Cell Division

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  • Cell Division
    • Mitosis
      • used for:   growth,  repair/ replacement and asexual reproduction
      • Stages
        • 1. Prophase (prepare)
          • - chromatin condense to chromosomes  - nuclear envelope disappears      - spindle apparatus radiates from centrioles and move toward poles
        • 2. Metaphase (middle)
          • - in middle of cell (equator) chromosomes line up            -  the spindle fibres attach to chromosomes - the  chromosomes are now maximally condensed
        • 3. Anaphase (apart)
          • - the centromeres split                - spindle contracts and the chromatids are pulled towards the poles of the cell  -each chromatid is pulled along by its centromere
        • 4. Telophase (two)
          • - the chromatids reach the poles              - become less condensed (back to chromatin)       -two new nuclear envelopes form
        • 5. Cytokinesis
          • - the cleavage furrow is formed (gap between the two new cytoplasm)     - the cytoplasm divides creating the two new cells
      • One sister chromatid from parent cell passes into each daughter cell. So no change in chromosome number
    • division of one cell into two daughter cells
    • Meiosis
      • Cells that divide by meiosis include:          -sperm cells    -egg cells       -pollen
      • occurs in sexual reproduction. creating four haploid cells from one diploid one.  includes two cycles of division
      • Division cycle 1 stages
        • 1. Prophase 1 (crossing over)
          • same as mitosis but chromosomes arrange into homologous pairs (bivalents) and exchange segments of DNA at the chiasmata (crossing over) this means the gene combo of a chromosome can be changed. no crossing over between sister chromatids.
        • 3. Anaphase 1
          • - the spindles contract and chromosomes are pulled to the poles         - bivalents split but centromere doesn't
        • 4. Telophase 1
          • - the cleavage furrow appears and the cell splits by the end of the stage         - cells produced by division 1 are haploid            - in some organisms the nuclear envelope returns but not in most
        • Division Cycle 2 stages
          • 1. Prophase 2
            • - if formed nuclear envelope breaks             - centrioles replicate and spindles grow outward at right angles from the old ones
          • 2. Metaphase 2
            • chromosomes move to equator            - the spindle fibres attach to the chromosomes= random assortment of sister chromatids
          • 3. Anaphase 2
            • - the centromere splits                - sister chromatids of each chromosomes separate and are pulled to opposite poles
          • 4. Telophase 2
            • - the chromosomes reach opposite poles                - cytokinesis occurs             - the two cells produced by division one divide to form four haploid daughter cells  - nuclear envelopes reappears
      • what causes genetic variation in daughter cells? crossing over and random assortment


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