The cell cycle is regulated by regulatory proteins such as **cyclins**. These regulatory proteins ensure that each stage of the cell cycle is performed at the right time, which helps maintain the fidelity of DNA replication and chromosome segregation between two daughter cells.

These regulatory proteins can be categorized into two types. One type of regulatory proteins are *internal regulators*. These regulators ensure that DNA is correctly replicated and that the mitotic spindle is properly connected to each chromosomes as the cell cycle goes on.

The other type of regulatory proteins are called *external regulators*. These regulators adjust the rate at which the cell divides according to the needs of the organism. For instance, if nutrients and energy are scarce, then the regulators slow down the rate of cell division.

When cells do not respond to the signals that regulate their growth, they are called cancer cells. Cancer is a disorder in which the body’s cells could no longer control growth. As a result, these cells replicate uncontrollably and form tumors, which are masses that can damage other tissues that surround it.

If cells are placed far from each other, they begin filling the empty spaces. When cells come in contact with other cells, they usually stop their growth. This indicates that cells have the ability to continue or stop their growth. A similar thing happens when a skin is cut. The cells that are located along the edges of the injury are dividing and producing new cells rapidly. This event indicates that the process of healing is now occurring. Once the skin is completely healed and restored, the cells will stop producing new cells and return to their normal state.

Cancer is considered as the disease of the cell cycle because cancer cells fail to respond to any external regulator and internal regulator that ensure an orderly cell growth. As a result, the loss of growth control breaks down the normal cell cycle.

Normally, a cell in mitosis already contains a protein called cyclin which triggers cell division. If cyclin is injected to a cell that has already entered mitosis, the cyclin would allow the cell mitosis to progress quickly.