The bending rigidity of mitotic chromosomes

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 ∼10^-22 N · m² for newt and ∼10^-23 N · m² 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 10² to 10³ 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 ≈ YR⁴, 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.