DNA and Cell Division

• DNA:

  • double strand of molecules that coils up into a double helix
  • exists in chromosomes
• Chromosomes:

  • 1 long molecule of data
  • can be divided into sections called genes
  • a single chromosome is a condensed molecule of DNA
• Gene:

  • a section of DNA
  • 1 molecule of DNA will contain thousands of genes
• Each human cell contains 46 chromosomes which occur in 23 identical pairs

  Each pair has the same genes

  Different forms of the same gene are called alleles which determine appearance
• Mitosis:

  process by which a cell divides into 2 genetically identical daughter cells
• Step 1 of Mitosis:

  1. chromosomes are replicated exactly
  2. number of sub-cellular structures increase
• Step 2 of Mitosis:

  1. chromosomes line up
  2. chromosomes are pulled apart to opposite ends of the cell
• Step 3 of Mitosis:

  1. cytoplasm divides
  2. new cell membrane forms
  3. cell divides into 2 genetically identical daughter cells
• Cell Differentiation:

  once a new cell has been made; it need to differentiate (specialise) as it grows to do its particular function

  it will only make the proteins and cell structures it need to do its job
• EMBRYONIC STEM CELLS can become any type of specialised cell
• Stem Cells:

  • not specialised - can become another specialised cell
  • can divide by mitosis to make more stem cells (unlike specialised cells)
  • some stem cells differentiate to become specialised
• Adult Stem Cells:

  • an undifferentiated cell found among differentiated cells in tissue/organs
  • roles in living organism:
  • maintain and repair tissue in which they are found
  • small number of adult stem cells in each tissue
  • once removed from body, capacity to divide is limited (ability to generate large numbers of adult stem cells is difficult)
  • BONE MARROW - example of tissue containing adult stem cells
  • can only treat a few diseases as the cells are already partly-specialised
• Embryonic Stem Cells:

  Advantages:

  • can create many embryos in a lab
  • painless
  • can differentiate into ANY type of cell

  Disadvantages:

  • 'death' of embryo - ethical
  • embryo cannot consent - ethical
  • may not work
• Adult Stem Cells:

  Advantages:

  • patient can consent
  • can treat some diseases
  • doesn't kill donor
  • reliable
  • quick recovery time (for patient)

  Disadvantages:

  • risk of infection from procedure
  • only treats some diseases
  • can be painful
• Embryonic and Adult Stem Cells:

  Advantages:

  • treats problem

  Disadvantages:

  • risk of viral infection from procedure
  • some stem cells can grow out of control (become cancerous)
• Stem Cells from Meristems in Plants:

  • used to produce clones of plants quickly and economically
  • rare species can be clones to protect them from extinction
  • crop plants with special features (eg. disease resistance) can be cloned to produce large numbers of identical plants for farmers