Key Concepts of Section 19.5

• Mitotic CDKs induce entry into mitosis in all eukaryotes.

• Mitotic CDKs are held inactive until the completion of DNA replication by inhibitory phosphorylation of the CDK subunit.

• Mitotic CDKs promote their own activation through positive feedback loops leading to the rapid inactivation of Wee1 kinase and activation of Cdc25 phosphatase.

• Mitotic CDKs induce nuclear envelope breakdown in most eukaryotes by phosphorylating lamins.

• Centrosome duplication occurs during S phase. Mitotic CDKs induce the separation of the duplicated centrosomes, which initiates mitotic spindle formation.

• Sister chromatids attach to the mitotic spindle via their kinetochores in a bi-oriented manner, with one sister kinetochore attaching to microtubules emanating from one spindle pole and the other one to microtubules nucleated by the other spindle pole.

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  Cells sense bi-orientation of sister chromatids through a tension-based mechanism. When kinetochores are not under tension, the protein kinase Aurora B phosphorylates the microtubule-binding subunits of the kinetochore, which decreases their microtubule binding affinity.

• Chromosomes must be compacted for segregation.

• Condensins, protein complexes that are related to cohesins, facilitate chromosome condensation and are activated by mitotic CDKs.