DB-Lec 7&8

DB-Lec 7&8 - Biology 442 Biology Developmental...

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Biology 442 Biology 442 Developmental Biology Developmental Biology Lecture 7 & 8 – Cleavage Lecture 7 & 8 – Cleavage
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Cleavage Cleavage : : A series of rapid mitotic divisions that A series of rapid mitotic divisions that divide the enormous volume of the egg into divide the enormous volume of the egg into numerous smaller cells called numerous smaller cells called Blastomeres Blastomeres . . In most systems the rate of cell division and the placement of the blastomeres relative to each other is under the control of RNA and proteins stored in the egg. During cleavage the cytoplasmic volume of the egg does not increase. Instead the zygote cytoplasm is divided into increasingly smaller cells. Mammals are the exception.
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Cleavage divisions are very rapid: Drosophila: 10 min per division Xenopus : 35 min per division Humans are an exception: 24 hours for first division Rapid Division is achieved by leaving Growth Phases out of the cell cycle M G1 S G2 Normal cell cycle M S Cleavage cell cycle
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Cleavage Cell Cycle Cleavage Cell Cycle No Growth – embryonic volume remains No Growth – embryonic volume remains the same the same No RNA synthesis – maternal stores No RNA synthesis – maternal stores provide most of needed materials provide most of needed materials Cells are non-motile during cleavage Cells are non-motile during cleavage In many organisms the cleavages are In many organisms the cleavages are synchronous – all cells divide at once. synchronous – all cells divide at once.
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Outline for Cleavage Lectures Outline for Cleavage Lectures Body Axes Body Axes Cleavage patterns and factors that influence them (yoke). Cleavage patterns and factors that influence them (yoke). Cleavage patterns of representative animals. Cleavage patterns of representative animals. sea urchins sea urchins amphibians amphibians mammals mammals birds/reptiles birds/reptiles insects insects Spatial control of cleavage – positioning and orientation of Spatial control of cleavage – positioning and orientation of mitotic spindles mitotic spindles Regulation of the cleavage cell cycle and the transition to Regulation of the cleavage cell cycle and the transition to a normal cell cycle at the end of cleavage – a normal cell cycle at the end of cleavage – Midblastula Midblastula transition transition
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Body Axes: Body Axes: A body has a top and a bottom, a back A body has a top and a bottom, a back and a front, left and right and you can cut it and and a front, left and right and you can cut it and look at it at different planes look at it at different planes
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Factors that influence cleavage patterns Factors that influence cleavage patterns 2 major factors determine the cleavage planes that divide the fertilized egg into blastomers 1. Factors in the egg cytoplasm determine the orientation of the mitotic spindles and the timing of their formation.
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DB-Lec 7&8 - Biology 442 Biology Developmental...

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