Body plans

• Morphogenesis= regulation of the pattern of anatomical development
• Homeobox gene= group of genes which all contain a homeobox.

  Is a section of DNA 180 based pairs long.

  • A homeodomain binds to DNA and switches other genes on or off.
  • Regulatory genes 
  • Pax6 is one of the homeobox genes, when mutated it causes a form of blindness in humans.
• Hox genes= are one group of homeobox genes that are only present in animals. They are responsible for the correct positioning of body parts.

  • In animals the hox genes are found in gene clusters.
  • The order in which the the genes appear along the chromosome is the order in which their effects are expressed in the organism.
• Body plans are usually represented as cross-sections through the organism showing the fundamental arrangement of tissue layers.

  • Diploblastic animals have two primary tissue layers and triploblastic animals have three primary tissue layers.
  • Animals are segmented, these segments have multiplied over time and are specialised to perform different functions.
  • Hox genes in the head control the development of mouthparts and hox genes in the thorax control the development of wings, limbs or ribs.
• Body plans are usually represented as cross-sections through the organism showing the fundamental arrangement of tissue layers.

  • Diploblastic animals have two primary tissue layers and triploblastic animals have three primary tissue layers.
  • Animals are segmented, these segments have multiplied over time and are specialised to perform different functions.
  • Hox genes in the head control the development of mouthparts and hox genes in the thorax control the development of wings, limbs or ribs.
• • Individual vertebrae and associated structures have all developed from segments in the embryo called somites. They are directed by hox genes to develop in a particular way depending on their position in the sequence.
• Symmetry

  • Radial symmetry is seen in diploblastic animals like jellyfish. They have no left or right sides only a top and a bottom.
  • Bilateral symmetry which is seen in most animals means the organisms have both left and right sides and a head and tail.
  • Asymmetry is seen in sponges which have no lines of symmetry.
• Role of mitosis= to increase the number of cells leading to growth.
• Role of apoptosis= shapes different body parts by removing unwanted cells and tissues. Cells undergoing apoptosis can also release chemical signals which stimulate mitosis and cell proliferation leading to the remodelling of tissues.
• Factors affecting expression of regulatory genes:

  • Stress can be defined as the condition produced when the homeostatic balance within an organism is upset. This can be due to externally factors like a change in temperature or light intensity. Internal factors can change due to the release of hormones.
  • These factors have greater impact during the growth and development of an organism.
  • Drugs like thalidomide given to pregnant women for morning sickness, later discovered that it prevented the normal expression of a particular hox gene, resulted in babys having shorter limbs.
• Apoptosis:

  1. DNA becomes denser and more tightly packed.
  2. Nuclear envelope breaks down.
  3. Chromatin condensing vesicles form which contain hydrolytic enzymes.
  4. Blebs transform into vesicles (apoptotic bodies) contain cell parts.
  5. Phogocytes engulf and digest cells via phagocytosis.