Practice question

Isolation ionic conditions profoundly affect visualization of chromatin hierarchical levels. In low salt buffer like 1 millimolar EDTA or 10 millimolar Tris-HCl, electrostatic repulsion between linker DNA segments prevents folding and H1-mediated compaction weak, leaving extended 11 nanometer beads-on-string fibers easily observed by electron microscopy with clear spacing. Adding NaCl to 100 millimolar or Mg2+ to 2 millimolar plus stoichiometric H1 induces coiling into 30 nanometer dense fiber. High salt beyond 0.6 molar strips histones, detergent solubilizes membranes, heat denatures proteins. Therefore optimal 11 nanometer fiber preservation requires low salt maintaining integrity while preventing higher order coiling.