A pathway for mitotic chromosome formation

Abstract

Mitotic chromosomes fold as compact arrays of chromatin loops.To identify the pathway of mitotic chromosome formation, we combined imaging and Hi-C analysis of synchronous DT40 cell cultures with polymer simulations. Here we show that in prophase, the interphase organization is rapidly lost in a condensin-dependent manner, and arrays of consecutive 60-kilobase (kb) loops are formed.During prometaphase, ~80-kb inner loops are nested within ~400-kb outer loops.The loop array acquires a helical arrangement with consecutive loops emanating from a central "spiral staircase" condensin scaffold.The size of helical turns progressively increases to ~12megabases during prometaphase. Acute depletion of condensin I or II shows that nested loops form by differential action of the two condensins, whereas condensin II is required for helical winding.