Background Essay: How Cells Divide: Mitosis vs. Meiosis

From a human perspective, it appears nature has come up with some ingenious ways to overcome the obstacles it has faced. Take the evolution of sex, for instance. Nature took a system by which parent cells reproduced simply by dividing (asexual reproduction), and altered it to allow two parent cells to combine to create offspring (sexual reproduction). For the latter process to work, however, the parent cells had to start with only half the amount of DNA, so that when they combined, the resulting daughter cell would contain one complete set: half from one parent and half from the other. This process is called meiosis. What's the difference between meiosis and mitosis? Mitosis is a stage in the cell cycle, the sequence of events cells undergo as they grow and divide. During mitosis, the nucleus of a cell divides to create two new nuclei, each containing an identical copy of DNA. (Cytokinesis is the next stage during which the cell actually pinches in two.) Almost all of the DNA duplication in your body is carried out through mitosis. Meiosis, in contrast, is the process by which sex cells (sperm and eggs) are created. While the other cells in your body contain 46 chromosomes: 23 from your father and 23 from your mother, your egg (or sperm) cells contain only half that number -- a total of 23 chromosomes. When an egg and sperm unite to make a fertilized egg, the chromosomes add up to 46. How exactly does meiosis mix and halve chromosomes? Find out through this feature, which provides a step-by-step, side-by-side comparison of meiosis and mitosis.