6.7
M-Cdk guida la transizione alla mitosi
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Q1: What is M-Cdk and how does it form?
M-Cdk is a protein complex consisting of Cdk1 (cyclin-dependent kinase 1) bound to M cyclin. It forms when M cyclin accumulates during G2 phase and early mitosis. M cyclin levels peak at this time, allowing it to bind to the stable Cdk1 protein, creating the active M-Cdk complex that drives the transition into mitosis.
Q2: How is M-Cdk activated in the cell?
M-Cdk activation is a multi-step process. First, CAK (CDK-activating kinase) phosphorylates the complex at an active site. However, Wee1 kinase phosphorylates two inhibitory sites, keeping M-Cdk inactive. The phosphatase Cdc25 then removes these inhibitory phosphates and suppresses Wee1 activity, fully activating M-Cdk for mitotic progression.
Q3: What mitotic processes does M-Cdk control during prophase?
During prophase, M-Cdk drives chromosome condensation, the shortening and compaction of chromosomes necessary for proper segregation during cell division. M-Cdk also initiates formation of the mitotic spindle, which is essential for separating chromosomes into two daughter cells. These early mitotic processes are critical for successful cell division.
Q4: How does M-Cdk affect the nuclear envelope and sister chromatids?
During prometaphase, M-Cdk helps degrade the nuclear envelope, allowing the nucleus to break apart and chromosomes to access spindle machinery. In metaphase, M-Cdk mediates the attachment of sister chromatids to opposite poles of the mitotic spindle, ensuring proper chromosome segregation and accurate distribution to daughter cells.
Q5: What role does M-Cdk play in organizing cellular structures during mitosis?
M-Cdk promotes multiphase reorganization of the Golgi apparatus, which is critical for correct spindle formation and proper organelle segregation between daughter cells. Throughout mitosis, M-Cdk also reorganizes the actin cytoskeleton, which determines spindle orientation and the axis of cell division, ensuring coordinated cellular reorganization.
Q6: Are other kinases involved in the G2/M transition besides M-Cdk?
Yes, polo-like kinases and Aurora kinases also regulate the G2/M transition. Plk1 is necessary for bipolar mitotic spindle formation and phosphorylates proteins that separate spindle poles. Aurora-A regulates spindle formation and stabilization, while Aurora-B enables sister chromatid attachment to the spindle, working alongside M-Cdk.
Q7: Why do M cyclin levels fluctuate while Cdk levels remain stable?
M cyclin gene transcription increases as cells approach the G2/M transition, causing M cyclin accumulation and binding to stable Cdk1 protein. After mitosis, M cyclin is degraded, while Cdk1 levels persist throughout the cell cycle, allowing different cyclins to bind and regulate distinct phases. This ensures precise cell cycle control.