Cells keep from growing too large by undergoing cell division, also known as mitosis. The mitosis process involves a cell splitting off into two daughter cells that contain the same DNA as the original cell.
Mitosis occurs because the outside part of a cell cannot grow as fast as the inside part of the cell. The cell exterior is necessary for transporting oxygen and food to the cell interior. When a cell grows, the interior develops at a quicker rate than the exterior of the cell. As such, the interior develops to the point where the exterior of the cell can no longer transport food and oxygen at a sustainable rate. To remedy this, the cell undergoes mitosis to make the transport of nutrients more efficient through two smaller cells.
The beginning stage of mitosis is called prophase. In prophase, a cell's chromosomes condense and structures called centrioles begin to move in opposite directions within the cell. The next stage is metaphase, which results in the chromosomes forming a line within the middle of the cell.
After metaphase, the chromosomes are pulled apart by spindle fibers and pulled in opposite directions of the cell. The separated chromosomes are genetically identical and are referred to as chromatids. The final stage is telophase, which involves the formation of a nuclear envelope and the completion of two new cells containing the same DNA.