8.2.1 Most of a cell’s DNA is not translated

• What are stem cells?

  Undifferentiated / unspecialised cells capable of:

  1. Dividing (by mitosis) to replace themselves indefinitely
  2. Differentiating into other types of (specialised) cells

  
• Describe how stem cells become specialised during development


  ● Stimuli lead to activation of some genes (due to transcription factors)

  ● So mRNA is transcribed only from these genes and then translated to form proteins

  ● These proteins modify cells permanently and determine cell structure / function
• Describe totipotent cells

  ● Occur for a limited time in early mammalian embryos

  ● Can divide AND differentiate into any type of body cell (including extra-embryonic cells eg. placenta)
• Describe pluripotent cells


  ● Found in mammalian embryos (after first few cell divisions)

  ● Can divide AND differentiate into most cell types (every cell type in the body but not placental cells)

  
• Describe multipotent cells

  ● Found in mature mammals

  ● Can divide AND differentiate into a limited number of cell types

  • Example: multipotent cells in bone marrow can divide and differentiate into different types of blood cell

  
• Describe unipotent cells, using an example

  ● Found in mature mammals

  ● Can divide AND differentiate into just one cell type

  • Example: unipotent cells in the heart can divide and differentiate into cardiomyocytes (cardiac muscle cells)

  
• Explain how stem cells can be used in the treatment of human disorders

  ● Transplanted into patients to divide in unlimited numbers

  ● Then differentiate into required healthy cells (to replace faulty / damaged cells)

  Examples:

  ● Potential treatment of Type 1 diabetes by creating healthy islet cells that produce insulin

  ● Bone marrow stem cell transplant for SCD / blood cancers

  1. Destroy patient’s bone marrow before treatment → so no faulty cells are produced
  2. Transplant stem cells from healthy person → divide and differentiate into healthy cells
• Explain how induced pluripotent stem (iPS) cells are produced

  1. Obtain adult somatic (body) cells (non-pluripotent cells or fibroblasts) from patient
  2. Add specific protein transcription factors associated with pluripotency to cells so they express genes associated with pluripotency (reprogramming)
  3. Transcription factors attach to promoter regions of DNA, stimulating or inhibiting transcription
  4. Culture cells to allow them to divide by mitosis

  • Once made, iPS cells can divide and differentiate into healthy cells to be transplanted into the same patient

  
• Evaluate the use of stem cells in treating human disorders - Advantages 


  For

  ✓ Can divide and differentiate into required healthy cells, so could relieve human suffering by saving lives and improving quality of life

  ✓ Embryos are often left over from IVF and so would otherwise be destroyed

  ✓ iPS cells unlikely to be rejected by patient’s immune system as made with patient’s own cells

  ✓ iPS cells can be made without destruction of embryo and adult can give permission
• Evaluate the use of stem cells in treating human disorders - Disadvantages 


  Against

  𝖷 Ethical issues with embryonic stem cells as obtaining them requires destruction of an embryo and potential life (embryo cannot consent)

  𝖷 Immune system could reject cells and immunosuppressant drugs are required

  𝖷 Cells could divide out of control, leading to formation of tumours / cancer