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Section F Mitosis and Meiosis 2013,April Department of Cell Biology School of Basic Medical Sciences Xinjiang Medical University Nafeisha Kadeer
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Cell division increases the number of somatic (body) cells, and consists of: Mitosis (division of nucleus) Cytokinesis (division of cytoplasm)
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The cell cycle
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The Mitotic Stage Following interphase is the M stage, including mitosis and cytokinesis. During mitosis, sister chromatids of each chromosome separate, and become the nuclei of the two daughter cells. The cell cycle ends when cytokinesis, the cleaving of the cytoplasm, is complete.
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Mitosis has four phases: prophase metaphase anaphase telophase Mitosis centromere
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Mitosis
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Late Interphase
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Early Prophase
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Late Prophase
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Metaphase
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Anaphase
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Telophase
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Cytokinesis
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Which stage of Mitosis you can see in this picture?
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Meiosis Meiosis involves the same four phases seen in mitosisprophase metaphase anaphase telophase The occur during both meiosis I and meiosis II. The period of time between meiosis I and meiosis II is called interkinesis. No replication of DNA occurs during interkinesis because the DNA is already duplicated.
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Meiosis
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Meiosis I continued
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Meiosis II (similar to mitosis)
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Prophase I very long and complicated, divided into 5 substages: 1.Leptotene(thin threads)stage 2.Zygotene(yoked threads)stage 3.Pachytene(thick threads)stage 4.Diplotene(double threads)stage 5.Diakinesis
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1.Leptotene(thin threads)stage
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2.Zygotene(yoked threads)stage Synapsis; Bivalent
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3.Pachytene(thick threads)stage Crossing over ; Tetrads
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Synapsis and crossing-over 4 strands
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4.Diplotene(double threads)stage Chiasmata
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4.Diplotene(double threads)stage
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5.Diakinesis
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Metaphase I
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Anaphase I
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Telophase I
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Prophase II
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Metaphase II
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Anaphase II
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Telophase II
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MEIOSIS: Reducing the Chromosome Number Meiosis reduces the chromosome number such that each daughter cell has only one of each kind of chromosome. The process of meiosis ensures that the next generation will have: 1. the diploid number of chromosomes 2. a combination of traits that differs from that of either parent.
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In humans, both sperm cells and the egg cell have 23 chromosomes each. Following fertilization of the egg cell by a single sperm, the zygote has 46 chromosomes, the diploid number found in human somatic cells. The 46 chromosomes represent 23 pairs of homologous chromosomes.
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Comparison of Meiosis with Mitosis 1. Before mitosis and meiosis, DNA replication occurs only once during the interphase prior to cell division. 2. Mitosis requires one division Meiosis requires two divisions. 3. Two diploid daughter cells result from mitosis Four haploid daughter cells result from meiosis.
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4. Daughter cells from mitosis are genetically identical to parental cells Daughter cells from meiosis are not genetically identical to parental cells. 5. Mitosis occurs in all somatic cells for growth and repair. Meiosis occurs only in the reproductive organs for the production of gametes. Comparison of Meiosis with Mitosis
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Meiosis I: Prophase I - pairing of homologous chromosomes Metaphase I – homologous pairs line up at metaphase plate Anaphase I – homologous chromosomes separate Telophase I – daughter cells are haploid Mitosis: Prophase has no such pairing Metaphase – chromosomes align at metaphase plate Anaphase – sister chromatids separate Telophase – diploid cells Comparison of Meiosis I to Mitosis
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Comparison of Meiosis II to Mitosis The events of meiosis II are like those of mitosis except in meiosis II, the nuclei contain the haploid number of chromosomes. At the end of telophase II of meiosis II, there are four haploid daughter cells that are not genetically identical. At the end of mitosis, there are two diploid daughter cells that are identical.
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Summary Cell division increases the number of body cells. Cells goes through a cell cycle. The human life cycle includes both mitosis and meiosis. Each species has a characteristic number of chromosomes.
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Mitosis 1. produces daughter cells that are identical to the parental cell. 2. has four phases designed to maintain the chromosome number. 3. is used for growth and repair of tissues. Meiosis 1. reduces the chromosome number. 2. includes two nuclear divisions. 3. results in non-identical haploid gametes. Summary
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