Mitosis and apoptosis are essential to the successful growth and proper maintenance of a multicellular organism. Organisms grow by increasing the cell size and increasing cell number. During the first parts of the cell cycle, cells tend to increase in size. However, there is a limit to how large a cell can grow because of the surface-area-to-volume ratio, which shrinks as a cell grows. Once a cell becomes too large, its plasma membrane can no longer efficiently support its growing interior volume. A solution to this problem is to divide and make two smaller cells. Mitosis is a process of cell division that results in two cells that are genetic clones of the parent cell. Mitosis is a form of division, where replicated chromosomes are carefully organized and separated in preparation for the division of the rest of the entire cell. This process results in two cells that are genetic clones of the parent cell. This means that one cell duplicates its genetic material and then splits into two cells. This increases the number of cells that are present in the organism. Cell growth can be exponential, meaning that one cell divides into 2, which then divide again to make 4, which then divide again to make 8, then 16, then 32, and so on. Animals start their existence as a fertilized egg cell called a zygote. After going through many rounds of the cell cycle, the adult form comprises trillions of cells.
Plants also perform the cell cycle to increase in size and cell number. The largest of the sequoia trees living in California started its life within a tiny seed and now stands 83 meters tall and 5.3 meters wide. Plants tend to concentrate their growth in meristems. A meristem is a specialized tissue that undergoes repeated, continuous cell division. In some plants, such as basil, growth occurs from the top of the stem and continues upward. This means that if the plant were cut halfway along the stem, it would fail to grow anymore. In contrast, plants such as grasses have their meristems at the bottom, meaning the tops of the plants can be cut off and cell division will continue, adding more new cells to the bottom. This is why a freshly cut lawn grows back a week later.
Mitosis and the cell cycle are also important to the replacement of old cells and the repair of damaged tissues. Human skin cells slough off continuously. If new cells were not formed to replace those that are lost, the outer tissue layers of the body would wear away, and blisters and sores would form. When epidermal and dermal tissue, which are superficial and deep skin layers, respectively, is damaged from an accidental cut, new tissue needs to form to pull the parted sides back together.
Formation of new cells through mitosis is important for growth, but timed destruction of cells is also key to correct development. Apoptosis is the programmed death of a cell, including destruction of its DNA and organelles. Certain body structures, such as mouse paws, initially form through cell division but are shaped into their final structure by carefully orchestrated cell destruction to remove excess cells. Frog tadpoles also lose their tails through programmed cell death as they develop into tailless adults. When a cell has become damaged or reached the end of its life, a series of events is triggered that causes internal components of the cells to be packed away into chambers called vesicles (pieces of cell membrane). This causes the cell to shrink in size and form many lobes (a process called blebbing). These vesicles are then digested by specialized cells called phagocytes, which eat other cells. This process continues until the entire cell has been destroyed. Apoptosis starts with signals that can come from either inside or outside the targeted cell. A cell may be signaled to enter apoptosis if it is no longer needed, if it is damaged or infected, or after it has undergone cell division so many times that its telomeres (protective chromosome caps) have shortened to a critical length. When a cell enters uncontrolled cell death, called necrosis, it usually splits open and spills its contents, which can cause inflammation and problems for nearby cells. The main purpose of apoptosis is to protect surrounding cells that would otherwise be damaged from necrosis of neighboring cells. In adult organisms, there is a balance between the formation of new cells and the destruction of old cells that keeps the number of cells relatively stable.