A prerequisite for chromosome segregation in all living organisms is the topological unlinking of sister DNA molecules, called DNA decatenation. Decatenation is performed by DNA topoisomerases that work by transiently breaking strand(s) in one DNA double helix and passing another double helix through the temporarily created gate. How DNA topoisomerases manage to recognize linkages between sister DNA molecules and how they promote decatenation of sister chromatids (but not catenation) is still largely unknown.The driving hypothesis of this project is that condensin promotes chromosome decatenation by guiding the unlinking activity of DNA topoisomerases. Condensin is a member of the family of SMC (Structural Maintenance of Chromosomes) protein complexes that is conserved from bacteria to humans. It forms large, ring-like structures that bind to and organize chromosomes. Efficient separation of sister chromosomes in the bacterium B. subtilis depends on the condensin complex. However, so far the precise role of condensin in chromosome segregation and its mechanisms are unclear.To test our hypothesis, we will establish a minichromosome in bacteria that segregates in a condensin-dependent manner and measure its decatenation in vivo in the presence and absence of condensin. We will investigate the mechanism by which condensin organizes DNA within the (mini-)chromosome using techniques like chromosome conformation capture (3C) and electron microscopy. Finally, we will attempt to reconstitute for the first time the entrapment of DNA double helices by ring-like SMC protein complexes using purified components. Our results will be pivotal for understanding the action of SMC proteins in general with important implications in the separation and segregation of chromosomes in bacteria, the shaping of mitotic chromosomes and the resolution of sister chromatids during mitosis in eukaryotes.