Fig. 2: Characterization of nonlinear chromosomal elasticity.

a, Stretch curves of chromosome in the presence (+) and absence (−) of polyamines, where the linear stiffness K₀ and the critical force F_c of the decondensed chromosome are indicated. b, Box plots of the linear stiffness (top) and the critical force (bottom) in the absence or presence of polyamines or PEG (linear stiffness: P = 0.19 for polyamines (two-sided t-test for related samples, N = 25 chromosomes), P = 0.03 for PEG (N = 12 chromosomes); critical force: P = 0.03 for polyamines, P = 0.09 for PEG). The box plot marks the quartiles of the distribution, the whiskers show the whole distribution and the centre line marks the median. The differences between the results for − Polyamines and − PEG are presumably due to the use of chromosomes from different preparations. c, The relative change in linear stiffness \({K}_{0}^{{\rm{cond}}}/{K}_{0}^{{\rm{dec}}}\) as a function of the relative change in critical force \({F}_{{\rm{c}}}^{\,{\rm{cond}}}/{F}_{{\rm{c}}}^{\,{\rm{dec}}}\) shows a linear relationship that agrees well with the predictions for a flexible polymer K₀ ∝ F_c, whereas it clearly deviates from the prediction for a semiflexible polymer \({K}_{0}\propto {F}_{{\rm{c}}}^{2}\). The error bars show the s.e.m., N = 12 chromosomes for PEG and N = 25 chromosomes for polyamines. d, Stretch curves of a chromosome before histone depletion and after various times of treatment with 1 M salt, measured in regular buffer containing polyamines. We see a drastic elongation, but the general curve shape and the mechanical stability are maintained. e, The relative change in linear stiffness \({K}_{0}^{{\rm{cond}}}/{K}_{0}^{{\rm{dec}}}\) as a function of \(({F}_{{\rm{c}}}^{{\rm{cond}}}/{F}_{{\rm{c}}}^{{\rm{dec}}})/ (\ell {(120\,{\rm{pN}})}^{{\rm{cond}}}/\ell {(120\,{\rm{pN}})}^{{\rm{dec}}})\) shows a linear scaling. Filled symbols compare chromosomes before and after histone depletion, and open symbols compare chromosomes before histone depletion with chromosomes during histone depletion (while in a 1 M salt buffer) (N = 10 chromosomes). f, Schematic illustrating how the mechanical properties of a mitotic chromosome composed of a flexible chromatin fibre depend on the physico-chemical environment. The chromosomes in a–c have been isolated from U2OS cells, the chromosome in d is from HCT116 cells and the chromosomes in e are from both.