There are several safeguards built into the cell division process to assure that cells do not divide unless they have completed the replication process correctly and that the environmental conditions in which the cells exist are favorable for cell division. Among others, there are systems to determine the following:
Is the DNA fully replicated?
Is the DNA damaged?
Are there enough nutrients to support cell growth?
If these checks fail, normal cells will stop dividing until conditions are corrected. Cancer cells do not obey these rules and will continue to grow and divide.
Now that we have discussed the cell cycle, we will briefly address the ways in which cells are signaled to divide.
Most cells in the body are not actively dividing. They are performing their functions, such as the production of enzymes to digest food or helping to move the arms or legs. Only a small percentage of cells are actually going through the process just described.
Cells divide in response to external signals that 'tell' them to enter the cell cycle. These signals may take the form of estrogen or proteins such as platelet derived growth factor (PDGF). These signaling molecules, depicted as an X-shaped molecule in the animation below, bind to their target cells and send signals into the nucleus. The result is that the genes responsible for cell division are turned on and the cell divides. For example, a cut in the skin leads certain blood cells, platelets, to produce a growth factor (that causes the skin cells to reproduce and fill the wound. Cell division is a normal process that allows the replacement of dead cells.