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Life Sci 2 - Chapter 20

Life Sci 2 - Chapter 20 - CHAPTER 20 ANIMAL DEVELOPMENT...

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CHAPTER 20: ANIMAL DEVELOPMENT: FROM GENES TO ORGANISM Development Begins with Fertilization The sperm and the egg make different contributions to the zygote - egg well stocked w/ nutrients, ribosomes, variety of molecules including mRNAs, & mitochondria DNA - sperm contributes centrioles along w/ haploid nucleus Fertilization causes rearrangements of egg cytoplasm - nutrient molecules are dense & concentrated by gravity in lower half, vegetal hemisphere - haploid nucleus of egg located opposite end, animal hemisphere - cortical (outermost cytoplasm) of animal hemisphere heavily pigmented, underlying less so - specific sperm-binding site only in animal hemisphere - sperm enters -> cortical cytoplasm rotates toward site -> reveals band of pigmented cytoplasm - gray cresent: band of pigmented cytoplasm on opposite side of sperm entry important in development - sperm entry -> ventral (belly) region, gray crescent -> dorsal (back) region & posterior end of embryo Rearrangements of egg cytoplasm set the stage for determination - fertilization - β-catenin (solid color filling embryo) is distributed throughout cytoplasm - CSK-3 (dots in embryo) which targets β-catenin for degradation, also found throughout cytoplasm - protein that inhibits GSK-3 is contained in vegetal pole vesicles - cortical rotation - vesicles in vegetal pole move on microtubule tracks to side opposite sperm entry - dorsal enrichment inhibitor - the vesicles release GSK-inhibiting protein… - dorsal inhibition of GSK-3 - …so GSK-3 cannot degrade β-catenin on the dorsal side… - … but does degrade on the ventral side - dorsal enrichment of β-catenin - thus there is a higher β-catenin concentration in the dorsal cells of the early embryo Cleavage: Repacking the cytoplasm - cleavage: transformation of diploid zygote into mass of cells through rapid series of cell divisions - cleavage proceeds w/ rapid DNA replication & mitosis, but no cell growth & gene expression - morula: solid ball of smaller and smaller cells - blastocoel: central fluid-filled cavity in morula
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