⚡ Core Concept: Mitotic Cytoskeletal Dynamics & Equal Chromosomal Segregation
Mitosis (M-phase) partitions identical duplicated sister chromatid genomes into two daughter nuclei via the microtubule-based mitotic spindle apparatus. Mitotic progression involves five coordinated stages (Prophase, Prometaphase, Metaphase, Anaphase, Telophase) concluding with actin-myosin contractile cytokinetic cleavage.
1. The Five Morphological Stages of Eukaryotic Mitosis
Mitotic events proceed sequentially under tight spatiotemporal regulation:
Figure 2: Sequential Cytological Stages of Mitosis & Cytokinesis
Mitotic Cytology
Figure 2: Mitotic progression from chromatin condensation and nuclear lamina disassembly through metaphase plate alignment, cohesin cleavage during anaphase, and contractile ring cytokinesis.
2. Molecular Choreography: Kinetochores & Mitotic Motors
Chromosome segregation relies on specialized multi-protein complexes and three classes of spindle microtubules:
- Kinetochore Microtubules: Anchor directly to the trilaminar kinetochore plate assembled upon centromeric α-satellite DNA (marked by the centromeric histone variant CENP-A). The Ndc80 complex forms the primary load-bearing microtubule attachment.
- Interpolar Microtubules: Interdigitate at the spindle midzone; kinesin-5 (Eg5) motor homotetramers cross-link antiparallel microtubules and push spindle poles apart (Anaphase B).
- Astral Microtubules: Radiate toward the cell cortex where dynein motors exert pulling forces on the centrosomes.
3. Anaphase Onset: Cohesin Cleavage & The SAC
Sister chromatids are held together along their arms and centromeres by the ring-shaped Cohesin complex (Smc1, Smc3, Scc1/Rad21, Scc3). Mitotic cohesin removal occurs in two waves:
- Prophase Pathway: Plk1 and Aurora B phosphorylate cohesin along chromosome arms, causing arm dissociation while centromeric cohesin is shielded by Shugoshin (Sgo1) and PP2A.
- Metaphase-Anaphase Transition: Once every kinetochore achieves amphitelic (bipolar) tension, the Spindle Assembly Checkpoint (SAC) is extinguished, releasing APC/C-Cdc20. APC/C ubiquitinates Securin, unleashing active Separase protease, which cleaves the Scc1 subunit, allowing instant poleward chromatid movement (Anaphase A).
Antimitotic Chemotherapeutic Drugs
Taxanes (Paclitaxel): Bind and hyper-stabilize β-tubulin polymers, preventing spindle depolymerization. Vinca Alkaloids (Vincristine, Vinblastine): Depolymerize microtubules. Both mechanisms chronically trigger the SAC, arresting neoplastic cells in mitosis and inducing apoptosis via Bim and Mcl-1 degradation.
