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2011_Questions_Week_5_Answers

2011_Questions_Week_5_Answers - 1 You dissect the animal...

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1. You dissect the animal and vegetal portions of a Xenopus blastula stage embryo and recombine them in an explant assay. After incubating for several days, describe the cell identity portions derived from the animal and vegetal tissues. -The animal cap will be converted to mesoderm. The endoderm will give rise to endodermal derivatives only. 2. You experimentally, deplete embryos for b-catenin, and repeat the assay from #1 (don't worry about possible secondary effects from losing cadherin-based cell adhesion). What will be the cell identity of the animal cap and vegetal portions? Why? -A loss of Beta-catenin in the vegetal endoderm means there will be no dorsalizing signal secreted from the endoderm to the mesoderm. This means that the vegetal region of the embryo will give rise only to ventral endodermal tissue types. In turn, this will cause the animal cap region to be specified to ventral mesoderm. 3. List two of the general mesodermal inducing factors? -Any of the following: Vg1 and VegT, TGF-beta/nodal related genes (Xnr, activin). Remember that nodal signaling is a subset of the TGF-beta superfamily 4. What is the primary purpose of the Nieuwkoop center? -To induce the organizer cells (remember that organizer cells are mesodermal, but are induced by the vegetal-specific Nieuwkoop center. The combination of beta-catenin and Vg1/nodal also directly induces gene expression of some of the BMP antagonists expressed by the organizer cells. Remember, B-catenin induces dorsal tissue identity but does not directly induce mesoderm. 5. Describe the role of Dishevelled in the Wnt pathway. How is it different in canonical and non-canonical pathways? -Wnt ligand binding to frizzled activates dishevelled, causing the stabilization of Beta catenin and the activation of Wnt signaling. In part, this is done by Dishevelled recruiting GBP, which then binds
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to and inactivates Glycogen Synthase Kinase 3 (GSK3). See slide 8 from 2/1/11 lecture and board work showing a dominant negative version ACTIVATES canonical Wnt signaling. We also discussed this in the experiment in which a DNGSKB injected keller explant does not rescue convergent extension in embryos also injected with a version of Dsh that specifically inhibits the PCP pathway (Xdd). This showed that canonical Wnt signaling
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