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The presented paper reviews the latest literature data on SMCs proteins (Structural Mainte- nance of Chromosomes) which contribute to regular chromatid segregation in mitosis and meiosis. SMC proteins are high molecular weight proteins with ATPase activity. These proteins are highly conserved in eukaryotes and prokaryotes. The structure of SMCs proteins is very specific, each SMC subunit contais two globular domains and a helical domain, called arm. Conservative motifs Walker A and Walker B are located at the N-terminal and C-terminal ends of the head domain. ATP binds to Walker A and Walker B of one subunit and the C-motif of the second subunit. C-motif is a part of C-terminal domain. SMCs can be classified into subfamilies, which associate with one another in particular pairs to perform their specific functions. SMCs are crucial components of condensins and cohesins. A single condensin or cohesin particle is composed of SMCs heterodimer and 3 or 2 non-SMC proteins, respectively. Cohesins are four- subunit complexes MCD1/RAD21/SCC1, SMC1, SMC3 i SCC3/SA1/SA2. The role of cohesins is holding sister chromatids together during mitosis and meiosis. They consists of SMC1-SMC3 heterodi- mer and Scc1 subunit which connects the head domains of SMC1 and SMC3, altogether forming a tripartite ring-like structure. Two models for the action of cohesin were proposed. One of them is referred as an embrace model. According to this cohesin complexes embrace two DNA duplexes to hold the sister chromatids together until metaphase. Proteolytic cleavage of Scc1 by separase at the start of anaphase opens the ring and release two sister chromatids. Condensins play key role in chromosomes condensation. They are five-subunit complexes containing SMC2-SMC4 heterodimer. This constitues the core of two types of condensin complexes, condensin I and condensin II. The condensin II associates with chromatin in prophase. Condensin I is cytoplasmic and interact with chromosomes after nuclear envelope break- down. Condensin II is required for chromosome condensation in early prophase, whereas condensin I is required for the complete dissociation of cohesin from chromosome arms, for chromosome compaction and for normal timing of progression through metaphase. Therefore both types are essential for proper chromosome segregation. Except for condensines and cohesines, there are other proteins which are impor- tant in chromosome segregation. Another protein required for proper chromosome segregation is SUMO. The SUMO is small ubiquitin related modifier is a member ubiquitin-like protein family. SUMO is engaged in cyclosome regulation. Cyclosome (APC/C – Anaphase Promoting Complex /Cyclosom), cau- ses degradation of securine, which release separase and affect the conformation of Pds5 protein setting free cohesins. CENP-F is a facultative centromeric protein, which in cooperation with other centromeric proteins and mitotic spindle checkpoint proteins affects chromosome segregation.

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The Editorial Board
Andrzej Łukaszyk - przewodniczący, Zofia Bielańska-Osuchowska, Szczepan Biliński, Mieczysław Chorąży, Aleksander Koj, Włodzimierz Korochoda, Leszek Kuźnicki, Aleksandra Stojałowska, Lech Wojtczak

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