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Unformatted text preview: PRACTICE EXAM 20 pts: 1. With the aid of a diagram, indicate how initial dorsal-ventral polarity is created in fruit fly and frog embryos. Fly Embryo Frog Embryo 5 pts In the fly embryo: the maternal Dorsal gradient activates D/V genes in a threshold dependent fashion. High levels of Dorsal activate ventral mesoderm genes (e.g. snail, twist), lower levels activate neuroectodermal genes (e.g. AS- C, sog, rho), and the absence of dorsal permits expression of dorsal genes (e.g. dpp). 5 pts In the frog embryo: the point of sperm entry, which is located at the equator in the animal hemisphere (e.g. near the vegetal hemisphere), induces cortical rotation away from the point of fertilization. Latent dorsalizing factors, which are believed to lie at the vegetal pole, are thought to be swept to a location opposite to that of the sperm entry point where they are activated to define the dorsal pole of the embryo (ventral being the original point of sperm entry). The activated dorsalizing factors lead to high levels of ß-catenin in the nucleus of dorsal cells which then functions as a morphogen to activate different genes along the D/V axis in a threshold dependent fashion. In what respects are these pattern forming mechanisms similar and how are they different? (two sentences maximum) 3 pts Similarities: Both embryos rely on a transcription factor morphogen (Dorsal in flies and ß-catenin in frogs) to activate tissue specific patterning genes in different D/V territories and maternal factors play a role in establishing D/V polarity. 2 pts Differences: An extrinsic factor (the point of sperm entry) plays a critical role in establishing the frog D/V axis but not that of the fly and the morphogens initiating D/V patterning are not related (e.g. Dorsal and ß-catenin). Explain briefly how VegT functions in vegetal cells to induce mesoderm in adjacent cells of the animal hemisphere....
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- Spring '09
- DNA, pts, Neural tube, neural identity genes