The physics of DNA stretching

Recent progress in experimental micromanipulation techniques allows single biomolecules, such as DNA, to be mechanically distorted under controlled conditions. As well as providing a wealth of new biochemical information, these experiments are also driving advances in statistical physics and non-equilibrium thermodynamics as theorists extend well established statistical methods to include a description of complex biomolecules at the single molecule level. This article will describe the physical ideas that are necessary to understand the wide range of experiments that stretch individual DNA molecules. These experiments operate in three distinct time regimes relative to thermal noise and therefore require three different branches of physics to understand them. Experiments, which stretch long DNA helices, are well described by equilibrium thermodynamics up until the point at which the molecule is irreversibly distorted, where non-equilibrium ideas apply. Short DNA sequences are only marginally stable at room temperature, therefore their response to an external force is dominated by thermal effects. These experiments are able to probe the kinetics of thermally activated processes in biomolecules in more detail than has been previously possible.