Eukaryotic chromosomes undergo dramatic changes and movements during mitosis. These include the individualization and compaction of the two copies of replicated chromosomes (the sister chromatids) and their subsequent segregation to the daughter cells. Two multisubunit protein complexes termed ‘cohesin’ and ‘condensin’, both composed of SMC (Structural Maintenance of Chromosomes) and kleisin subunits, have emerged as crucial players in these processes. Cohesin is required for holding sister chromatids together whereas condensin, together with topoisomerase II, has an important role in organizing individual (...) axes of sister chromatids prior to their segregation during anaphase. SMC and kleisin complexes also regulate the compaction and segregation of bacterial nucleoids. New research suggests that these ancient regulators of chromosome structure might function as topological devices that trap chromosomal DNA between 50 nm long coiled coils. BioEssays 25:1178–1191, 2003. © 2003 Wiley Periodicals, Inc. (shrink)