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Background - Coggle Diagram

- - - - homeotic genes
      - segment polarity genes
  - - - hox regulator genes that segments anterior posterior axis
      - encodes homeodomain
        
        Helix loop helix structure
        
        60 AA
    - - Hox gene 6-8 more recent due to less sequence similarity with other older Hox genes.
    - - polycomb repressors control colinearity of hox genes in mamals through chromtain manipulation.
        
        Inhibiting transcription of Hox by preventing transcription factors from binding to promoters through the gaps
        
        polycomb tighten DNA winding around histones
      - trithorax
        
        loosen chromatin packaging to enable transcription of Hox
      - Temporal colinearity
        
        Activators and repressors act in opposite directions along expression of Hox cluster.
      - Spatial colinearity
        
        Spatial colinearity occur in mammals
        
        Regulatory sequences are between the hox genes
- - - - Determines which tissue and time hox genes are expressed
  - - - atennapedia hox cluster
      - biothorax complex downstream of atenna pedia cluster
      - Hox cluster expression on chromosome 3 corresponds to anterior-posterior axis development.
      - Expression chronology
        
        1) lab gene
        
        4) Dfd gene
        
        5) Scr gene
        
        6 Antp gene
        
        7 Ubx
        
        8 abdA
        
        9-13) Abd-B
        
        cad last
      - Atavism studies
        
        ubx mutant express double T2 instead of T3 4 pairs of legs similar to arachnids.
        
        Hox gene modification are able to induce atavism (devolution back to ancestor)
    - - Hox genes deivided into cluster a, b, c, d on different chromosomes.
      - Most of the additional Hox genes in mouse are duplications
      - abd-B (abdomen development) duplication prevalent
      - Hox gene cluster duplication on different chromosomes different between species
    - - Fossil evidence suggest hox gene duplication pre cambrian era led to diversification of body plans from bilaterian organisms.
      - Urbilateralian ancestor are ancestor of the bilaterian clade of animals.
        
        German ur for original
      - Pre-Cambrian mechanism
        
        repetitive factors form around Hox genes
        
        Unequal crossover during meiosis duplicated hox genes
        
        Continued unequal crossover led to hybrid hox genes with new sequences whilst old hox genes are conserved.
    - - Colinearity and clusters less vital in drosophila genome due to strands containing sequences in opposite orientation.
        
        In mammals, hox genes share regulator sequences
      - In mammals, Hox being less spread out and clustered
        
        All genes transcribed in same orientation
        
        Fewer introns than drosophila