Kinetochores are large protein structures that form over the specialized centromeric chromatin regions of chromosomes and to which microtubules from the mitotic spindle attach in mitosis and meiosis. While the inner kinetochore proteins are present through interphase of the cell cycle, the outer kinetochore proteins are assembled upon cell division. Each of the replicated sister chromatids has its own kinetochore, facing in opposite directions. The attachment of sister kinetochores to opposite poles of the mitotic spindle ensures segregation of sister chromatids to opposite poles of the cell, and thus proper distribution to daughter cells. Kinetochores also contain proteins that regulate M phase progression and direct chromosome movements during mitosis (or meiosis), including checkpoint kinases and motor proteins. In addition to serving as attachment sites for microtubules, kinetochores contribute to sister chromatid cohesion.
Immunofluorescent staining of kinetochores is difficult to distinguish from nuclear bodies in interphase cells, as both appear as dots in the nucleus. In mitotic cells, however, proteins that localize to kinetochores can give rise to distinct dots on the condensed chromosomes. Early in mitosis, INCENP is localized to chromosome arms, after which it concentrates at kinetochores.
Read more about the proteome of the kinetochore as a substructure of the nucleoplasm.