Microtubules are the structural backbone of the mitotic spindle, the machine that physically separates duplicated chromosomes so each daughter cell gets a complete set. They do far more than just pull chromosomes apart: microtubules build the spindle from scratch, capture chromosomes and align them at the cell’s equator, sense and correct attachment errors, and ultimately position the site where the cell pinches in two. Without microtubules, cell division cannot happen, which is exactly why some of the most effective cancer drugs work by disrupting them.
How Microtubules Are Built
Microtubules are hollow tubes assembled from repeating protein subunits called tubulin. Each building block is a pair of two related proteins, alpha-tubulin and beta-tubulin, locked together. These pairs stack end to end to form long filaments, and about thirteen of those filaments wrap around to create the tube. The result is a structure roughly 25 nanometers across, stiff enough to push and pull on cellular structures but flexible enough to bend without snapping.
What makes microtubules special during mitosis is their instability, and that is a feature, not a bug. A microtubule can grow steadily for a while and then abruptly switch to rapid shrinking, a behavior called dynamic instability. This switching depends on a small energy-carrying molecule, GTP, bound to each tubulin subunit. Shortly after a subunit joins the growing tip, it burns its GTP. As long as new GTP-carrying subunits keep arriving fast enough, a small cap of unburned subunits stabilizes the tip and the microtubule keeps growing. Experiments using fluorescently labeled tubulin showed that as few as about 40 GTP-bound subunits at the tip are enough to hold a microtubule together. When that cap is lost, the tube rapidly disassembles.1Cell Press (Current Biology). Direct Measurement of the Size of the GTP Cap in Microtubules This constant cycle of growth and catastrophic shrinkage lets the cell quickly remodel its microtubule network when it enters mitosis.
Where Microtubules Come From During Division
Microtubules do not just sprout randomly. In most animal cells, they are born at structures called centrosomes, which sit at opposite ends of the cell during mitosis and act as the two poles of the spindle. At the heart of each centrosome is a ring-shaped complex made of a special form of tubulin, gamma-tubulin. This gamma-tubulin ring complex serves as a template: new microtubules nucleate from it and grow outward, their fast-growing “plus” ends radiating toward the center of the cell while the slow “minus” ends stay anchored at the pole.2PubMed Central. γ-Tubulin complexes in microtubule nucleation and beyond Anchoring proteins at the centrosome physically hold the gamma-tubulin ring complexes in place, and blocking those anchor proteins with antibodies measurably reduces the number of microtubules a centrosome can produce.3PubMed Central. Centrosomal proteins CG-NAP and kendrin provide microtubule nucleation sites by anchoring gamma-tubulin ring complex
Centrosomes are not the only way to build a spindle, though. Cells also generate microtubules near chromosomes themselves. A signaling molecule called RanGTP accumulates around chromosomes and releases spindle assembly factors that had been locked up by transport proteins. Those freed factors nucleate, stabilize, and organize microtubules close to the chromosomes. Even though only a fraction of the assembly factors in the cell’s interior become active near chromatin, cooperative and self-reinforcing interactions among them are enough to reliably produce a bipolar spindle.4PubMed Central. The RanGTP gradient – a GPS for the mitotic spindle This chromosome-driven pathway is a critical safety net: it ensures that even if centrosomes are damaged or absent, cells can still divide.
Capturing and Connecting to Chromosomes
Once the spindle forms, microtubules must find and grab each chromosome. Each duplicated chromosome has a specialized protein structure called a kinetochore, built on a region of DNA that acts as a handle. The kinetochore is where the physical connection between a microtubule and a chromosome actually happens. At the core of that connection is a protein complex called Ndc80, which directly binds to the microtubule surface. The Ndc80 complex does not just hold on passively: it can maintain its grip on a microtubule even as the microtubule shortens, making it the leading candidate for the long-sought coupler that converts microtubule shrinkage into chromosome movement.5PubMed Central. The Ndc80 complex: integrating the kinetochore’s many movements
The process of capture is partly a search-and-find operation. Microtubules growing out from the spindle poles cycle through growth and shrinkage, probing in many directions. When a microtubule tip happens to contact a kinetochore, it gets caught. This “search and capture” is surprisingly fast given that chromosomes can be scattered throughout the cell. The chromosome-driven microtubule nucleation pathway described above helps, because microtubules generated near chromosomes can be incorporated into kinetochore connections, shortening the time it takes for all chromosomes to become attached.
