Physical Approaches to the Study of DNA

New micromanipulation techniques now enable physicists and biologists to study the behavior of single biomolecules such as DNA. In particular, it is possible to measure the elastic response of individual DNA molecules to changes in the double helix's supercoiling. The force versus extension diagram for torsionally relaxed DNA is continuous and allows one to evaluate the persistence 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 value, the molecule is locally destabilized and changes conformation. This structural transition occurs at stretching forces which can be exerted in vivo by molecular motors and at degrees of supercoiling found in the cell, and may have implications for DNA structure and function within the nucleus.