Cell Division and Growth

• Cell cycle

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

  • It is described as the lifecycle of the cell
• State the three stages of the cell cycle

  • Interphase

  • Mitosis

  • Cytokinesis
• Interphase


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

  • Double helix ‘unzips’ exposing two strands

  • DNA bases align next to complementary bases on DNA

  strands

  • Complementary base pairs join

  • Two identical DNA molecules formed
• Chromosome


  A linear DNA molecule tightly coiled around proteins
• What happens to chromosomes during DNA replication?

  The DNA in the ‘arm’ of each chromosome (chromatid) is replicated
• Mitosis


  A form of cell division that produces two identical ‘daughter’ cells, both genetically identical to the parent cell
• Why is mitosis important in organisms?

  • Asexual reproduction

  • Growth

  • Repair of damaged cells

  • Cell replacement
• The four stages of mitosis

  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase
• Prophase

  • DNA condenses, chromosomes become visible

  • Nuclear membrane dissapears
• Metaphase

  Chromosomes line up along the cell equator
• Anaphase


  • Spindle fibres attach to each chromosome

  • ‘Arms’ of each chromosome pulled to opposite poles

  • Chromatids separated
• Telophase

  • Nucleus of cell divides

  • New membrane forms around each set of chromosomes
• What does cytokinesis involve?

  • Division of the cell membrane and cytoplasm

  • Two genetically identical ’daughter’ 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
• What are 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
• What does the 95th percentile mean?

  95% of measurements will be below the value of the 95th percentile
• What can doctors 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)
• Describe growth in animals

  • Cell division occurs in all body cells. It occurs at a slower rate in adults than in younger animals as growth stops and cell division is only needed for replacement and repair

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

  • Cell division can only occurs in meristematic tissue. The rate of cell division remains the same throughout a plant’s life

  • Meristematic stem cells can differentiate into any type of cell for as long as the plant lives

  • Cell elongation occurs in all cells. Cells expand and enlarge enabling growth of the plant
• 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
• Why is cell differentiation important?

  It 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 type of cell
• 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
• What is the primary function of adult stem cells?

  Replacement of dead cells e.g. replacement of red blood cells which only live for 120 days
• Where are stem cells found in plants?

  Meristems
• Where is 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 type of cell during the life of a plant
• How can stem cells be used 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 can embryonic stem cells be collected from?

  • Donor stem cells removed from embryos grown in vitro

  • Patient’s own stem cells are 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 using stem cells in medicine

  1. Transplanted stem cells could cause tumours
  2. Finding suitable stem cell donors is a difficult task
  3. Stem cells may be rejected by the body (immunosuppressants taken)
  4. Potential side effects
  5. • Long term risks of stem cells unknown
  6. 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 reproductive cloning of humans