Body plans

• what are homeobox genes?

  • group of genes containing a homeobox- highly conserved in plants, animals and fungi, control development of a body plan of an organism
  • section of DNA 180 base pairs long
• what type of gene is a homeobox gene?

  a regulatory gene
• why are homeobox genes considered regulatory genes?

  • code for a part of proteins which is highly conserved
  • this part is called a homeodomain, and it binds to DNA, switching other genes on/off
• what are hox genes?

  • a group of homeobox genes that are only present in animals
  • responsible for the correct positioning of body parts (controls the body plan)
  • hox genes are found in gene clusters- mammals have four gene clusters on different chromosomes
  • the order in which the genes appear on the chromosome is the order in which their effects are expressed in an organism
• describe the aspect of body plans

  • cross-sections through the organism
  • shows arrangement of tissue layers
  • eg- start off as segments and then multiply and specialise over time to perform different functions
• what are the roles of mitosis?

  • growth
  • repair
  • replacement
• what is the role of apoptosis?

  • shapes different body parts
  • removes unwanted cells and tissues
  • cells undergoing apoptosis can release chemical signals which stimulate mitosis, stimulating the remodelling of tissues
• what type of genes regulate mitosis and apoptosis?

  hox genes
• what factor affects the expression of regulatory genes?

  environmental, both internal and external
• state examples of external environmental stimuli that control the expression of regulatory genes

  • stress- when homeostatic balance within an organism is upset
  • can be due to changes in temperature
  • changes in light intensity
  • drugs
• state examples of internal environmental stimuli that control the expression of regulatory genes

  • release of hormones
  • psychological stress
• what is radial symmetry?

  • symmetry along a plane, at any angle through a central axis
  • when organisms have no left or right, only top and bottom
• what is bilateral symmetry?

  • symmetry along a central axis, eg 2 arms and two legs in humans
  • when organisms have both a left and right, and head and tail, not just a top and a bottom
• how would you identify whether the same hox gene within different organisms was responsible for a particular characteristic?

  • isolate that gene from one species using PCR/restriction enzymes
  • test the gene in the tissue of a different species to monitor how that tissue will develop
  • positive results mean that the tissue develops whatever body part the gene codes for eg- eyes growing on legs
  • if the gene is switched off, then that body part won't develop
  • use DNA sequencing to compare the DNA sequence of the gene in different organisms
• is it true that all hox genes are homeobox genes but not all homeobox genes are hox genes?

  • yes, because hox genes are a form of homeobox gene
  • yes because it's present in vertebrates
  • yes, because they are present in hox clusters
  • other forms of homeobox genes are present in other clusters