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Introduction: Organisms that reproduce sexually do so using specialized cells called gametes (sperm and egg). These cells fuse during reproduction to form a new organism with a complete complement of chromosomes, half from the mother and half from the father. In order for the offspring to get the correct number of chromosomes, each of the gametes needs to contain only half the total number of the adult organisms. This is achieved through the process of meiosis.
Unlike mitosis (cell division), which increases the number of body cells an organism, which have the same chromosome number as the parent cell, meiosis produces cells with half as many chromosomes.

Meiosis involves a series of chromosome actions and movements and the ultimate splitting of the cell. However, meiosis also involves two cell divisions per event, whereas mitosis only involves one.

The first step of meiosis involves the reduction of the number of chromosomes within the nucleus. Prophase I is also called the reduction division for this purpose. Also, during Prophase I, little pieces of the ends of each chromosome switch places with its counterpart on its homologue. This "crossing over" is done to increase genetic diversity. By mixing up the chromosomes, the offspring will be slightly different than their parents and hopefully more fit for the environment.

This diagram illustrates the movement of the chromosomes and what happens to each cell during meiosis. Note that in sperm cell production, one cell produces four more, while in egg cell production, one cell produces one more with three dead cells called polar bodies.

Directions for this Lesson: Answer the practice questions and then watch the videos to learn more about how the process of meiosis happens.

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