All life begins with a single cell. Just like living organisms go through a life cycle (which we just saw with corals in the previous section), all cells go through a cell cycle.
How do we go from a single cell to a complex, multicellular organism? The answer is cell division. Cell division is when a parent cell divides into two daughter cells. From one cell, the cell divides, creating two cells. Then the two new cells divide creating four cells, and so on and so on. Our bodies are made of trillions of cells and they are constantly dividing!
Cell division is necessary for life. Not only do cells divide as organisms grow and develop, but more cells are created to repair damaged or old and worn out cells. For instance, if you scrape your knee, you bleed, a scab forms, and eventually new skin replaces the scab. This all occurs because new cells are being created. Your skin cells divide to create more skin cells to replace the ones that were damaged.
Cell theory states that cells can only arise from other living cells. The theory also states that all organisms are composed of one or more cells and cells are the most basic unit of life.
Organisms from the Domain Eukarya have complex cells that divide and replicate in a cell cycle. Eukaryotes have two different cell cycles: meiosis and mitosis. Each of these cell cycles plays a vital role in the larger life cycle of an organism.
As we dive into cell cycles, refer to the Appendix if you need to review the parts of a chromosome or cell.