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How do cells promote faithful chromosome segregation in mitosis? While there is only one way for mitosis to go right, there are many ways for it to go wrong. For example, in early mitosis, if there are incorrect contacts between microtubules and chromosomes, chromosomes can become misaligned, which can lead to incorrect segregation of sister chromatids. In late mitosis, how is the cell certain that the time is right to perform cytokinesis? The chromosome passenger complex (CPC) is a molecular guardian angel that acts at many stages of mitosis to safeguard the fidelity of the process. At the start of mitosis, the CPC localises all over the chromosomes and acts to modify chromatin, during mitosis it moves to the chromosome centromeres to prevent incorrect microtubule attachments and before cytokinesis the CPC finds its way to the central spindle. Therefore, a question of ongoing research is how does the CPC elegantly re-localise throughout mitosis to save the day? How are homologous chromosomes held together, and then separated in meiosis I? You might remember from above that it is the protein cohesin that holds together sister chromatids in metaphase of mitosis and metaphase II of meiosis. However, in meiosis I homologous chromosomes must be held together in metaphase I, before these ties are swiftly broken during anaphase I. This feat is performed by a miraculous cellular zipper called the synaptonemal complex (SC). This zipper must be strong enough to hold chromosomes together, but it must also be disassembled equally efficiently, otherwise homologous chromosomes will not accurately segregate in anaphase I, leading to a potentially disastrous genetic inequality in the daughter cells. How exactly this zipper disassembles is a hot topic of research.
