Meiosis is a special type of cell division that takes place only in cells involved in sexual reproduction. In higher organisms like humans, these are immature sperm cells in males and developing eggs in females. All other cells in your body use a different type of cell division, called mitosis, to produce new cells. For example, cells in your skin divide regularly by mitosis to keep a new supply of skin cells available at all times.
Mitosis: Identical New Cells
When a cell begins to divide by mitosis, it makes a second copy of the DNA in its nucleus. Chromosomes contain this DNA, and humans have 46 chromosomes. Once DNA production is done, there are still 46 chromosomes, but each is twice the normal size. Next, chromosomes line up across the cell's center and each chromosome splits in half, with one half moving to each end of the cell. Finally, a new membrane forms across the middle of the cell, making two new cells, each containing 46 new chromosomes. In the absence of spontaneous changes to DNA, called mutations, mitosis produces two new cells that are identical to the parent cell.
Meiosis: Genetic Diversity
In all your cells, one member of each of the 23 pairs of chromosomes came from your father and one from your mother. When a developing egg or sperm cell begins meiosis, it doubles the size of each of its chromosome by making new DNA. Then, unlike mitosis where chromosomes divide, in meiosis one member of each pair of chromosomes moves to each end of the cell, which then divides into two new cells. Called the first meiotic division, the new cells have only 23 chromosomes. They are also genetically different from the parent cell. For example, because the pairs sort randomly, one cell could have a gene for eye color from your father but a gene for hair color from your mother.
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To complete meiosis, the second meiotic division occurs in these new cells, when each chromosome splits in half, much like in mitosis. So after two meiotic divisions, each parent cells produces four new cells, each with 23 chromosomes but half the normal amount of DNA. When fertilization takes place, a male sperm and a female egg fuse, producing an embryo with 46 chromosomes and the full amount of DNA.
Because genes on chromosomes are shuffled like the cards in a deck during the first meiotic division, meiosis produces cells genetically different from the parent cell. A very special process, is occurs in any organism that uses sexual reproduction, including animals, humans and even some plants.