Mitotic Chromosome Condensation Driven by a Volume Phase Transition

Procedures were devised for the reversible decondensation and recondensation of purified mitotic chromosomes. Computational methods were developed for the quantitative analysis of chromosome morphology in high throughput, enabling the recording of condensation behavior of thousands of individual chromosomes. Established physico-chemical theory for ionic hydrogels was modified for application to chromosomal material and shown to accurately predict the observed condensation behavior. The theory predicts a change of state (a “volume phase transition”) in the course of condensation, and such a transition was shown to occur. These findings, together with classical cytology showing loops of chromatin, lead to the description of mitotic chromosome structure in terms of two simple principles: contraction of length of chromatin fibers by the formation of loops, radiating from a central axis; and condensation of the chromosomal material against the central axis through a volume phase transition. One sentence summary The mitotic chromosome is an axially scaffolded ionic hydrogel, undergoing a volume phase transition to achieve a condensed state.