Fig. 1: Optical tweezers experiment to examine in situ ion-mediated chromosome condensation and histone depletion.

a, Schematic of the experiment: the multichannel flow cell enabled us to maintain different ionic conditions and concentrations in the main channels 1–3 (where channel 1 also contains streptavidin-coated microbeads) on the one hand and in side channel 5 on the other hand. Chromosomes were introduced into the flow cell via channel 4. b, Illustration of the two manipulations carried out: reversible ion-mediated chromosome condensation (left) and irreversible depletion of histones (right). The contour length (green) and persistence length (yellow) are indicated in the right panel. c, Fluorescence images of eGFP-tagged H2B histones of a chromosome in the condensed (+ polyamines) and decondensed (− polyamines) states. Scale bar, 2 μm. d, Schematic illustrating the physics of ion-mediated chromosome condensation: the strong negative charge of chromatin needs to be compensated by counterions, leading to an osmotic pressure Π that drives water into the chromosome (top). When these counterions are replaced by polyamines, which act as more effective counterions, the chromosome condenses (bottom). e, Box plot of the relative length change of chromosomes from U2OS and HCT116 cells caused by polyamines, PEG and histone depletion. The box plot marks the quartiles of the distribution, the whiskers show the whole distribution and the centre line marks the median. To obtain a robust estimate of the size change, we compared the length of the chromosome at 50 pN, ℓ(50 pN), before and after changing the solvent conditions. For polyamines, we found a mean length ratio of 121 ± 5% (decondensed/condensed, mean ± standard error of the mean (s.e.m.), N = 25 chromosomes); for PEG, 143 ± 8% (N = 12 chromosomes); for histone depletion, 355 ± 66% (N = 10 chromosomes). f, Normalized length of chromosomes from U2OS cells at a constant force of 250 pN. Without polyamines or PEG, chromosomes frequently elongated stepwise and often disintegrated to the point where they could not support a force of 250 pN (red dots) (N = 7 chromosomes per condition).