Ch 6 Learning Obj

Ch 6 Learning Obj - Describe Drosophila development through...

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Describe Drosophila development through gastrulation. (6.1-6.5) o First, the sperm enters an egg that has already been activated during ovulation a few minutes before fertilization. As the oocyte squeezes through a small orifice, calcium channels open. The oocyte nucleus then resumes meiotic divisions and cytoplasmic mRNAs become translated, even without fertilization. The sperm can only enter at one site - the micropyle at the future dorsal anterior region of the embryo. The micropyle is a tunnel in the chorion (eggshell) that allows sperm to pass through one at a time. By the time the sperm enters the egg, the egg has already begun to specify axes. There is competition between sperm based on the length of the sperm tails - some of which are longer than the entire adult fly. The entire sperm is incorporated into the egg, including the tail. The sperm cell membrane does not fully break down until the entire sperm is inside the oocyte. Karyokinesis occurs 13 times before the cell eventually undergoes superficial cleavage (around the central yolk). Cleavage in the Drosophila forms a syncytium, a single cell with many nuclei. The first 13 divisions are the syncytial blastoderm and the mid-blastoderm transition occurs from around 11-13. The 14th cycle is the first cycle as the cellular blastoderm. Gastrulation occurs shortly after the mid-blastula transition. The first movements of gastrulation segregate the presumptive meso-, endo-, and ectoderm. The mesoderm folds inward to form the ventral furrow that eventually pinches off to become a ventral tube within the embryo. The endoderm invaginates to form two pockets at the anterior and posterior end of the ventral furrow. The pole cells are internalized with the endoderm and the embryo bends to form the cephalic furrow. The ectodermal cells on the surface and the mesoderm undergo convergence and extension, migrating toward the ventral midline to form the germ band. These will form the trunk of the embryo. The band extends posteriorly and wraps around the top (dorsal) surface of the embryo. At this point, the cells destined to form the most posterior larval structures are located immediately behind the future head region. Body segments appear, dividing ecto- and mesoderm. The germ band retracts, placing presumptive posterior segments at the posterior tip of the embryo. At the dorsal
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This note was uploaded on 11/10/2010 for the course BIOS 7676 taught by Professor Sanders during the Spring '10 term at LA Tech.

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Ch 6 Learning Obj - Describe Drosophila development through...

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