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Chapter 47

Chapter 47. Preformation vs. epigenesis. Preformation: embryo development is miniature versions of adult like Russian nesting dolls Epigenesis: relative formless egg develops into adult. Fertilization in mammals. Acrosomal Reaction

phillip-gilliam
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Chapter 47

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  1. Chapter 47

  2. Preformation vs. epigenesis • Preformation: embryo development is miniature versions of adult like Russian nesting dolls • Epigenesis: relative formless egg develops into adult

  3. Fertilization in mammals • Acrosomal Reaction • Sperm reaches egg; acrosome releases hydrolytic enzymes by exocytosis. • Enzymes activate cytoskeletal elements that that elongate the end structure and penetrate the jelly coat of the egg. • Tip of acrosome has proteins that bind to receptors on the vitelline layer just exterior to the plamsa membrane. In some animals, there is a “lock and key” recognition to assure that they are only fertilized by the proper species. • Egg and sperm plasma membranes fuse. • This causes an electrical response in the egg plamsa membrane. • This depolarization prevents other sperm from penetrating the egg (fast block to polyspermy) • Fusion also causes vessicles inside egg to exocytosis contents which catalyze a hardening of the extracellular matrix of the cell thus acting as the slow block to polyspermy) • In mammals, fusion of egg and sperm nuclei does not occur until after the first mitotic division.

  4. Apoptosis: programmed cell death; necessary for proper development • accounts for the lack of webbing between our fingers and toes (which is their in embryos) • totipotent: a cell that has retained the potential to form all parts of the animal • determination: progressive restriction of a cell’s developmental potential. • Induction: the ability of one group of cells to influence the development of an adjacent groups of cells • Tissue-specific proteins: proteins found only in a certain type of cell • Genomic equivalence: all cells contain the same genes • Maternal effect genes: phenotypes expressed by these genes are exclusively the product of the mother’s contribution; usually mutations in genes that are necessary for the egg polarity to be set up properly. • Homeotic genes: genes that determine the anatomical identify of different segments of an individual.

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