Body plans & Homeobox genes

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  • Body plans & Homeobox genes
    • Homeobox genes are responsible for determining the body pattern of an organism
      • Tend to come in groups called HOX  CLUSTERS
      • They are regulatory genes
    • A homeobox is a section of DNA which can turn other genes on/off by binding to DNA.
      • 60 amino acids long
      • Necessary to turn homeobox genes on/off.
        • Well conserved across plants, animals and fungi.
    • HOX genes only present in animals
      • Control positioning of body parts
      • Mammals have 4 Hox clusters on different chromosomes
        • The order that they appear in along a chromosome is the order in which their effects are expressed
    • Body plans usually shown as cross sections through an organism.
      • Shows fundamental arrangement of tissue layers
        • Diploblastic animals- 2 primary tissue layers
        • Tripoblastic animals- 3 primary tissue layers
      • Animals are segmented- these segments have multiplied over time and are specialised for various functions.
    • Most animal body shapes have symmetry
      • Bilateral symmetry is seen in most animals. They have both left/right sides, as well as a head and tail.
      • Radical symmetry is seen in diploblastic animals such as jellyfish. They have no left/right sides, only a top and bottom.
      • Assymetry is seen in sponges, which have no lines of symmetry.
    • Mitosis (cell division) and apoptosis (programmed cell death) are essential for shaping  organisms.
      • Hox genes regulate mitosis and apotosis.
      • In removing unwanted cells by apoptosis, cells cells can also release chemicals which cause mitosis and cell proliferation, thus remodelling tissues.
    • Expression of regulatory genes is affected by the environment
      • Internal and external influences lead to stress, which can be defined as the condition produced when homeostatic balance within an organism is disturbed
        • Internal factors
          • Hormonal changes
          • Psychological stress
        • External factors
          • Temperature change
          • Change in light intensity
    • Expression of regulatory genes is affected by drug use.
      • Drugs can also affect the activity of regulatory genes
      • Thalidomide  (morning sickness drug in 1950's) prevents expression of a hox gene, leading to birth of babies with shortened limbs.
        • Now used to treat some forms of cancer.

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This is wrong- it is not radical symmetry, it is RADIAL. Radial means circular. Please fix this it is difficult to revise from something that is wrong

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