Diagrammatic representation of mitosis, the mitotic apparatus, and different types of kinetochore attachments. A. During the first stage of mitosis (prophase), the replicated chromosomes, still enclosed by the nuclear envelope, undergo condensation, while the replicated centrosomes move to diametrically opposing positions around the nucleus. Nuclear envelope breakdown marks the beginning of prometaphase, when kinetochores establish attachment with microtubules of the mitotic spindle. At the end of prometaphase, the chromosomes become aligned at the spindle equator forming the metaphase plate, and the cell is said to be in metaphase. The sister chromatids separate from each other and move to opposite poles of the mitotic spindle in anaphase. During the last stage of mitosis, telophase, the nuclear envelope begins to reassemble around the recently segregated chromosomes. Mitotic chromosome segregation is followed by cytokinesis, in which an actin-myosin contractile ring cleaves the cytoplasm to generate two individual daughter cells. B. Kinetochores and chromosomes can establish different types of attachments with microtubules during the early stages of mitosis. Monotelic attachment occurs when one sister kinetochore is attached to microtubules and the other sister is unattached. This is a typical first step in establishment of attachment during prometaphase. When the unattached sister kinetochore binds microtubules from the opposite spindle pole, the chromosome establishes amphitelic attachment. Amphitelic attachment is the only type of attachment that ensures correct chromosome segregation. Due to the stochastic nature of kinetochore-microtubule interactions, chromosomes can occasionally establish erroneous attachments. These include syntelic attachment, in which the two sister kinetochores bind microtubules from the same spindle pole, and merotelic attachment, in which a single kinetochore binds microtubules from both spindle poles instead of just one. Persistence of merotelic attachment into anaphase causes a chromosome segregation defect in the form of a lagging chromosome (see text and Figure 2 for details).