Cell Division
Cell Division
Cell division is the process by which one cell divides to make two. It is the mechanism that enables an organism to grow, repair damaged tissue, and replace dead cells. There are two different forms of cell division: mitosis and meiosis. Mitosis (my-TOH-sis) is the division of a cell nucleus (a cell's control center) to produce two identical cells. Meiosis (may-OH-sis) is a form of cell division that produces differing sex cells. Mitosis is used to grow, replace, and repair with exact copies. Meiosis is used to produce an entirely new individual.
Without cell division, an organism could no longer grow, reproduce, or repair itself. Every day, the human body makes billions of new cells. Yet each human being began life as a single cell that was formed by the union of a sperm and an egg. Once that single cell began dividing (first into two, then into four, then eight, and so on), the process continued until a complete individual was formed. In organisms that are still growing, like seedlings or children, cells divide very rapidly, but as an organism grows older, many of its cells lose their ability to divide. Thus when cells are dying faster than they can be replaced, the organism begins to feel and show the effects of the aging process, and it looks and acts older. Older people develop wrinkles because their skin and muscle tone is lost as fewer cells are replaced. Older people also cannot heal themselves as quickly as they did when young. All cells have a basic cycle of life that they go through, according to their specialty. A healthy young person's skin cells complete one cycle every twenty-four hours, but a person's brain cells go through only so many cycles and then stop forever.
PHASES OF MITOSIS
Despite the duration of an individual cell's life cycle, each cell goes through the same process when it divides. Most cells are produced by mitosis. In mitosis, a single cell goes through a process in which it eventually produces an identical cell called a "daughter cell." Each daughter cell then grows and soon becomes capable of dividing and producing yet another daughter cell. Mitosis takes place in four stages—prophase, metaphase, anaphase, and telophase—during which each chromosome copies itself, the nucleus divides in two, and the whole cell splits into two identical daughter cells. Each new cell receives a set of chromosomes identical to those of the original cell.
During the first phase of mitosis, called the prophase, the cell's chromosomes become shorter and thicker and duplicate themselves, appearing as double-stranded structures. These joined copies are called chromatids. The membrane around the nucleus also begins to disintegrate. During the metaphase, each pair of joined chromosomes line up across the center of the cell and attach themselves to tiny tubes called spindle fibers. In anaphase, the third stage of mitosis, the chromatids (joined chromosome pairs) are pulled apart by the spindle fibers and move toward opposite ends of the cell as it begins to divide. Actual division occurs in the telophase, when an envelope surrounds each set of chromatids to form a new nucleus in each. Finally, the cell splits in two as the cyotplasm (the cell's jelly-like fluid) turns inward and pinches together, resulting in the production of two new, identical cells. Despite minor differences, mitosis is basically the same for plant and animal cells.
PHASES OF MEIOSIS
Where mitosis makes two identical cells, meiosis produces differing cells. Meiosis takes place whenever reproductive cells such as sperm, pollen, or egg cells are produced. The goal of meiosis is to reduce by half the number of chromosomes, so that when two different reproductive cells join together to form a new organism, it will have the exact same number of chromosomes as its parent. If this halving of chromosomes did not happen, the new cell produced would have twice the number of chromosomes that it should have. For example, in order to be human, an individual must have forty-six chromosomes. Without meiosis, that number would be ninety-two chromosomes after fertilization. Because of meiosis, fertilization will result in an offspring with the exact same number of chromosomes as the parents, getting one-half from each.
Unlike mitosis, meiosis has only two major stages that result in the creation of four reproductive cells. Besides halving the number of chromosomes, meiosis also performs another major function. It allows genetic material to be "shuffled" since chromosomes cross over each other and swap genes before the cell divides. This is a random exchange of genetic
material that assures that an entirely new individual will be produced after fertilization. Because of this shuffle of genetic instructions, each reproductive cell is given its own unique set of instructions (making sure, for example, that no two egg cells will have the exact same combination of genes). This partly explains why brothers or sisters of the same parents have different characteristics. Eventually, when two of these unique cells are joined sexually (sperm and egg) to form a new individual (which further mixes the genetic instructions), a unique organism is created. The exception to this is, of course, the case of identical twins (two complete individuals with the exact same genetic makeup). Identical twins occur after fertilization when a human embryo spontaneously splits during the first cleavage (division) and forms two separate cells.
[See alsoCell; Chromosome; DNA; Meiosis; Mitosis ]