Cell Division

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  • Cell Division
    • Mitosis
      • Interphase- during the longest stage of mitosis, the cell's DNA and organelles are copied, so that there is enough content for two cells to be produced.
      • Prophase- in this stage, the chromosomes condense into two short strands called chromatids (chromatids are genetically identical chromosomes joined at the centromere region). The cell's centrioles start to form the spindle at opposite 'poles' of the cell.
      • Metaphase- spindle fibres form between the poles and join at the equator (the widest part of the spindle). The nuclear envelope breaks down, and the centromeres of the chromatids attach to spindle fibres at the equator.
      • Anaphase- spindle fibres begin to shorten, pulling the chromatids apart in opposite directions. One chromatid goes to each pole. Anaphase ends when the chromatids reach the poles and the spindle breaks down.
      • Telophase- the nuclear envelope reforms around each pole and the chromosomes unravel, so that the genetic information is sorted into separate nuclei.
      • Cytokinesis- this is the actual splitting of the cells, when the cell membrane constricts as protein filaments on the outside of the cell contract until the cell is divided.
    • Stem Cells
      • Totipotent- these are cells with the potential to develop into any type of cell (they can become a total organism).
        • Stage in human development-after 3 complete cel cycles the zygote contains 8 identical totipotent cells. Because of their totipotency, they can bceome identical twins, triplets etc.
      • Pluripotent- these cells can become most, but not all, cell types.
        • Stage in human development- 5 days after conception, a blastocyst has formed. The outer layer will go on to become the placenta, and the inner cells (50 cells) form the embryos tissues. These cells are pluripotent embryonic cells.
      • Stem Cells in Medicine
        • Potential uses- stem cells are undifferentiated, so if they were encouraged to grow into a specific type of cel tissue, you could grow new organs. This would reduce the need for organ donors and could save lives.
        • Technique- pluripotent embryonic stem cells are removed from 'spare' embryos from fertility clinics (women are given drugs to stimulate over-ovulation, so many of the eggs produced aren't needed).
          • These stem cells are isolated and grown in a culture with the hope that they can produce new organs.
        • Problems- organs can be rejected by the body because it is a foreign object. Normal methods like tissue typing and immuno-suppressant drugs can be used. A new method has been found; therapeutic cloning- a dilpoid cell is removed from the patient, fused with an ovum with it's nucleus removed and then grown in a culture. This zygote would be geneticall identical to the patient, so the rejection risk is much lower.
        • Ethical implications- as embryos are potentially human beings, some people believe that taking the stem cells and 'killing' the embryo is wrong. Adult stem cells can be used for research, but scientists believe they could learn more from thee embryonic stem cells.


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