B1 - Cell division

• Chromosomes


  DNA molecules combined with proteins that contain genes which provide the instructions for protein synthesis
• Gene


  A section of DNA which codes for a protein
• The nucleus of human body cells contains 46 chromosomes
• The nucleus of gametes contains 23 chromosomes
• Chromosomes in the nucleus of a body cell

  • Arranged in pairs - 46 chromosomes form 23 pairs
• Cell cycle


  1. Replication of DNA and synthesis of organelles

  2. Mitosis

  3. Division of cell
• Mitosis


  Produces identical cells which all have the same genetic information, and produces additional cells for growth and repair
• First stage of cell cycle

  Cells grow and increase in mass, replicate DNA and synthesise more organelles (eg. mitochondria, ribosomes)
• Second stage of cell cycle

  Each chromosome in a pair is pulled to opposite poles of the cell, then the nucleus divides
• Third stage of cell cycle

  The cytoplasm and cell membrane divide - two identical daughter cells are produced
• Stem cell


  An unspecialised cell which is capable of differentiating into other cell types and of self-renewal
• Function of stem cells in embryos

  • Embryonic stem cells can replicate themselves and differentiate into many other types of cells, and may be able to treat conditions such as paralysis and diabetes by dividing to replace damaged cells
• Function of stem cells in adult bone marrow

  • Adult stem cells can differentiate into several cell types to replace dead or damaged tissues, for example bone marrow stem cells can form many different types of blood cell
• Function of stem cells in plant meristems


  • Meristem stem cells retain the ability to differentiate into any type of plant cell throughout their lifespan - they can differentiate into any cell which is required by the plant
• Therapeutic cloning


  A cloning method where an embryo is produced with the same genetic makeup as the patient, so the stem cells which originate from the embryo will not be rejected by the patient's immune system, and can be used to treat certain medical conditions
• Advantages of cloning plants using meristem stem cells

  • Can prevent rare plants from becoming extinct, can produce large numbers of plants with a favourable characteristic, can produce identical plants for research
• Issues associated with the use of stem cells

  • Many embryonic stem cells are sourced from aborted embryos - some people have an ethical/religious objection
  • Development of stem cell therapies is slow, expensive and difficult
  • Adult stem cells infected with viruses could transfer infections to patients
  • If donor stem cells do not have a similar genetic makeup to the patient, an immune response could be triggered