Physics News Update
Number 352, December 22, 1997 by Phillip F. Schewe and Ben Stein
The American Institute of Physics Bulletin of Physics News
ATTACHING A SINGLE DNA MOLECULE TO A SILICON SURFACE can now be done with heat instead of chemicals (which are usually specific to certain surfaces or sites), promising a more general and powerful method for depositing single DNA molecules onto specific locations on silicon chips. Such "DNA chips" are expected to provide a new approach to developing bio-sensors or bio-electronic circuitry. Researchers at Rockefeller University (G.V. Shivashankar, firstname.lastname@example.org, phone 212-327- 8160) first attach a single DNA molecule to a latex bead in water. They then use a focused laser beam known as an "optical tweezer" to trap the bead and hold the DNA molecule in place. Next an atomic force microscope tip comes in contact with the bead. Meanwhile, the laser stays on in order to attach the bead to the probe tip. The composite tweezer-and-AFM tool allows great manipulation flexibility, retains the biological functionality of the DNA, and offers the possibility of studying DNA and protein interactions (Applied Physics Letters, 22 Dec 1997).
THE PHYSICS OF POSTURE. In order for a person to stand up straight a number of sophisticated sensory systems must work together: the vestibular (inner ear), the proprioceptive (sense of touch), and the visual. In order to understand this process better, scientists at Boston University (contact Carson Chow, 617-353-1491, email@example.com until Jan 1; firstname.lastname@example.org thereafter) have put subjects on a special force-sensitive platform which records the minutiae of their swaying motions. Subjecting this digitized information to a statistical analysis, the Boston researchers conclude that the way in which the posture control mechanism strives to maintain an upright posture is the same whether the subject is merely swaying in a random fashion standing at ease or is being perturbed by a slight external push. This issue had been of great interest to those who treat patients with apparent balance problems. (Lauk et al., upcoming article in Physical Review Letters.)
THE SPRING CONSTANT OF A SINGLE POLYMER CHAIN has been measured by researchers at the Niels Bohr Institute in Denmark (Henriette Jensenius, email@example.com). A measure of the stiffness of a spring or spring-like object, the spring constant has previously been determined for long polymers such as DNA molecules, but not for smaller polymers which are tens or hundreds of times shorter in length and much softer. In the present experiment, a micron-size bead in water spontaneously tethers itself to a glass plate by means of a polystyrene chain with a length of about 50 nanometers. By monitoring the distance between the bead and the plate, the researchers then studied the spring-like behavior of the polymer chain as it stretched and compressed in the fluid solution. For polymers cross-linked with some additional scaffolding, the researchers measured a spring constant of 1.5 x 10-3 N/m. For non-cross-linked chains, the researchers measured a value of 2.5 x 10-4 N/m, larger than expected by theory. For comparison, a Slinky has a spring constant of 1 N/m, and an atomic force microscope's cantilever, the plank-like structure whose tip scratches the surface of the sample to be imaged, has a spring constant of 0.1 N/m. (H. Jensenius et al., Phys. Rev. Lett., 22 December 1997.)