genetic control of body plan development

• • the body plan of an organism is controlled by homeotic sequences/genes
• • homeotic sequences/genes ensure our cells, tissues and organs develop in the right places. several of these genes contain sequences called homeobox sequences/genes
• • homeobox sequences/genes are regulatory genes and contain 180 DNA bp. these genes determine the body plan of an organism (morphogenesis) as it grows from a zygote to a complete organism
• the proteins made from homeobox genes are transcription factors which contain homeodomains that bind to DNA. the homeodomains found in the proteins are 60 amino acids long.
• the shape of the proteins, that homeodomains are found in, have a H-T-H shape. part of the homeodomain sequence recognises the TATA sequence of an enhancer region of a gene to be transcribed
• • homeobox sequences/genes are very similar in different organisms. they are said to be homologous to eachother.
  • for example, despite the fact that the last common ancestor of the fruit fly and mouse lived one billion years ago, their homeobox genes are very similar. they are highly conserved
• homeobox genes have also been found in plants and fungi. all plants share a similar set of homeobox genes, as do fungi. however, these genes are not homologous with those found in animals
• Hox genes are homeobox genes that are only found in animals, they are expressed in early embryonic development
• sequential and temporal linearity- hox genes are arranged in the order that they are expressed in, the order of when the various body part develops = colinearity
• regulating the hox genes

  1. mRNA supplied from the egg cytoplasm (egg polarity)
  2. Gap genes
  3. pair role genes
  4. hox genes
• mitosis

  • during differentiation different genes are switched off/deactivated/not expressed whilst other genes are switched on/activated/expressed
  • a study in 1962 showed that cells have a finite number of divisions before they die. This number is around 50 and is called the Hayflick constant.
• apoptosis

  • programmed cell death
  • important in metamorphosis
  • also important in separating webbed toes and fingers
• apoptosis

  • controlled by signals both from inside and outside the cells (hormones, cytokines, from the immune system and growth factors). Nitric oxide can disrupt the electron transport chain
  • apoptosis should be balanced with mitosis
• apoptosis steps

  1. a normal cell
  2. at the start of apoptosis the cell begins to 'bleb' and the nucleus starts to disintegrate
  3. cell fragments are produced with intact plasma membranes and containing organelles
  4. cell fragments are ingested and digested by phagocytic cells