page_title: Spindle checkpoint
section_title: Cell division: duplication of material and distribution to daughter cells
hierarchical_section_title: Spindle checkpoint / Background on sister chromatid duplication, cohesion, and segregation / Cell division: duplication of material and distribution to daughter cells
context_page_description: The spindle checkpoint, also known as the metaphase-to-anaphase transition, the spindle assembly checkpoint, or the mitotic checkpoint, is a cell cycle checkpoint during mitosis or meiosis that prevents the separation of the duplicated chromosomes until each chromosome is properly attached to the spindle. To achieve proper segregation, the two kinetochores on the sister chromatids must be attached to opposite spindle poles. Only this pattern of attachment will ensure that each daughter cell receives one copy of the chromosome. The defining biochemical feature of this checkpoint is the stimulation of the anaphase-promoting complex by M-phase cyclin-CDK complexes, which in turn causes the proteolytic destruction of cyclins and proteins that hold the sister chromatids together.
context_section_description: When cells are ready to divide, because cell size is big enough or because they receive the appropriate stimulus, they activate the mechanism to enter into the cell cycle, and they duplicate most organelles during S (synthesis) phase, including their centrosome. Therefore, when the cell division process will end, each daughter cell will receive a complete set of organelles. At the same time, during S phase all cells must duplicate their DNA very precisely, a process termed DNA replication. Once DNA replication has finished, in eukaryotes the DNA molecule is compacted and condensed, to form the mitotic chromosomes, each one constituted by two sister chromatids, which stay held together by the establishment of cohesion between them; each chromatid is a complete DNA molecule, attached via microtubules to one of the two centrosomes of the dividing cell, located at opposed poles of the cell. The structure formed by the centrosomes and the microtubules is named mitotic spindle, due to its characteristic shape, holding the chromosomes between the two centrosomes. Both sister chromatids stay together until anaphase; at this moment they separate from each other and they travel towards the centrosome to which they are attached. In this way, when the two daughter cells separate at the end of the division process, each one will receive a complete set of chromatids. The mechanism responsible for the correct distribution of sister chromatids during cell division is named chromosome segregation.
  To ensure that chromosome segregation takes place correctly, cells have developed a precise and complex mechanism. In the first place, cells must coordinate centrosome duplication with DNA replication, and a failure in this coordination will generate monopolar or multipolar mitotic spindles, which generally will produce abnormal chromosome segregation, because in this case, chromosome distribution will not take place in a balanced way.
caption: Three types of cell division: binary fission (taking place in prokaryotes), mitosis and meiosis (taking place in eukaryotes).
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