The Bending Rigidity of Mitotic Chromosomes

Abstract

The bending rigidities of mitotic chromosomes isolated from cultured N. viridescens (newt) and Xenopus epithelial cells were measured by observing their spontaneous thermal bending fluctuations. When combined with simultaneous measurement of stretching elasticity, these measurements constrain models for higher order mitotic chromosome structure. We measured bending rigidities of B ϳ10 Ϫ22 N ⅐ m 2 for newt and ϳ10 Ϫ23 N ⅐ m 2 for Xenopus chromosomes extracted from cells. A similar bending rigidity was measured for newt chromosomes in vivo by observing bending fluctuations in metaphase-arrested cells. Following each bending rigidity measurement, a stretching (Young's) modulus of the same chromosome was measured in the range of 10 2 to 10 3 Pa for newt and Xenopus chromosomes. For each chromosome, these values of B and Y are consistent with those expected for a simple elastic rod, B Ϸ YR 4 , where R is the chromosome cross-section radius. Our measurements rule out the possibility that chromosome stretching and bending elasticity are principally due to a stiff central core region and are instead indicative of an internal structure, which is essentially homogeneous in its connectivity across the chromosome cross-section.

Mitotic chromosomes are chromatin networks without a mechanically contiguous protein scaffold

“…A number of results indicate there is no appreciable restructuring of chromatin during or shortly after chromosome extraction into extracellular buffer. First, elasticity of chromosomes in the extracellular medium (17,18) is close to that measured in vivo (19,22,23). Second, we observe no morphological change of chromosomes during extraction from cytoplasm to extracellular buffer (17)(18).…”

“…However, such ''lost'' protein structures do not contribute significantly to the mechanical properties of mitotic chromosomes, because their bending and stretching properties in vivo are similar to those measured in the extracellular medium (23). Repetition of the experiments of this paper in mitotic cell extracts may be able to further address this question.…”

The energy components of stacked chromatin layers explain the morphology, dimensions and mechanical properties of metaphase chromosomes

“…Furthermore, chromosome bending requires a lateral deformation of the chromosome (figure 5d), but again according to the multilayer chromosome model, presumably this local deformation gives rise to the dissociation of nucleosome-nucleosome interactions between layers. As this mechanism is equivalent to the mechanism of elongation, this can explain the experimental results obtained with N. viridescens and X. laevis chromosomes [35] showing that the Young modulus calculated from bending results is equivalent to that found in stretching experiments.…”

The influence of chromosome flexibility on chromosome transport during anaphase A

“…Experimental data bearing on the second issue are somewhat contradictory. Simple in vitro bending and stretching experiments seem to indicate a thin, stiff core at the center of the chromosome (28), but later in vivo experiments are in good agreement with the predictions of a homogenous chromosome (26), like that considered here.…”

“…With regard to the first issue, experimental studies have shown that for chromosomes of both newt lung epithelial cells and Xenopus A6 cells the bending and stretching moduli are roughly uniform along the length of the chromosome and are, in particular, no different in the vicinity of the centromere than elsewhere on the chromosome (26). Of course, the situation in Drosophila early embryos may be different, and studies of chromosome flexibility in Drosophila were unable to rule out this possibility (27).…”