Calcium ions function as a booster of chromosome condensation

Chromosome condensation is essential for the faithful transmission of genetic information to daughter cells during cell division. The depletion of chromosome scaffold proteins does not prevent chromosome condensation despite structural defects. This suggests that other factors contribute to condensation. Here we investigated the contribution of divalent cations, particularly Ca²⁺, to chromosome condensation in vitro and in vivo. Ca²⁺ depletion caused defects in proper mitotic progression, particularly in chromosome condensation after the breakdown of the nuclear envelope. Fluorescence lifetime imaging microscopy-Förster resonance energy transfer and electron microscopy demonstrated that chromosome condensation is influenced by Ca²⁺. Chromosomes had compact globular structures when exposed to Ca²⁺ and expanded fibrous structures without Ca²⁺. Therefore, we have clearly demonstrated a role for Ca²⁺ in the compaction of chromatin fibres.