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Chapter 12 Meiosis

O during meiosis i the homologs in each chromosome

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o During meiosis I, the homologs in each chromosome pair separate from each other. One homolog goes to one daughter cell; the other homolog goes to the other daughter cell. o The end result of meiosis I is that each of the two daughter cells has one of each type of chromosome instead of two, and thus half as many chromosomes as the parent cell had. o The diploid ( 2n ) parent cell produces two haploid ( n ) daughter cells. Each chromosome still consists of two sister chromatids, however- meaning that chromosomes are still replicated. o During meiosis II, sister chromatids from each chromosome separate. One sister chromatid goes to one daughter cell; the other sister chromatid goes to the other daughter cell. o The cell that starts meiosis II has one of each type of chromosome, but each chromosome has been replicated. o The cells produced by meiosis II also have one of each type of chromosome, but now the chromosomes are unreplicated. o Sister chromatids separate during meiosis II, just as they do during mitosis- meiosis II is actually equivalent to mitosis occurring in a haploid cell. Chromosome movements during meiosis I and II are coordinated by kinetochore microtubules that attach at the centromere of each chromosome. Movement is driven by motor proteins located at the kinetochore- the point of attachment. The outcome of meiosis is a reduction in chromosome number. For this reason, meiosis is known as a reduction division. In most plants and animals, the original cell is diploid and the four daughter cells are haploid. These four haploid daughter cells, each containing one of each homologous chromosome, eventually go on to form egg cells or sperm cells via a process called gametogenesis. When two gametes fuse during fertilization, a full complement of chromosomes is restored. The cell that results from fertilization is diploid and is called a zygote. In this way, each diploid individual receives both a haploid chromosome set from its mother and a haploid set from its father.
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The Phases of Meiosis I Meiosis begins after chromosomes have been replicated during S phase. During early prophase I, chromosomes condense, the spindle apparatus forms, and the nuclear envelope begins to disappear. One crucial event that occurs during this phase is that homologous chromosome pairs come together in a process called synapsis. The structure resulting from synapsis is called a tetrad, and consists of two homologous chromosomes with each homolog consisting of two sister chromatids. Chromatids from different homologs in a pair are referred to as non-sister chromatids. During late prophase I, the non-sister chromatids begin to separate at many points along their length. They stay joined at certain locations, and form an x-shaped crossover structure called a chiasma. At least one chiasma forms in every pair of homologous chromosomes; there are often several chiasmata.
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