ch 44 - Ch 44 How Does Fertilization activate development...

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Ch. 44: How Does Fertilization activate development? Fusion of sperm and egg (fertilization) accomplishes several things: Ion fluxes across the membrane, sets up blocks to stop more sperm from entering, changes pH of cytoplasm, increases egg metabolism and stimulates protein synthesis, and initiates rapid cell division The sperm and the egg make different contributions to the zygote o Everything the zygote needs in early stage comes from mother’s egg cell o Sperm contributes its haploid nucleus, and the centriole Centriole contributes centrosome (which organizes the mitotic spindle for cell division) and creates microtubules (used for cilia and cell signaling) Rearrangements of egg cytoplasm set the stage for determination o When sperm enters zygote, the egg cytoplasm are organized from polarity o Nutrients in an unfertlizaed frog egg are dense yolk granules that are concentrated by gravity in lower half (vegetal hemisphere) o Haploid nucleus is at other end, called animal hemisphere o Frog egg is radially symmetrical o Sperm binding sites are on the surface or the animal hemisphere, and when sperm binds and enters, its converted to bilateral symmetry Cortical cytoplasm rotates toward site of sperm entry, which touches animal and vegetal poles, which produces a band of pigmented cytoplasm on opposite side of sperm entry Band is “gray crescent” created When this gray crescent forms, the centriole initiates cytoplasmic regorgnaization The movement of cytoplasm, proteins, and organelles changes distribution of signals Changes around distribution of B-catenin and a protein kinase called GSK-3 (the GSK-3 phosphorylates B- catenin, degrading it), and a GSK-3 inhibitor After sperm enters, the inhibitor moves along microtubules, disabling GSK3 This results in a higher concentration of B-catenin on the dorsal side How Does Mitosis Divide Up the Early Embryo? Cleavage repackages the cytoplasm
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o Cleavage is the sequence of early cell division that transforms the diploid zygote into a mass of undifferentiated cells o The embryo does mitosis but cell does not grow, gets packed Develops Central fluid-filled cavity called blastocoel, and then embryo is called blastula, with individual cells inside called blastomeres o The patterns of cleavage influences how these blastula form Complete cleavage-little yolk inside embryo allows for complete cleavage, but since vegetal pole contains more yolk, its not an even division. Results in blastomeres of vegetal hemisphere being larger than animal If mitotic spindles of division form parallel or perpendicual to the animal-vegetal axis, radial cleavage occurs If the mitotic spindles are not at oblique angles to animal-vegetal axis, spiral cleavage occurs
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