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Meiosis is a form of cell division that produces daughter cells with half the number of chromosomes as the parent cell. To achieve this reduction in chromosome number, chromosomes behave in a manner that is different from their behavior during mitosis. This tutorial describes the chromosome structures involved in meiosis and how these structures separate.
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An Overview of Meiosis (p. 28)
Before considering each stage of meiosis, it is useful to see an overview of the process.
Normally, all members of a eukaryotic species have a specific number of chromosomes, called the diploid number (2 n ), in each somatic cell. These chromosomes occur in pairs, with each member of the pair having the same length, centromere location, and number of gene sites. Members of each pair are called homologous chromosomes, and normally one of the homologs is inherited from the mother and one from the father.
In the first steps of meiosis, the homologous chromosomes pair, or synapse.
The synapsis of homologous pairs is a feature of meiosis but not of mitosis. In mitosis, each member of a homologous pair behaves autonomously.
As the chromosomes condense further, the previous doubling of each chromatid at an earlier stage becomes apparent. Thus, one now sees that the bivalent actually consists of four chromatids. The resulting structure is called a tetrad. The duplicated pairs of chromatids are called sister chromatids, whereas chromatids from different homologs of the tetrad are called nonsister chromatids.

Now the first of two divisions in meiosis takes place. The components of a tetrad separate into two dyads. Each dyad is composed of two sister chromatids joined at a common centromere. This division is called reductional division because the number of centromeres is halved for each dyad. Reductional division is part of meiosis I.
The first meiotic division is now followed by a second division. This division is similar to the one in mitosis; each dyad spits into two monads. Each monad is composed of one chromosome from the four chromatids in the original tetrad. Thus, a total of four monads are produced, corresponding to four haploid ( n ) cells. Equational division is part of meiosis II.
The difference between the results of meiosis and mitosis is that mitosis produces cells with the full diploid (2 n ) complement of chromosomes, and meiosis produces cells with only a haploid number ( n ) of chromosomes.
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Meiosis differs from mitosis in that the daughter cells produced contain half the number of chromosomes as the parent cell. This reduction occurs because chromosomes separate twice during meiosis. Homologous chromosomes pair, or synapse, and the resulting tetrad separates into two pairs of sister chromatids during the first meiotic division. This first division is called reductional division because the number of centromeres (each representing one chromosome with two chromatids) is reduced by one-half following this division.
The second meiotic division is similar to mitosis in that the sister chromatids separate; this second division is also called equational division.
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Define haploid and diploid.
Explain what homologous chromosomes are.
Identify the chromosomes that make up a dyad and a tetrad.
Describe the two divisions that occur during meiosis.
Compare and contrast meiosis and mitosis.
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bivalent diploid dyad equational division haploid homologous chromosomes monad nonsister chromatids reductional division sister chromatids synapsis tetrad