Cells grown in a petri dish tend to divide until they form a thin layer covering the bottom of the dish. If cells are removed from the middle of the dish, the cells bordering the open space will begin dividing until the have filled the empty space. What does this experiment show?
a. Cell division is a completely random process.
b. Once cells divide, they can never divide again.
c. The controls on cell growth and division can be turned on and off.
d. There is only a limited amount of DNA available to a given group of cells.
Answer: c