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Meiosis is a process of reduction division in which the number of chromosomes per nucleus is halved and homologous chromosomes are separated. Gamete formation in animals is a typical example of an event in which meiosis plays a key role. In this animation, you can watch a diploid (2N) animal cell go through two rounds of meiotic divisions in order to form four haploid (N) gametes. Note that when meiosis begins, each chromosome has already been replicated and exists as two sister chromatids held together at the centromere.

During the first meiotic division, you will notice that the homologous chromosomes, shown in black and red, align and stick together in a process called synapsis. This allows spindle fibers to attach to the synapsed homologs (tetrads) and to move them as a group to the equator of the cell. As anaphase begins, the homologs can then be oriented such that they are pulled apart to opposite poles of the cell. Following telophase and cytokinesis, two new daughter cells are formed. Note that each of these daughter cells now has half an many chromosomes (N) as the parental cell (2N)! Thus, the first meiotic division is also called the reduction division.

The second meiotic division closely resembles mitosis. During this division each of the nuclei generated during the first meiotic division splits to form two more nuclei. As you can see, the total number of chromosomes per nucleus does not change during this second division. Thus, four haploid gametes are produced.





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