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
• Arrangement of chromosomes in body cell nucleus

  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
  • Produces additional cells for growth and repair
• First stage of cell cycle

  Longest stage - 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 embryonic stem cells

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

  • Can differentiate into several cell types to replace dead or damaged tissues
  • Bone marrow stem cells can form many different types of blood cell
• Function of stem cells in plant meristems


  • 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
  • The stem cells which originate from the embryo will not be rejected by the patient's immune system, so 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