Sporophylls and Spore Variations

Sporophylls produce clusters of sporangia known as “sori”

Groups of sporophylls form cone-like structures called “strobili” 

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The entire process of mitotic CDKs activation and entry into mitosis involves two positive feedback loops and coordination between events at the centrosome, in the cytoplasm, and in the nucleus.

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Cdc25B, activating an initial small pool of cyclin B/1 at the centrosome, which is facilitate centrosome maturation.

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Initial pool of active cyclin B-CDK1 phosphorylates Cdc25C, this recruits Pin1 and Plk1, which recognize Cdc25C through their phospho-binding domains. 

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Pin1 and Plk1 modify and phosphorylate Cdc25C to stimulate its phosphatase activity

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The pool of active Cdc25C is created, that activates the large pool of cytosolic cyclin B-CDK1, which activates more cytosolic Cdc25C as part of positive feedback loop 1

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At the same time that Cdc25C is being activated, mitotic CDKs and Plk1 phosphorylate Wee1, which targets it for destruction through a phosphobinding SCF protein-ubiquitin ligase. This forms positive feedback loop 2