Physical approaches to the study of DNA

New micromanipulation techniques now enable physicists and biologists to st udy the behavior of single biomolecules such as DMA. In particular, it is p ossible to measure the elastic response of individual DNA molecules to chan ges in the double helix's supercoiling. The force versus extension diagram for torsionally relaxed DNA is continuous and allows one to evaluate the pe rsistence length of the polymer. When the molecule is supercoiled, however, stretching leads to the buildup of torsional stress in the double helix's axis. When the twist energy thus generated increases beyond a critical valu e, the molecule is locally destabilized and changes conformation. This stru ctural transition occurs at stretching forces which can be exerted in vivo by molecular motors and at degrees of supercoiling found in the cell, and m ay have implications for DNA structure and function within the nucleus.