If cytokinesis took place before mitosis, the two daughter cells would end up with only half the required genetic material and, unable to function, would die. In eukaryotic cells, cytokinesis normally happens just after or during the last stage of mitosis, known as telophase.
Mitosis is the mechanism by which a cell creates two identical copies of itself. This process occurs in eukaryotes. Animal species use mitosis for growth since the moment of conception, when cells divide rapidly. Adult individuals use mitosis for repair of damaged tissue. The product of mitosis is two sets of chromosomes within a parent cell, which is ready to split in two. Once the two sets of chromosomes on two sides of the parent cell start forming nuclei, cytokinesis divides the cytoplasm in half and two daughter cells are created.
Although cytokinesis normally does not happen before mitosis is over, other things can go wrong. For instance, there might not be enough cytoplasm for two daughter cells. Or a chromosomal mutation might have happened, which would deem the two new cells nonviable or even capable of causing a disease. For these instances, checkpoints exist in the life cycle of a cell. These checkpoints are controlled by particular proteins, as discovered by Ted Weinert of the University of Arizona and Fred Hutchinson of the Cancer Research Center in Washington.