Chromatin condensation refers to the process by which the chromatin in the nucleus changes from a loose filament to a compact chromosome during the pre-dividing period. In the process of mitosis or meiosis, chromatin has a lower compression ratio, so chromatin filaments are looser and spread throughout the nucleus. From the early stage of division, chromatin filaments begin to spiral and condense into visible chromosomes. The process of chromatin turning into chromosomes at this stage is called chromatin condensation.
Early in the mitosis of metazoans in the early stage, visible thread-like structures began to form at the interface of uniformly distributed chromatin. The driving force for this initial phase of chromatin compaction is controversial. Condensates because they consume delayed processes [condensation plays an important role at this stage. However, the details of their activation and their mechanisms remain unclear. The pentameric condensin complex consists of two proteins of the chromosomal structure maintenance (SMC) family and three non-SMC subunits, one kleisin and two HEAT repeat proteins. The SMC protein forms a long anti-parallel coiled coil, flanked by the hinge domain required for the complex formation between the ATPase head domain and the SMC subunit. The head domain interacts with the kleisin subunit, so that both the SMC and the kleisin subunit form a closed loop, while the HEAT-repeat protein mainly associates with its complex with its kleisin subunit through their interactions. This loop may capture two DNA strands on the same chromosome, similar to the way adhesive proteins (structure-related complexes involved in sister chromatid cohesion, DNA repair, and transcriptional regulation) surround sister chromatids. In addition to kleisin-mediated loop formation, the ATPase domains of SMC proteins interact with each other during ATP binding and dissociate during hydrolysis. The purpose of the ATPase cycle is largely unknown; however, interference mutations can eliminate the function of condensin in the body.
Despite important events in the life cycle of eukaryotes, our understanding of the mechanism of mitotic chromatin condensation and deagglomeration is still vague. Chromosome condensation appears to be a multi-layered process in which different mechanisms work at different levels. Condensins are key factors, but their exact role / function remains controversial. H4 tail-mediated nucleosome interactions are regulated by the deacetylation of H4 K16, which is important for chromatin concentration in germinating yeast, and a similar mechanism may also help mitotic chromatin compaction in metazoans. The condensation of chromatin is embedded in the mitotic exit regulatory network. Phosphatases may be required to reduce mitotic phosphorylation on key factors, but the nature of these targets has yet to be determined. AAA-ATPase participates in polycondensation of chromatin: p97 removes Aurora B from chromatin; the complex of RuvB-like ATPases RuvBL1 and RuvBL2 is essential for chromatin deconcentration, probably due to its chromatin remodeling activity.
References:1. Hammond CM.; et al. Linear plasmids and chromosomes in bacteria. Molecular Cell Biology. 2017, 18 (3): 141–158.
2. Hinnebusch J.; et al. "Linear plasmids and chromosomes in bacteria. Molecular Microbiology.1992, 10 (5): 917–22.