Elasticity measurements show the existence of thin rigid cores inside mitotic chromosomes

Chromosome condensation is one of the most critical steps during cell divis ion. However, the structure of condensed mitotic chromosomes is poorly unde rstood. In this paper we describe a new approach based on elasticity measur ements for studying the structure of in vitro assembled mitotic chromosomes in Xenopus egg extract. The approach is based on a unique combination of m easurements of both longitudinal deformability and bending rigidity of whol e chromosomes. By using specially designed micropipettes, the chromosome fo rce-extension curve was determined. Analysis of the curvature fluctuation s pectrum allowed for the measurement of chromosome bending ridigity. The rel ationship between the values of these two parameters is very specific: the measured chromosome flexibility was found to be 2,000 times lower than the flexibility calculated from the experimentally determined Young modulus. Th is requires the chromosome structure to be formed of one or a few thin rigi d elastic axes surrounded by a soft envelope. The properties of these axes are well-described by models developed for the elasticity of titin-like mol ecules. Additionally, the deformability of in vitro assembled chromosomes w as found to be very similar to that of native somatic chromosomes, thus dem onstrating the existence of an essentially identical structure.