Mitotic chromosome formation and the condensin paradox

During cell division, the chromatin is compacted and resolved into discrete mitotic chromosomes whose proper formation is essential for the faithful distribution of the replicated genome to the daughter cells. Chromatin within mitotic chromosomes is packaged in an orderly and reproducible fashion, but the nature of this higher-order structure has remained elusive, as have the mechanisms of its establishment. Here we provide an overview of how the functional dissection of a non-histone protein complex, condensin, has contributed to our understanding of mitotic chromosomes. Recent studies have revealed that mitotic chromosome formation involves two events: chromatin compaction and establishment of a stable intrinsic higher-order structure. Surprisingly, condensin is only required for the second of these events.