Stretching sub-micron DNA fragments with optical tweezers

Optical tweezers have become an important tool for the manipulation of single biomolecules. However, their application to stretching biopolymers is usually limited to molecules that are several microns in length because conventional optical tweezers manipulate molecules laterally in the focal plane of the microscope objective, a mode in which steric hindrances from the attached microsphere and the surface are substantial. In order to study the properties of short DNA fragments that are typically 1000 bp long, we used optical tweezers in the axial direction to pull microsphere away from the cover glass surface. The microsphere was held in the linear region of the optical potential where the optical force is least sensitive to the bead position. By varying the laser intensity, different stretching forces were applied to the DNA molecule, and the axial position of the tethered microsphere was obtained from its diffraction pattern. The results indicate that the wormlike chain model is still valid for such short DNA fragments.