Mitosis Mechanics: The Spindle and Its Microtubules

Core Concept

The mitotic spindle is a bipolar, dynamic machine built from microtubules and motor proteins that segregates duplicated chromosomes into two daughter cells. Its microtubules fall into three functional classes: kinetochore microtubules, which attach chromosomes to the spindle poles; interpolar (polar) microtubules, which overlap in the spindle midzone and push the poles apart; and astral microtubules, which radiate toward the cell cortex to position the spindle and define the cleavage plane. Microtubule dynamic instability plus the action of kinesins, dynein, and other motors let the spindle self-organize, capture chromosomes, align them, and then pull them apart.

Key Components

• Microtubules: Dynamic polymers of α/β-tubulin with a fast-growing plus end and a slower minus end.
• Centrosome/spindle pole: Main microtubule-organizing center (contains γ-tubulin ring complexes) at each pole.
• Kinetochore microtubules: Attach to kinetochores; lose/gain tubulin to move chromosomes.
• Interpolar microtubules: Overlap antiparallel at the midzone; separated by motors to elongate the spindle.
• Astral microtubules: Extend to the cortex; anchor and orient the spindle.
• Motors: Kinesin-5 (Eg5, slides antiparallel MTs), kinesin-4/10 (chromokinesins), kinesin-14 (minus-end-directed), cytoplasmic dynein, kinesin-13 (MCAK, depolymerizer).
• Kinetochore: Chromosome attachment site; the Ndc80 complex is the core microtubule binder.

Mechanism / How It Works

1. Spindle assembly: In prophase, duplicated centrosomes separate and nucleate microtubules. Microtubules grow and shrink by dynamic instability — stochastic switching between growth and catastrophe — a "search-and-capture" process that finds kinetochores.
2. Capture and biorientation: Kinetochore microtubules bind kinetochores via the Ndc80 complex. Chromosomes initially attach to one pole (monotelic) and, through error correction by Aurora B and tension, become bioriented (each sister attached to opposite poles).
3. Congression: Motor proteins and kinetochore microtubule dynamics move chromosomes to the spindle equator (the metaphase plate).
4. Anaphase A: Once cohesin is cleaved, kinetochore microtubules shorten, primarily by plus-end depolymerization (pulling) combined with minus-end flux, moving sister chromatids toward opposite poles.
5. Anaphase B: Interpolar microtubules slide apart via kinesin-5 and are lengthened, while astral microtubules pull the poles toward the cortex; the spindle elongates, further separating the chromosome sets.

Energy and Directionality

The spindle is a GTP/ATP-driven machine. Tubulin polymerization is powered by GTP hydrolysis (the tubulin-GTP cap stabilizes growing ends; GTP hydrolysis to GDP promotes catastrophe), and motor proteins move by ATP hydrolysis. Chromosome movement in anaphase A is driven largely by microtubule depolymerization itself — energy released from the GTP-hydrolyzed lattice — and by motor activity. The process is directional and irreversible: once separase cleaves cohesin and the APC/C commits the cell, sister chromatids cannot be reattached; the cell is committed to completing segregation.

Experimental Evidence / Technique

• Live-cell imaging with fluorescent tubulin reveals spindle dynamics and chromosome movements in real time.
• Laser microsurgery of individual spindle fibers or kinetochores (e.g., severing kinetochore fibers) shows each fiber's contribution to force.
• Drug perturbations: Taxol stabilizes microtubules, nocodazole depolymerizes them, and monastrol inhibits kinesin-5 (producing monopolar spindles) — each maps a specific mechanical role.
• Micromanipulation/traction force measurements quantify forces at kinetochores and poles.
• In vitro reconstitution with purified tubulin, motors, and DNA-coated beads shows that bipolar spindles can self-organize without centrosomes (chromosome-driven assembly).

How it works

1. Spindle assembly: In prophase, duplicated centrosomes separate and nucleate microtubules. Microtubules grow and shrink by dynamic instability — stochastic switching between growth and catastrophe — a "search-and-capture" process that finds kinetochores.
2. Capture and biorientation: Kinetochore microtubules bind kinetochores via the Ndc80 complex. Chromosomes initially attach to one pole (monotelic) and, through error correction by Aurora B and tension, become bioriented (each sister attached to opposite poles).
3. Congression: Motor proteins and kinetochore microtubule dynamics move chromosomes to the spindle equator (the metaphase plate).
4. Anaphase A: Once cohesin is cleaved, kinetochore microtubules shorten, primarily by plus-end depolymerization (pulling) combined with minus-end flux, moving sister chromatids toward opposite poles.
5. Anaphase B: Interpolar microtubules slide apart via kinesin-5 and are lengthened, while astral microtubules pull the poles toward the cortex; the spindle elongates, further separating the chromosome sets.

Common confusions

• Anaphase A vs. B: A moves chromosomes toward poles (kinetochore MT shortening); B separates the poles (spindle elongation). They overlap but are mechanistically distinct.
• Centrosome is not absolutely required: Some cells (plants, oocytes) build functional spindles without centrosomes via chromosome-driven assembly.
• Kinetochore vs. centromere: The centromere is the DNA region; the kinetochore is the protein complex assembled on it.
• Plus vs. minus ends: Kinetochore MTs attach at their plus ends; minus ends are at the poles. Depolymerization at the plus end (and flux at the minus end) drives anaphase A.

Quick review

• Spindle = bipolar array of dynamic microtubules + motors; three classes: kinetochore, interpolar, astral.
• Assembly: search-and-capture by dynamic instability; biorientation via error correction (Aurora B).
• Anaphase A (chromosome-to-pole) + Anaphase B (pole-to-pole) = segregation.