![]() ![]() Dynamically unstable microtubules probe the space (“search”), and get stabilized once they contact a kinetochore (“capture”). In somatic cells of higher eukaryotes, kinetochores assemble on each sister chromatid during prophase, and become accessible for the microtubules of the spindle upon nuclear envelope breakdown. ![]() Kinetochore Geometry in Mitosis Back-to-Back Orientation of Sister Kinetochores in Mitosis We will discuss such dynamic aspects of spindle attachment separately-although of course geometry of kinetochores and spindle attachment are interdependent. The cell needs additional mechanisms that ensure correct orientation with respect to the spindle poles. Proper kinetochore “geometry” is not sufficient for correct chromosome attachment, since, for example, transient malorientation of chromosomes on the mitotic spindle is frequent during prometaphase even if sister kinetochores are properly assembled on opposite sides of the chromosome. By morphology/geometry we mean how the kinetochores are arranged on the chromosome, e.g., the back-to-back orientation of sister kinetochores on mitotic chromosomes. We will treat two aspects of kinetochore orientation separately, the geometric/morphological aspect and the dynamic aspect. In this review, we will discuss recent advances in the molecular biology of kinetochore orientation. Aneuploidy is also a hallmark of many cancers, and might contribute to tumorigenesis ( The excess number of chromosomes results from chromosome missegregation during meiosis, and the aneuploidy is passed on from the resulting germ cell to the embryo. Prominent examples are the clinical syndromes caused by excess copies of chromosome 21 (Down syndrome), chromosome 13 (Patau syndrome), 18 (Edward syndrome) or the sex chromosomes (Klinefelter and Turner syndrome). Aneuploidy is deleterious for cells, not only because it can cause essential genetic information to get lost, but also because imbalances in chromosome number severely perturb gene dosage and thereby cellular function. Defects in the kinetochore, in cohesion, or in any of the factors that promote biorientation lead to chromosome missegregation and hence aneuploidy (an abnormal number of chromosomes) in the daughter cells. In anaphase, cohesin is cleaved by the protease separase allowing the sister chromatids to move to opposite poles. One kinetochore is assembled on each of the two sister chromatids of a chromosome, and both sister kinetochores become attached to opposite spindle poles by metaphase. In mitosis, chromosomes condense and the two copies become visible as “sister chromatids”. ![]()
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