The 6-membered cyclic form of sugars is usually called the “pyranose” form in reference to the cyclic ether pyran.

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This combination of two positive feedback loops:

one that activates a CDK1 activator (Cdc25C)

one that inhibits a CDK1 inhibitor (Wee1)

results in the abrupt and irreversible transition from G2 into mitosis.

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Active cytoplasmic cyclin B-CDK1 translocates into the nucleus, where it phosphorylates nuclear lamins to cause nuclear envelope breakdown and, together with Plk1 and the Aurora kinases, induces chromosome condensation and formation of the mitotic spindle.

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Plk1 is critical for formation of the mitotic spindle as well as for chromosome segregation.

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The Aurora kinase family members Aurora A and Aurora B play key roles in mitotic spindle formation and in ensuring that chromosomes attach correctly to the mitotic spindle so that they are segregated accurately during mitosis.

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Once mitotic CDKs are activated at the end of G2 , they phosphorylate specific serine residues in all 3 nuclear lamins.

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This phosphorylation, together with phosphorylation of lamin B by protein kinase C, causes depolymerization of the lamin intermediate filaments.