Mitosis and Meiosis

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Cell division may happen by either mitosis or meiosis, depending on what type of cell is invovled. Mitosis is a process by which a cell divides to form two daughter cells. They each have the same exact number and kind of chromosomes as the parent cell. Meiosis occurs in the primary sex cells leading to the formation of viable egg and sperm cells. They reduce the number of chromosomes to half in each gamete so that when they are getting furtilized, the species chromosome number is kept even. Mitosis happens in the reproduction of unicellular organisms and in the addition of cells to a tissue or organ in a multicellular organism. There are four stages of mitosis. The first stage is the prophase. In this stage the chromosomes become shorter and thicker and they each consist of two chromatids. The nuclear membrane begins to disintegrate. The centrioles in animal cells, fungi, algae and some other ferns double and a pair move toward each pole and the chromosomes begin to move toward the equator of the cell. The second stage is called the metaphase. In this stage the centrioles move to the poles. The chromosomes line up at the equator of the spindle and the spindle fibers are attached to the centromeres connecting them to the poles of the spindle. The third stage is called the anaphase. By this time, the centromeres split apart. The chromatid pairs of each chromosome separate from each other and they move quickly in opposite directions, one toward each pole. The last stage is called the telophase. The chromosomes that separated apart reach the poles. The pole is the place where the new nucleus of each daughter cell will be located. The spindle fibers extending from the poles to the centromeres disappear and those fibers that lie in the plane between the rows of chromosomes remain for a longer time. A nuclear membrane reforms around each bundle of chromosomes at the poles. At the center of animal cells, the cytoplasm turns inward, pinching the old...
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