Weak internucleosomal interactions in chromatin as revealed by optical trapping. (a) The experimental setup uses DNA packaged into chromatin, with a bead coupled to each end of the DNA. One bead is held in place with a micropipette, and the other is held by an optical trap formed by a focused laser beam. The optical trap can be moved to apply a specific force on the DNA, and the length to which the DNA is stretched is measured directly. (b) Fiber length is measured (from the known magnification) at different applied forces (in piconewtons), and the process can be reversed, showing that chromatin can stretch and snap back to its original structure.