Direct Observation of Histone-Induced DNA Shortening

We construct a system of magnetic tweezers and apply it to study the interaction between histones and DNA. The condensation of DNA by purified histones at low ionic strengths is directly monitored by recording the length of the DNA as a function of elapsed time. It is found that DNA condensates in a dynamic manner. The binding of histones to DNA is energetically favoured, but the tension applied on DNA tends to unravel the DNA-histone complex. The competition between the two processes determines the rate of the DNA condensation.