DIRECT LASER TRAPPING OF SINGLE DNA-MOLECULES IN THE GLOBULAR STATE

A sharply focused laser is able to trap small particles at the laser f ocal point due to the difference in refractive index of the particles and that of the surrounding medium, This technique, called laser trapp ing, can be used to manipulate animal or bacterial cells without any c ontact and has been widely applied in biological research. However, it has been difficult to trap biological macromolecules such as DNA mole cules, because these molecules give a low difference in refractive ind ex and cannot overcome Brownian motion, DNA molecules can be transform ed to a condensed globular state. This transformation results in a hig her refractive index of DNA due to its increased density. We demonstra te in this paper that a single DNA molecule can be optically trapped u sing a Nd:YAG laser (1064 nm wavelength) upon transformation from the coiled state to the globular state.