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How Cells Divide: Mitosis vs. Meiosis
by Rick Groleau

As viewed from a human perspective, nature has done some ingenious engineering to overcome some of the obstacles it has faced. Take the evolution of sex, for instance. To make the move from asexual to sexual reproduction, 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). It met this challenge by devising (again, speaking from a human perspective) a system by which parent cells incorporate genetic information from both of its parents but contain half the amount of DNA. With only half the DNA, when the parent cell combines with another parent cell, the proper amount of DNA is maintained. This solution is called meiosis.

Mitosis describes the process by which the nucleus of a cell divides to create two new nuclei, each containing an identical copy of DNA. (Cytokinesis describes the division of the rest of the cell.) Almost all of the DNA duplication in your body is carried out through mitosis. Meiosis, as described above, is the process by which certain sex cells are created. If you're male, your body uses meiosis to create sperm cells; if you're female, it uses meiosis to create egg cells. Others 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 equal 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.

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Rick Groleau is managing editor of NOVA Online.

This feature originally appeared on NOVA's "18 Ways to Make a Baby" Web site, www.pbs.org/nova/baby/.

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