contact guidance: some guidance cues require the navigating axon to come into physical contact with the cue and therefore act over a short range. Such guidance cues may either attract or repel growing axons. (a) in contact attraction the migrating axon follows a route demarcated by an attractive cue, shown in green. (b) in contact repulsion the axon is confined to a particular trajectory, minimizing its contact with the repellent cue, shown in red.

Major forms of chemotropism. (a) chemoattraction: the navigating axon senses a gradient of an attractive cue (green) and moves towards its source. (b) chemorepulsion: the axon senses a gradient of a repulsive cue (red) and turns to move down the gradient, away from the source. (c) Growth cones can also respond to repulsive cues by collapsing. in the example shown here, the growth cone first becomes exposed to a chemorepellent cue (left panel), causing it to withdraw all filopodia and collapse (middle). The growth cone can then start to move forward again, but in a different direction (right panel).

• during the syncytial blastoderm stage you can add plasmids with GAL4 or UAS. Some of them will integrate into germ cells.

• GAL4 = transcription factor

• UAS = DNA binding site specific to GAL4

1. to limit / target the expression , you engineer the plasmid to have GAL4 DNA just downstream of your tissue specific promoterexamples : TissueSpecificPromoter::GAL4

2. you engineer the UAS plasmid to have UAS-{InsertionalMutagenesisGene}examples : UAS::GFP or UAS::lacZ

3. add GAL4 plasmids to one line , and UAS plasmids to another line during syncytial blastoderm stage

4. cross the 2 different lines , their offspring will inherit both GAL4 and UAS constructs you embedded

5. during normal transcription of DNA , it will start at promoter sequences , transcribing GAL4

6. at this time , GAL4 gene product acts as a transcription factor and promotes DNA transcription at UAS sites.

7. now the InsertionalMutagenesisGene will be transcribed and translated

• Slit and Robo :

  • there is a time dependance on Robo expression

  • some axons always express Robo ( longitudinal )

  • some axons don't at first ( commissural ) , allowing them to avoid slit repulsion and cross the midline

    • then they start expressing Robo so they don't go backwards

  • Slit = ligand

    • expressed along the midline

  • Robo = receptor

    • Gain of Function = axons can’t cross at all

    • Loss of function = 100% crossing of axons with slit

• Netrin and DCC / Frazzled :

  • Netrin = ligand

    • expressed along the midline

    • binds to DCC receptors on growth cones and cause attraction

  • DCC / Frazzled = receptor