Cell division

1.2 Substance movement into & out of cells

1.4 Higher-level organisation

Cell division

  1. Chromosomes

    1. The nucleus of a cell contains chromosomes, which carry many genes that control the characteristics of the body
    2. Many genes have different forms called alleles, which may produce different characteristics
    3. In cells, chromosomes are normally found in pairs

  2. Mitosis

    1. Cells divide by mitosis to produce replacement cells or additional cells during growth
    2. When a cell divides by mitosis, a copy of the genetic material is made and the cell divides once to form two genetically identical cells

  3. Meiosis

    1. Cells in reproductive organs divide to form gametes by meiosis
    2. When a cell divides by meiosis, a copy of the genetic material is made and the cell divides twice to form four gametes, each with a single set of chromosomes
    3. Gametes join at fertilisation to form a single cell with a new pair of chromosomes, and repeatedly divides by mitosis to form many cells and as an organism develops, these cells differentiate to form different types of cells

  4. Stem cells

    1. Stem cells are undifferentiated cells that can differentiate into specialised cells and can divide to form new stem cells
    2. Many plant cells retain the ability to differentiate throughout life
    3. Most types of animal cells differentiate at an early stage so in mature animals, cell division is mainly restricted to reparation & replacement, but stem cells can be taken from human embryos or adult bone marrow
    4. Therapeutic cloning is a medical treatment that takes a body cell from the patient and extracts the nucleus to put into a denucleated egg cell, which is then stimulated with an electric shock so that is begins to divide by mitosis, producing stem cells that can differentiate into helpful specialised cells that have the same genes as the patient and so won’t be rejected by their body
    5. Stem cell therapy may be able to help many medical conditions, such as differentiating stem cells into new nerve cells to treat paralysis

    6. However, there is controversy over the use of stem cells:

      For Against
      Embryos are not equivalent to human life, only the potential The creation of a human embryotic stem cell line requires the destruction of a human embryo, which some consider to be murder

  5. Tumours

    1. Tumours result from the abnormal uncontrolled division of cells
    2. Benign tumours do not invade other tissues but can be dangerous if they grow in tissues such as those in the brain and compress it
    3. Malignant tumours, or cancer, invade healthy tissue and may metastasise, whereby they enter the bloodstream and circulate to other parts of the body, forming secondary tumours
    4. Tumours can be caused by chemical carcinogens, such as those found in tobacco1 smoke and asbestos, and by ionising radiation, such as UV and X-rays
  1. Tobacco is the cause of about 22% of cancer deaths ↩