All organisms contain units of life called cells. Cells divide, providing structure for all living things, whether a single-celled bacteria, leaf cells in a plant, or skin cells in a cheetah. When a single-celled organism divides, it is undergoing reproduction. Cell division is the process by which one cell becomes two new cells. These cells may or may not be identical in nature. In multicellular organisms, cell division plays many roles and is the reason complex organisms such as humans are able to grow from a single, fertilized egg cell. Cell division continues throughout the lifetime of an organism, functioning in tissue growth, repair, and maintenance. Cell division is a major component of the cell cycle, the life of a cell, from its beginning to the time when it divides to produce a new cell. The phases of cell division can be superimposed over the phases of the cell cycle.
Most of the lifetime of a cell is spent in growth. The cell gets larger until it is almost twice its original size. When it is time to divide, the cell replicates its genetic material, segregates the copies to opposite ends of the cell, and then pinches down the middle, forming two new cells. Each new cell is identical to the original cell, with a full complement of genetic material. The process of cell division that results in two cells that are genetic clones of the parent cell is called mitosis and occurs in most types of cells. In sexually-reproducing organisms, a different process, called meiosis, creates sex cells. In meiosis, the cells resulting from division contain only half the organism's genetic material. This allows daughter cells to combine with sex cells from another parent, forming a new organism with unique genetic material.