Positive superhelical torsion might be introduced by condensin complexes, presumably at the short nucleosome-free regions condensin preferentially binds to. Torsion could then spread along the chromatin fiber by topo IIa-mediated conversion of the handedness of nucleosomal linker DNA crossings until over-coiling results in the formation of plectoneme loops that compact chromosomes. Binding of histone H1 to linker DNA and the nucleosome dyad, close to the histone H3 tail, might limit the propagation of torsion, and hence H1 might need to be removed from chromosomes upon its phosphorylation. Whether phosphorylation of histone H1 indeed loosens the protein’s association with chromatin is, however, not yet clear. The reports that a non-phosphorylatable version of histone H1 exhibits decreased binding to Xenopus chromosomes and that a phospho-mimicking mutation of a single Aurora B kinase target site within the N terminus of the human histone variant H1.4 results in lower H1.4 turnover on mitotic chromosomes argue rather against this hypothesis. Having ruled out changes in chromatin architecture as the sole determinant of mitotic chromosome architecture, we now focus on the mechanisms of action of condensin, topo IIa, cohesin, and other components of the chromosome condensation machinery. In light of the newly available data derived from the Vatalanib web analysis of chromatin topology in mitotic chromosomes by molecular and structural biology, biophysical, and polymer modeling approaches discussed above, we will attempt to synthesize a three-step framework for the formation of mitotic chromosomes. A three-step model for the formation of mitotic chromosomes Step 1: Linear chromatin looping Linear chromatin structures become first visible by light microscopy at the beginning of mitotic prophase. Knockdown of condensin SMC subunits in C. elegans embryos or in cultured chicken cells significantly delays the initial appearance of such thread-like structures, which suggests that condensin complexes must play a role in this process. Depletion of condensin II subunits, but not of condensin I subunits, causes the same effects in cultured human cells. Unusual chromatin structures can also be observed during prophase after deletion of the condensin II kleisin subunit in mouse neuronal stem cells, while chromosome morphology is not notably affected by deletion of the condensin I kleisin subunit. These observations suggest that condensin II is essential for initiating the formation of individualized chromosomes early during prophase. How condensin II is activated in early prophase is still incompletely understood. Activation is most likely initiated by 759 Bioessays 37: 755766, 2015 The Authors. Bioessays published by WILEY Periodicals, Inc. M. Kschonsak and PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19808515 C. H. Haering Prospects & Overviews …. Review essays Box 3 Cohesin complexes Cohesin complexes are similar in shape and subunit composition to condensin complexes. They are thought to hold together replicated sister chromatids by entrapping both sister DNAs within the ring-shaped architecture created by their Structural Maintenance of Chromosomes and kleisin subunits. Alternative models suggest that two or more cohesin rings, each encircling a single DNA helix, interact. Similar to condensin, the kleisin subunit binds to HEAT-repeat subunits called SCC3 or SA1/SA2 and PDS5. Cohesin rings lock around the sister DNAs at the nascent replication fork in a manner that depends on the stable interaction of the N termi