During the cell cycle, which accomplishes the replication of one cell into two daughter cells, there are several checkpoints which occur regulating this process. If it were not regulated, cells could reproduce in an out of control fashion, producing tumors and cancer. Each checkpoint verifies if the cell will continue to the next step in the cycle. At the checkpoint, DNA is checked for errors and the cell is either stalled at this stage and repairs made, or the cell may be destroyed. The first checkpoint is the G1 checkpoint. It is determined if the cell will continue to the S phase, or if it will be delayed at this stage or enter a resting stage. The G2 checkpoint verifies if the cell is ready to enter the M phase or mitosis. The metaphase checkpoint occurs during mitosis. If the chromatids are aligned in the center of the cell, and it passes this checkpoint, anaphase can occur and the chromatids separate into chromosomes migrating to opposite poles. Once the cell pinches apart, the daughter cells will again enter G1 and the cycle continues.
Normal cell cycle regulation, for most eukaryotic cells, is the process governing when a cell divides or doesn't, if there is something wrong with that process and this in turn leads to mutations or mistakes in the transfer of DNA for that division, it will lead to the death of that cell in order to prevent the development of mutant cells.
The four stages of this cycle include:
2-mitosis (or division of the nucleus)
4-division of the cytoplasm
These four stages have to take place each time the cell divides and any time the resulting daughter cells, from previous division, divide.
If there are problems with cell cycle regulation, those mutant cells can survive and even proliferate, leading to further problematic divisions, leading to things like cancerous or other types of dangerous or improperly functioning cells.
Aberrant DNA in these cells, which is passed on thanks to the lack of a properly functioning cell cycle regulation, can lead to all kinds of problems, not just within each particular cell but also to cells that aren't doing their particular job, aren't growing to the right proportions, basically anything and everything in a human or animal can be affected in a very negative way.