gene determination

• undifferentiated to determined to differentiated cell types
• determination means that a cell is committed to particular fate
• determination is the process leading up to observable differentiation of a cell
• differentiated stage is achieved through transcriptional regulation, involving gene cascade
• external signal triggers turning on of master regulatory gene

  causes cell specific genes to turn on to become a differentiated cell
• muscle differentiation

  • MyoD is activated by external signals
  • cell is determined (called myoblast cell)
  • MyoD activates expression of other muscle specific transcription factors
  • then activate genes for muscle proteins, blocking of cell division
  • non-dividing myoblast fuse together to form muscle fibres
• Drosophila development

  • main axes are determined
  • segments are established
  • filling in the details (such as organs)
• Drosophila segments

  • 3 for the head
  • 3 for the thorax
  • 9 for the abdomen
• bicoid means 2 tailed
• Bicoid mutant has no anterior end, just 2 posterior ends

  • example of egg polarity gene
  • is result of maternal effect gene
  • encodes transcription factors which initiate cascade of gene activations
• bicoid protein gradient determines the anterior end of the fly (higher concentration of the protein)
• transcription factors that determine polarity and position are called morphogens
• egg polarity genes initiate gene cascade, starting with segmentation genes (gap, pair-rule, segment polarity) then homeotic genes
• egg polarity genes determine

  anterior-posterior axis and induce gap genes
• gap genes determine 


  sub-dividing embryo into broad areas and induce pair-rule genes
• pair-rule genes determine 


  pairs of segments and induce segment polarity genes
• segment polarity genes determine

  anterior-posterior axis of each segment and induce homeotic genes
• homeotic genes identified as dominant mutations that changes the identity of body parts
• Hox genes determine identity of embryonic regions along the anterior-posterior axis
• Hox genes are colinear meaning 


  they occur in clusters, arranged in the same order as the regions they affect