Mitosis is a fundamental process of all living things. Cells divide to produce more cells. This is necessary for organisms that are large, because there is a limit on how big cells can be. Mitosis is also the basis for healing wounds and fighting off invading pathogens that cause sickness. Certain organisms lose millions of cells per day as part of their normal function. Mitosis allows old cells to be replaced by new ones, ensuring an army of fully functioning cells.
Organisms grow larger by producing more cells. Cells give rise to other cells through the process of mitosis. The human organism has trillions of cells, though it started as a single-celled zygote after the sperm and egg fused. Bones would not be able to elongate, hair would not be able to grow out and muscles would not be able to thicken without mitosis. Larger animals do not have larger cells, but more cells of the same size. This is because nutrients cannot efficiently diffuse through a large cell. This is called the surface-area-to-volume ratio dilemma.
Mitosis produces identical cells, which is crucial for replacing damaged cells due to injury. When a piece of skin is cut open, skin cells must divide and then fill the wound. In order to maintain its barrier function, such as keeping out water, the skin needs to fill a cut with more skins cells. The wounded area also needs a new supply of blood vessels that will provide the new skin with nutrients. Blood vessel cells also undergo mitosis in order to create new vessels.
Your immune system has cells called B lymphocytes or T lymphocytes. Each type has a specific function. B lymphocytes make antibodies that trap bacteria. T lymphocytes secrete proteins that destroy infected cells. When activated by an invasion of foreign particles, such as from bacteria, B and T cells undergo many rounds of mitosis to produce more battle-ready copies of themselves. They also undergo mitosis to produce memory cells that are prepared for the same invaders at a later time.
Replacement of Dying Cells
Certain organs constantly lose cells that need to be replaced by new cells. The skin is one such organ. Millions of skin cells die per day and need to be replenished. Skin stem cells undergo mitosis to make new mature skin cells. The inner lining of the large intestine is another example. These cells are replaced every week, so intestinal stem cells are often dividing to make replacements. Hair is third example. Hair grows from cells that divide at its base, which is embedded in the skin. By the time hair cells are part of the hair shaft that is above the scalp, they are dead.