The function of eukaryotic chromosomes is understood relatively well. Surprisingly little is known, however, about the role of chromosome structure and morphology in their performance as a template for replication and transcription. Even less is known about the input of higher order chromatin structure acquired in G2-M cell-cycle transition in the proper chromosome segregation in mitosis. The proper chromosome transmission in mitosis is facilitated by numerous cellular proteins. The recently discovered family of chromosomal proteins, the SMC family (Structural Maintenance of Chromosomes), represents a key subset of these polypeptides. The SMC proteins are present in most of the cellular organisms: Archaea, Bacteria, lower and higher eukaryotes. While most bacterial species, where sequence information is available, have only one SMC gene, budding yeast Saccharomyces cerevisiae have four SMC genes. SMC1, SMC2 and newly characterised SMC3 and SMC4, all are essential for viability. In mammals, judjing from information derived from human and mouse genome sequencing, there are also four types of SMC genes (SmcA, SmcB, SC2 and SmcD). The SMC function is critical in the process of chromosome condensation during the mitotic prophase and possibly in other chromatin mediated processes, including double strand break repair. SMC proteins are also indispensable for the proper disjoining of sister chromatids during the anaphase.
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