The Cell Cycle: Cell Growth and Division

Cell division is the biological basis for an organism’s growth. In simple unicellular organisms such as yeast (Saccharomyces), this process allows for the creation of a daughter organism. In complex organisms such as humans, cell division enables the development of an individual from the one-celled zygote stage to a fully functioning adult with many trillions of cells.

In order to support an individual’s growth, why do cells divide rather than simply expand in size? Incidentally, as a cell expands in size, so do its demands. For example, at a normal size, a cell is able to keep up with its needs for protein synthesis; however, with increased growth, the cell simply cannot deal with the increasing needs without creating extra copies of DNA.

Sufficient nutrient and gas exchange is another problem that a cell would encounter with unlimited expansion. Food, oxygen, and water enter a cell while waste products such as carbon dioxide exit through the cell membrane. The rate at which this exchange occurs is dependent on the surface area of the cell, or the total area of its cell membrane. However, the cell’s demands for gases and nutrients are dependent on its volume. A small cell contains a high ratio of surface area to volume. As a cell expands in volume, its surface area/volume ratio diminishes, resulting in slowed transfer of gases and nutrients.

In order to avoid these issues with size, a cell grows and eventually divides to form two “daughter” cells, each with its own complete copy of DNA. This process, called the cell cycle, is divided into four distinct stages. There are specific proteins found in each cell that signal the cell to progress (or not to progress) into the next stage. The stages of the cell cycle include, the early gap (G1) phase, synthesis (S) phase, late gap (G2) phase, and mitosis (M) phase. During the G1 phase, the cell grows in size and produces new proteins. G1 is followed by S phase during which new chromosomes are synthesized. After the DNA is duplicated, the cell produces new organelles during the G2 phase. Finally prepared for division, the cell enters M phase in order to undergo mitosis, or the division of the cell nucleus, leading to two daughter cells.