Topic 2

• Cell cycle


  A series of events that take place in a cell involving cell growth, DNA replication and cell division
• Cell cycle


  1. Interphase

  2. Mitosis

  3. Cytokinesis
• Interphase


  The longest stage of the cell cycle that involves cell growth, the synthesis of new organelles and DNA replication
• DNA replication


  1. Double helix 'unzips' exposing two strands

  2. DNA bases align next to complementary bases on the DNA strands

  3. Complementary base pairs join

  4. Two identical DNA molecules formed
• Chromosome


  A linear DNA molecule tightly coiled around proteins
• During DNA replication, the DNA in the 'arm' of each chromosome (chromatid) is replicated
• Mitosis


  A form of cell division that produces two diploid 'daughter' cells, both genetically identical to the parent cell
• Importance of mitosis in organisms

  • Asexual reproduction
  • Growth
  • Repair of damaged cells
  • Cell replacement
• Stages of mitosis


  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase
• Prophase


  1. DNA condenses, chromosomes become visible

  2. Nuclear membrane disappears
• Metaphase


  Chromosomes line up along the cell equator
• Anaphase


  1. Spindle fibres attach to each chromosome

  2. 'Arms' of each chromosome pulled to opposite poles

  3. Chromatids separated
• Telophase


  1. Nucleus of the cell divides

  2. New membrane forms around each set of chromosomes
• Cytokinesis


  • Division of the cell membrane and cytoplasm
  • Two genetically identical 'daughter' cells produced
• A cell divides by mitosis once every 2 minutes. After 10 minutes, there will be 32 identical cells
• Cancer


  • Non-communicable disease
  • Uncontrolled cell division (due to damaged DNA) results in the formation of a primary tumour
  • Tumour cells break off and spread to other tissues forming secondary tumours
• Percentile charts


  • A chart used to monitor growth
  • Measurements (e.g. fetal length or head circumference) can be compared to the expected values at a certain age
• 95th percentile


  95% of measurements will be below the value of the 95th percentile
• What doctors can determine from percentile charts

  • Slower growth than normal (below the bottom line)
  • Faster growth than normal (above the top line)
  • Abnormal growth (irregular growth patterns)
• Growth in animals


  • Cell division occurs in all body cells, but at a slower rate in adults
  • Most cells differentiate at an early stage and become specialised, but some adult stem cells retain their ability to differentiate
• Growth in plants


  • Cell division can only occur in meristematic tissue, and the rate remains the same throughout a plant's life
  • Meristematic stem cells can differentiate into any cell type for as long as the plant lives
  • Cell elongation occurs in all cells, enabling growth of the plant
• Stem cells


  Cells that are unspecialised and capable of differentiating into a range of cell types
• Cell growth in adults

  Occurs at a slower rate than in younger animals as growth stops and cell division is only required for replacement and repair
• Cell differentiation


  • Most cells differentiate at an early stage and become specialised
  • Some adult stem cells retain their ability to differentiate
• Growth in plants


  1. Cell division can only occur in meristematic tissue

  2. The rate of cell division remains the same throughout a plant's life

  3. Meristematic stem cells can differentiate into any cell type for as long as the plant lives

  4. Cell elongation occurs in all cells, enabling growth of the plant
• Stem cells


  Unspecialised cells 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
• 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
• Adult stem cells


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

  Replacement of dead cells e.g. replacement of red blood cells which only live for 120 days
• Stem cells in plants

  Found in meristems
• Meristem tissue


  Found in regions of the plant where cells are continuously dividing e.g. root tips, shoot tips
• Meristematic stem cells


  Stem cells found in meristems that are unspecialised and capable of differentiating into any cell type during the life of a plant
• Using stem cells in medicine

  1. Stem cells collected

  2. Stem cells stimulated to differentiate into specific cell types e.g. heart muscle cells

  3. Specialised cells transplanted into the patient

  4. Used to treat damage or disease e.g. heart disease
• 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
• Benefits of using stem cells in medicine

  • Treat damage or disease e.g. heart disease, type 1 diabetes
  • Treat diseases that would otherwise be untreatable
  • Used in scientific research
  • Growing organs for transplants
• Risks of stem cell use in medicine


  • Transplanted stem cells could cause tumours
  • Finding suitable stem cell donors is a difficult task
  • Stem cells may be rejected by the body (immunosuppressants taken)
  • Potential side effects
  • Long term risks of using stem cells unknown
  • Stem cells may become contaminated during preparation and when transplanted transmit infections to the patient, making them sicker
• Ethical issues related to the use of stem cells in medicine

  • The embryos that were used to provide stem cells are destroyed which is seen as unethical and a waste of potential human life
  • May lead to the reproductive cloning of humans