The complexity of the relationship between kinetochores and the mitotic spindle reflects the requirement for accurate distribution of the genetic material between dividing cells. The most common error in mitosis is the lack of separation between sister chromatids, resulting in one of the two daughter cells receiving both chromosome copies. This error, which occurs approximately once in 100,000 cell divisions, can take place if chromatids fail to attach to the correct spindle pole or if a pair of chromatids attaches to only one pole. In some cases, the two sister chromatids, although correctly assembled in the mitotic spindle apparatus, simply fail to separate at anaphase.