"This 3 level helix hierarchy is a model for how the 30 nm chromatin fiber is packaged into a condensed chromosome. The rubber band example demonstrates that the structure can form just using internal elastic forces and contact forces that prevent the fiber from passing through itself. The same physical mechanism could turn uncondensed chromatin into a condensed chromosome. Instead of twisting the chromatin, torsional stress could be induced by changing the environment ionic strength or by the action of proteins."