Cell Division and Stem Cells

• Chromosome


  A linear DNA molecule tightly coiled around proteins that contains genes
• Human body cells contain 46 chromosomes (23 pairs)
• Sex chromosomes


  A pair of chromosomes that determines sex: Males have an X and a Y chromosome, Females have two X chromosomes
• Mitosis


  A form of cell division that produces two 'daughter' cells, both genetically identical to the parent cell with the same number of chromosomes (46 chromosomes), involves one division
• Use of Mitosis

  • Asexual reproduction
  • Growth
  • Repair of damaged cells
  • Cell replacement
• Meiosis


  A form of cell division involved in the formation of gametes, chromosome number is halved (23 chromosomes), involves two divisions, producing four gametes
• Gametes


  Sex cells (sperm cells, egg cells) that contain half the chromosome number
• Meiosis for sexual reproduction

  • Increases genetic variation
  • Ensures that the resultant zygote has a full set of chromosomes
• Cancer


  Non-communicable disease, uncontrolled mitosis results in the formation of a primary tumour, tumour cells break off and spread to other tissues forming secondary tumours
• Stem cells


  Cells that are unspecialised and capable of differentiating into a range of different cell types
• Differentiation


  • The process by which stem cells become specialised (have a specific function)
  • Some genes switch on or off, determining cell type
• Importance of cell differentiation

  • Enables the formation of specialised tissues with specific functions e.g. muscle tissue
• Embryonic stem cells


  Stem cells found in very early embryos that are unspecialised and capable of differentiating into any cell type
• Function of embryonic stem cells

  • Enable the growth and development of tissues in human embryos
• Where embryonic stem cells can be collected from


  • Donor stem cells removed from embryos grown in vitro
  • Patient's own stem cells removed from the umbilical blood before birth
• Adult stem cells


  Stem cells that can differentiate into a limited range of cell types e.g. bone marrow stem cells
• Primary function of adult stem cells

  • Replacement of dead cells e.g. replacement of red blood cells which only live for 120 days
• Potential uses of stem cells in medicine

  • Treat disease e.g. heart disease, type 1 diabetes
  • Used to repair damaged tissue e.g. brain damage
  • Used in scientific research
  • Growing organs for transplants
• Ethical issues related to the use of stem cells in medicine

  • The embryos used to provide stem cells are destroyed which is seen as unethical and a waste of potential human life
  • Could lead to the 'farming' of embryos for stem cells
  • May lead to the reproductive cloning of humans
• Where meristem tissue is found


  • In regions of the plant where cells are continuously dividing e.g. root tips, shoot tips