Cell division

• Chromosomes

  The nucleus of a cell contains chromosomes, each of which carries a large number of genes made of DNA molecules
• The cell cycle

  1. Cell grows bigger

  2. DNA replicates (chromosomes are duplicated)

  3. Increase in number of sub-cellular structures, such as ribosomes and mitochondria

  4. A complete set of chromosomes is pulled to each end of the cell

  5. The nucleus divides to form two nuclei

  6. Cytoplasm and cell membrane divide to form two identical daughter cells
• Cell division by mitosis

  • Important for the growth and repair of cells, for example, the replacement of skin cells
  • Used for asexual reproduction
• Types of stem cells

  • Adult stem cells
  • Embryonic stem cells
• Adult stem cells

  Found in specific parts of the body in adults and children, can only differentiate into certain types of cells
• Embryonic stem cells

  Found in early human embryos, can differentiate into any type of specialised cell in the body
• Advantages of stem cells

  • Fewer ethical issues with adult stem cells as donors can consent
  • Adult stem cells are an already established technique for treating diseases such as leukaemia
  • Relatively safe to use as a treatment and donors recover quickly
  • Can treat a wide range of diseases as embryonic stem cells can form any specialised cell
  • May be possible to grow whole replacement organs
  • Usually no donor needed for embryonic stem cells as they are obtained from spare embryos from fertility clinics
  • Certain plant species can be cloned to prevent extinction and produce large numbers of plants with desirable traits
• Disadvantages of stem cells

  • Requires a donor for adult stem cells, potentially meaning a long wait time to find someone suitable
  • Adult stem cells can only differentiate into certain types of specialised cells so can be used to treat fewer diseases
  • Ethical issues with embryonic stem cells as the embryo is destroyed and each embryo is a potential human life
  • Risk of transferring viral infections to the patient with embryonic stem cells
  • Newer treatment so relatively under-researched, not yet clear if they can cure as many diseases as thought
  • Cloned plants are genetically identical, so a whole crop is at risk of being destroyed by a single disease or genetic defect
• Therapeutic cloning

  Cells from a patient's own body are used to create a cloned early embryo of themselves, and stem cells from this embryo can be used for medical treatments and growing new organs. These stem cells have the same genes as the patient, so are less likely to be rejected when transplanted.