Cleavage

Cleavage - 1) 2) Cleavage rapid cell division; constant...

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1) 2) Cleavage – rapid cell division; constant mitotic divisions - no cell growth or increase in organism size - blastomeres – undifferentiated, generic new cells formed by mitotic divisions a) Benefits: -smaller pieces build up to larger unit -better surface area/volume ratio Protostomes: spiral cleavage Deutorestome: radial cleavage b) Different cleavage patterns determined by amount/presence of yolk - cytokinesis is harder to perform with lots of yolk present i) holoblastic cleavage - entire volume of zygote is continually divided (1) equal holoblastic – equal-sized blastomeres result from the constant division -i.e. starfish; organisms with little yolk -microlecithal cells – little yolk; equal holoblsatic -isolecithal – even distribution of yolk (2) unequal holoblastic – mesolecithal (moderate amount of yolk) -i.e. frog: divisions tend to favor one end resulting in more blastomeres at the end with the yolk Animal pole: Vegetal pole: ii) meroblastic cleavage -only part of original volume is continuously divided -macrolecithal – lots of yolk -telolecithal – yolk favors one side i.e. birds, reptiles polyspermy – multiple sperm fusing with the embryo after plasomogamy: 3 events to prevent polyspermy occur simultaneously 1) fast electrical block - influx of Ca 2+ ions causing depolarization in place in 1-3 seconds
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Cleavage - 1) 2) Cleavage rapid cell division; constant...

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