistence length approaches the commonly quoted value of around 52nm.
The effective persistence length as a function of the contour length of the DNA. The black circles are data points taken in our lab for DNA of only a few hundred basepairs.