Reversible and irreversible unfolding of eukaryote chromosomes by force

We have carried out studies of metaphase chromosomes in primary explant cultures of newt lung epithelia, using in situ microsurgery and force measurement. The chromosomes are elastic over a five-fold range of extension, with a force constant of about one nanonewton (nN). For larger extensions, chromosomes are permanently elongated; if stretched beyond 30 times, their cross-sectional diameter is increased after relaxation. Fluorescence studies indicate that histone proteins are not being removed. We conclude that we are first unfolding and then breaking chromosome-folding proteins by extension.