If the cell cycle is not completed properly

Improper separation during anaphase results in a cell that has an abnormal number of chromosomes. Anaphase is part of mitosis, or the process of cell division. Anaphase is the time during which chromosomes that are lined up in the middle of a cell are pulled apart in two directions, resulting in two new cells. Errors during anaphase can result in the usual two cells after mitosis or one big cell because the two cells never split apart. In either case, the number of chromosomes is often wrong. There are different types of chromosomal abnormalities, ranging from one extra copy of one chromosome to multiple extra copies of all chromosomes.