B2 - Cell division

• Cell division by mitiosis produces 2 genetically identical daughter cells through the cell cycle
• Stage 1 of the cell cycle


  • the cell increases in size
  • DNA replicates
  • there is an increase in number of sub-cellular structures
• Stage 2 of the cell cycle


  • a complete set of chromosomes is pulled to each end of the cell
  • the nucleus divides to form two nuclei
• Stage 3 of the cell cycle


  • cytoplasm and cell membrane divide to form two genetically identical daughter cells
• The nucleus of a cell contains chromosomes
• • Chromosomes are made of DNA molecules
  • In normal human cells, chromosomes are found in 23 pairs
  • Each chromosome carries a large number of genes
• Stem cell


  An undifferentiated cell that can develop into one or more types of specialised cell
• There are two types of stem cells in mammals:

  • adult stem cells
  • embryonic stem cells
• Stem cells can be cloned to produce large numbers of identical cells
• Adult stem cells


  • Found in specific parts of adults and children, such as bone marrow
  • Can only differentiate to form certain types of cells
  • e.g. stem cells in bone marrow can only differentiate into types of blood cell
• Advantages of adult stem cells


  • fewer ethical issues
  • adults can consent to having their stem cells removed and used
  • an established technique for treating diseases like lukaemia
  • relatively safe to use as a treatment and donors recover quickly
• Disadvantages of adult stem cells


  • requires a donor
  • potentially means a long wait to find someone suitable
  • can only differentiate into certain types of cells
  • can only be used to treat certain diseases
• Embryonic stem cells


  • found in early human embryos
  • often taken from spare embryos from fertility clinics
  • can differentiate into any type of specialised cell in the body
• Advantages of embryonic stem cells


  • can treat a wide range of diseases
  • can differentiate into any type of specialised cell
  • may be possible to grow entire replacement organs
  • usually no donor needed since they are obtained from spare embryos from fertility clinics
• Disadvantages of embryonic stem cells


  • ethical issues
  • the embryo is destroyed, and the embryo is a potential human life
  • risk of transferring viral diseases to the patient
  • newer treatment, so relatively under-researched
  • not yet clear if they can cure as many diseases as originally thought
• Plant meristems


  • found in meristem regions in the roots and shoots of plants
  • can differentiate into all cell types
  • can be used to create clones of whole plants
• Advantages of plant meristems


  • rare species of plants can be clonned to prevent extinction
  • plants with desirable traits can be cloned to produce large numbers of identical plants
  • fast and low-cost production of large numbers of plants
• Disadvantage of plant meristems


  cloned plants are genetically identical, so a whole crop is at risk of being destroyed by a single disease or genetic defect
• Theraputic cloning


  • cells from a patient's body are used to create a cloned early embryo of themselves
  • stem cells from this embryo can be used for medical treatmennts and growing new organs
  • these stem cells have the same genes as the patient, so are less likely to be rejected when transplanted