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Animation: Meiosis

by Pearson
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In meiosis, a diploid cell (2n) undergoes two consecutive divisions. The first division, meiosis I, produces two haploid cells (n) with duplicated chromosomes. The second division, meiosis II, produces four haploid cells (n) with unduplicated chromosomes. Note that the haploid cells have only half as many chromosomes as the parent cell. Now, let's look at each phase of meiosis. Like mitosis, meiosis is preceded by an interphase, during which the chromosomes replicate. The centrosome also duplicates in preparation for cell division. Prophase I begins with condensation of the chromosomes. Homologous chromosomes, each made up of two sister chromatids, come together in pairs, aligned gene by gene. The DNA molecules of nonsister chromatids are broken and are rejoined to each other in a process called crossing over. The X-shaped regions where crossovers have occurred are called chiasmata (singular, chiasma). Meanwhile, other cellular components prepare for the division of the nucleus. The centrosomes move away from each other, and spindle microtubules form between them. The nuclear envelope and nucleoli disperse. Finally, spindle microtubules from one pole or the other attach to the kinetochores on the chromosomes. Microtubules move the homologous pairs toward the metaphase plate. At metaphase I, the pairs of homologous chromosomes are aligned on the metaphase plate. For each pair, kinetochore microtubules from one pole of the cell are attached to both chromatids of one homolog, while kinetochore microtubules from the other pole of the cell are attached to the chromatids of the other homolog. Thus, the homologs are poised to move to opposite poles of the cell. During anaphase I, homologs separate due to the breakdown of proteins that are responsible for sister chromatid cohesion along the chromatid arms. The homologs move toward opposite poles, guided by the spindle apparatus. Sister chromatid cohesion persists at the centromere, causing the chromatids to move as a unit toward the same pole. The chromosomes finish their journey during telophase I, and cytokinesis occurs, producing two haploid daughter cells. Note that in meiosis I, the homologous chromosomes have been separated. Each cell has a complete haploid set of duplicated chromosomes. Each chromosome is still composed of two sister chromatids, but one or both chromatids include regions of nonsister chromatid DNA. Meiosis is not over yet; it consists of two consecutive divisions. However, no chromosome duplication occurs between meiosis I and meiosis II. During meiosis II, the sister chromatids will be separated. During prophase II, a spindle forms in each cell. The chromosomes are each still composed of two chromatids associated at the centromere. The chromosomes are moved by microtubules toward the metaphase II plate of each cell. During metaphase II, the chromosomes are positioned at the metaphase plate. Because of crossing over in meiosis I, the two sister chromatids of each chromosome are not genetically identical. The kinetochores of sister chromatids are attached to microtubules extending from opposite poles. In anaphase II, the breakdown of proteins holding the sister chromatids together at the centromere allows the chromatids to separate. The chromatids move toward opposite poles as individual chromosomes. In telophase II, nuclei form at opposite poles of each dividing cell, the chromosomes begin decondensing, and cytokinesis splits the cells apart. Note that during meiosis II, the sister chromatids were separated. Meiosis has produced four daughter cells, each with a haploid set of unduplicated chromosomes. The four daughter cells are genetically distinct from one another and from the parent cell.
In meiosis, a diploid cell (2n) undergoes two consecutive divisions. The first division, meiosis I, produces two haploid cells (n) with duplicated chromosomes. The second division, meiosis II, produces four haploid cells (n) with unduplicated chromosomes. Note that the haploid cells have only half as many chromosomes as the parent cell. Now, let's look at each phase of meiosis. Like mitosis, meiosis is preceded by an interphase, during which the chromosomes replicate. The centrosome also duplicates in preparation for cell division. Prophase I begins with condensation of the chromosomes. Homologous chromosomes, each made up of two sister chromatids, come together in pairs, aligned gene by gene. The DNA molecules of nonsister chromatids are broken and are rejoined to each other in a process called crossing over. The X-shaped regions where crossovers have occurred are called chiasmata (singular, chiasma). Meanwhile, other cellular components prepare for the division of the nucleus. The centrosomes move away from each other, and spindle microtubules form between them. The nuclear envelope and nucleoli disperse. Finally, spindle microtubules from one pole or the other attach to the kinetochores on the chromosomes. Microtubules move the homologous pairs toward the metaphase plate. At metaphase I, the pairs of homologous chromosomes are aligned on the metaphase plate. For each pair, kinetochore microtubules from one pole of the cell are attached to both chromatids of one homolog, while kinetochore microtubules from the other pole of the cell are attached to the chromatids of the other homolog. Thus, the homologs are poised to move to opposite poles of the cell. During anaphase I, homologs separate due to the breakdown of proteins that are responsible for sister chromatid cohesion along the chromatid arms. The homologs move toward opposite poles, guided by the spindle apparatus. Sister chromatid cohesion persists at the centromere, causing the chromatids to move as a unit toward the same pole. The chromosomes finish their journey during telophase I, and cytokinesis occurs, producing two haploid daughter cells. Note that in meiosis I, the homologous chromosomes have been separated. Each cell has a complete haploid set of duplicated chromosomes. Each chromosome is still composed of two sister chromatids, but one or both chromatids include regions of nonsister chromatid DNA. Meiosis is not over yet; it consists of two consecutive divisions. However, no chromosome duplication occurs between meiosis I and meiosis II. During meiosis II, the sister chromatids will be separated. During prophase II, a spindle forms in each cell. The chromosomes are each still composed of two chromatids associated at the centromere. The chromosomes are moved by microtubules toward the metaphase II plate of each cell. During metaphase II, the chromosomes are positioned at the metaphase plate. Because of crossing over in meiosis I, the two sister chromatids of each chromosome are not genetically identical. The kinetochores of sister chromatids are attached to microtubules extending from opposite poles. In anaphase II, the breakdown of proteins holding the sister chromatids together at the centromere allows the chromatids to separate. The chromatids move toward opposite poles as individual chromosomes. In telophase II, nuclei form at opposite poles of each dividing cell, the chromosomes begin decondensing, and cytokinesis splits the cells apart. Note that during meiosis II, the sister chromatids were separated. Meiosis has produced four daughter cells, each with a haploid set of unduplicated chromosomes. The four daughter cells are genetically distinct from one another and from the parent cell.