Extension of a DNA Molecule by Local Heating with a Laser

Thermal convection and thermophoresis induced by $\mu \mathrm{m}$-scale local heating are shown to elongate a single DNA molecule. An infrared laser used as a point heat source is converged into a dispersion solution of DNA molecules, which is observed under a fluorescent microscope. The thermal convection around the laser focus manifests as extensional flow for the long DNA chain. A simulation of thermal convection that reproduces the experimental condition provides numerical support for the stretching caused by thermal convection. This DNA elongation technique is a novel method for manipulating the intact single DNA molecules, and it can be applied to a “lab on a chip”.

Figure 1

A single DNA molecule under laser irradiation. The intersection points of the white arrows in each panel indicate the laser focus. The free DNAs in the dextran solution just before laser illumination are displayed at the left end with negative time values. The horizontal time axis on the right-hand graph corresponds to the time specified under each picture. The graph shows the time development of the long-axis length for six independent DNA-stretching observations and the average line (black).

Figure 2

Experimental results of DNA elongation for light water and heavy water. The initial long-axis diameters of the DNAs in each solution for time values around zero are plotted in the lower left, and the diameters after laser irradiation are plotted on the right. The broken lines are linear regression fitting lines for each condition.

Figure 3

Results of the thermal convection simulation. The temperature difference of the cell correlates with the color index bar, and current vectors are denoted as arrows in the images. The heating spot is located at $(r,z)=(0,40)\mu \mathrm{m}$.

Figure 4

Distributions of flow velocity and strain/shear rates in the simulation result. The focus of the heating is at $(r,z)=(0,40)\mu \mathrm{m}$, and the plotted profiles consider the $r$ direction elements around the heat spot. (a) The contour lines of flow velocity profile $v_r$. (b) The contour line image denotes the distribution of the extensional strain $\partial v_r/\partial r$ and shear strain $\partial v_r/\partial z$ around the focus.