Fig. 3: AFM-based microrheology on native human mitotic chromosomes.

a Representative force curve (plotted as a function of time) obtained from a single measurement; the tip approaches the sample and penetrate until a force of ~150 pN is reached. After that, the tip is kept at a constant height for 0.5 s while the chromosome structure relaxes from the initial deformation. Subsequently, the tip is oscillated for 10 periods of time and then retracted from the chromosome. Measurements were repeated multiple times on different locations in order to probe the entire chromosome body. For each chromosome this procedure was repeated using three different oscillatory frequencies, 2, 20 and 200 Hz. The oscillatory measurements allowed calculating the Shear Storage Modulus (G’) and the Shear Loss Modulus, for the three oscillatory frequencies, (b, c) respectively. Each point in (b, c) represents the average G’ and G” obtained from all the measurements performed on a specific chromosome; error bars are the respective SEM. d Values of G’ and G” averaged over all the probed chromosomes and plotted as a function of the oscillation frequency; error bars are the SEM. The dashed gray line indicates a scaling of \({{{\rm{\omega }}}}^{1/2}\), corresponding to the high-frequency regime of the Rouse model. e Loss tangent as a function of the oscillation frequency; the black trace is obtained from the values of G’ and G” shown in (d) while the gray trace reports the values obtained by Meijering et al.¹⁵ using OT. f) Schematics showing that AFM operates perpendicularly to the chromosome long axis, hence mainly probing the mechanical response of the chromatin network. OTs and micropipettes operate parallel to the chromosome long axis, probing the contributions of the chromatin network and the protein-rich central scaffold in parallel. g) Average FRCs for the probed chromosomes, each curve is obtained by averaging the “Relaxation” part of all the curves performed on a single chromosome. The dashed lines describe the double exponential function (Equation 1) used to fit the curves.