Spindle Checkpoint
In order to preserve one cell's identity and its proper functioning, it is necessary to maintain constant the appropriate number of chromosomes after each cell division. Only one error generating cells with more or less chromosomes than expected (a situation termed aneuploidy), might lead in best cases to cell death, or alternatively it might generate catastrophic phenotypic results:
- In humans, Down syndrome appears in children carrying in their cells one extra copy of chromosome 21, as a result of a defect in chromosome segregation during meiosis in one of the progenitors. This defect will generate a gamete (spermatozoide or oocyte) with an extra chromosome 21. After fecundation, this gamete will generate an embryo with three copies of chromosome 21.
- In cancer cells, aneuploidy is a frequent event, indicating that these cells present a defect in the machinery involved in chromosome segregation, as well as in the mechanism ensuring that segregation is correctly performed.
The mechanisms verifying that all the requirements to pass to the next phase in the cell cycle have been fulfilled are called checkpoints. All along the cell cycle, there are different checkpoints. The checkpoint ensuring that chromosome segregation is correct is termed spindle assembly checkpoint (SAC), spindle checkpoint or mitotic checkpoint. During mitosis or meiosis, the spindle checkpoint prevents anaphase onset until all chromosomes are properly attached to the spindle. To achieve proper segregation, the two kinetochores on the sister chromatids must be attached to opposite spindle poles (bipolar orientation). Only this pattern of attachment will ensure that each daughter cell receives one copy of the chromosome.
Read more about Spindle Checkpoint: Discovery of The Spindle Assembly Checkpoint (SAC), Metaphase To Anaphase Transition, Spindle Assembly Checkpoint Overview, Spindle Checkpoint Defects and Cancer