Controls on Cell Division

How is the cell cycle regulated?

When scientists grow cells in the laboratory, most cells will divide until they come into contact with each other. Once they do, they usually stop dividing and growing. What happens if those neighboring cells are suddenly scraped away in the culture dish? The remaining cells will begin dividing again until they once again make contact with other cells. This simple experiment shows that controls on cell growth and division can be turned on and off.

Something similar happens inside the body. Look at Figure 10–14. When an injury such as a cut in the skin or a break in a bone occurs, cells at the edges of the injury are stimulated to divide rapidly. New cells form, starting the process of healing. When the healing process nears completion, the rate of cell division slows, controls on growth are restored, and everything returns to normal.

The Discovery of Cyclins For many years, biologists searched for a signal that might regulate the cell cycle—something that would “tell” cells when it was time to divide, duplicate their chromosomes, or enter another phase of the cell cycle.

In the early 1980s, biologists discovered a protein in cells that were in mitosis. When they injected the protein into a nondividing cell, a mitotic spindle would form. They named this protein cyclin because it seemed to regulate the cell cycle. Investigators have since discovered a family of proteins known as cyclins that regulate the timing of the cell cycle in eukaryotic cells.