Lecture 8 Chapter 47.1 Book notes

Lecture 8 Chapter 47.1 Book notes - Biology 110b Chapter...

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Biology 110b Chapter 47.1 Book notes (Page 988) Three successive stages to an animal’s body: 1. Cleavage- cell division creates a hollow ball of cells, the blastula, from the zygote. 2. Gastrulation- produces a three-layered embryo, the gastrula. 3. Organogenesis- generates rudimentary organs from which adult structures grow. Contact of the sperm with the egg’s surface initiates metabolic reactions within the egg that trigger the onset of embryonic development. Acrosomal reaction begins when a specialized vesicle at the tip of the sperm, called the acrosome , discharges hydrolytic enzymes. These enzymes digest the jelly coat of the egg, enabling an elongating sperm structure called the acrosomal process to penetrate the jelly coat. Vitelline layer , a meshwork of extracellular matrix molecules lying under the jelly coat. Contact of the tip of the acrosome with the egg membrane leads to the fusion of sperm and egg plasma membranes and the entry of the sperm nucleus into the cytoplasm of the egg. Depolarization occurs within about 1-3 seconds after a sperm binds to an egg, preventing additional sperm cells from fusing with the egg’s plasma membrane. Without this fast block to polyspermy , multiple sperm could fertilize the egg, resulting in an aberrant number of chromosomes in the zygote. (Page 989) Fusion of egg and sperm plasma membranes also triggers a series of changes in the egg that cause a longer-lasting block. Sperm binding appears to activate a signal transduction pathway involving two second messengers, IP 3 and DAG, that cause calcium to be released from the egg’s endoplasmic reticulum into the cytosol. The high concentration of Ca 2+ bring about the cortical reaction , fusion with the egg’s plasma membrane of numerous vesicles lying in the egg’s cortex, the area just beneath the membrane. These cortical granules , formed during oogenesis, now release their contents into the perivitelline space , the space between the plasma membrane and the vitelline layer.
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Biology 110b Chapter 47.1 Book notes Enzymes released from the cortical granules degrade proteins holding the vitelline layer to the plasma membrane, while mucopolysaccharides produce an osmotic gradient, drawing water into the perivitelline space and swelling it. The fertilization envelope resists the entry of additional sperm, and, along with other changes in the egg’s surface, function as a longer-term slow block to polyspermy . (Page 990)
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Lecture 8 Chapter 47.1 Book notes - Biology 110b Chapter...

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