Figure 5.

A topology model for chromosome compaction mediated by topoisomerase II and condensin. Proper chromosome compaction may result from a balance of self-catenation, imposed by topoisomerase II, within condensin-mediated DNA loops (condensin depicted as red circles and DNA strands in blue). Self-catenation may work as a loop-stabilizer between adjacent loops, thereby ensuring chromosome compaction. Removal of condensin I from mitotic chromosomes prevents the formation of loops, and consequently allows the formation of excessive chromosome entanglements by topoisomerase II (both intra and intermolecularly). Such massive amounts of chromosomal entanglements may lead to an overall increase in chromosome compaction. In turn, impairing topoisomerase II causes loss of chromosome compaction, especially along the longitudinal plane. This may be explained by the absence of self-entanglements promoted by topoisomerase II action, which prevents loop-stabilization and/or distal intramolecular connections.