How Attachment Errors Get Fixed
Not every initial attachment is correct. Sometimes both kinetochores of a duplicated chromosome end up connected to microtubules from the same pole, a situation that would send both copies to one daughter cell and leave the other with none. The cell has a quality-control system to detect and correct these mistakes, centered on an enzyme called Aurora B kinase. Aurora B senses tension: when a chromosome is properly connected to opposite poles, the microtubules pull the two kinetochores in opposing directions, creating tension across the chromosome. This tension physically separates Aurora B from its targets on the kinetochore, so the attachments remain stable.6PubMed Central. Aurora B Tension Sensing Mechanisms in the Kinetochore Ensure Accurate Chromosome Segregation
When the attachment is wrong and both kinetochores connect to the same pole, there is no opposing pull, so tension is low. Aurora B stays close to its targets and adds chemical tags that weaken the microtubule-kinetochore bond. The microtubule lets go, giving the chromosome another chance to be captured correctly. Experiments on bi-oriented kinetochores confirmed that even properly attached kinetochores primarily release their microtubules when Aurora B is artificially activated, demonstrating that this enzyme is the decisive switch.7Journal of Cell Biology. Tension promotes kinetochore–microtubule release by Aurora B kinase
Lining Chromosomes Up at the Middle
Before a cell can split its chromosomes, it first lines them all up along the spindle’s equator, a step called congression. Microtubules contribute to this alignment through at least two mechanisms. The first is straightforward tug-of-war: kinetochore microtubules from opposite poles pull a chromosome back and forth until the forces balance at the midpoint. The second mechanism involves motor proteins called chromokinesins that ride along chromosome arms. These motors walk toward microtubule plus ends, which point toward the equator, generating what is called the polar ejection force. Because microtubule density is highest near the poles, the push away from the poles is strongest close to them, nudging chromosomes toward the middle of the spindle.8Journal of Cell Science. Building an integrated model of chromosome congression
Two chromokinesins in human cells, known as Kid and KIF4A, contribute to this process in distinct ways. Knocking down Kid disrupts the orientation of chromosome arms and slows congression. Removing KIF4A instead increases the number and length of spindle microtubules and reduces the precision of the metaphase plate. Removing both at once causes severe attachment failures and can trigger a prolonged mitotic arrest.9Journal of Cell Biology. Human chromokinesins promote chromosome congression and spindle microtubule dynamics during mitosis The cell relies on both motors working together to get every chromosome into position quickly and accurately.
The Checkpoint That Prevents Premature Separation
Even one unattached kinetochore is enough to halt the entire process of chromosome separation. The spindle assembly checkpoint monitors attachment status and prevents the cell from moving into anaphase until every chromosome is properly connected to both poles. Unattached kinetochores recruit checkpoint proteins and catalyze the assembly of an inhibitory complex that blocks the signal to start separation. Only when the last kinetochore achieves a stable, tension-bearing microtubule attachment does the checkpoint switch off and allow anaphase to begin.10PubMed Central. Regulation of mitotic progression by the spindle assembly checkpoint The checkpoint is the reason why drugs that disrupt microtubules can trap dividing cells indefinitely in mitosis: without proper attachments, the checkpoint never silences.
Pulling Chromosomes Apart in Anaphase
Once the checkpoint is satisfied, the cell enters anaphase and chromosomes move toward opposite poles in two overlapping ways. The first, called anaphase A, involves shortening of the kinetochore microtubules. As tubulin subunits peel away from the shrinking end, the chromosome rides the depolymerizing tip poleward. Classic experiments showed that this movement requires no external energy source beyond the microtubule’s own disassembly: the energy stored in the polymer’s structure is sufficient to drag a chromosome along.11PubMed. Polewards chromosome movement driven by microtubule depolymerization in vitro
The second mechanism, anaphase B, pushes the two spindle poles farther apart, stretching the entire spindle and pulling the attached chromosomes with it. This elongation depends on a separate population of microtubules that overlap in the spindle midzone, pointing in opposite directions from each pole. Motor proteins slide these antiparallel microtubules apart, and dynein motors anchored at the cell cortex tug outward on microtubules radiating from each pole.12PubMed Central. Anaphase B Work in fruit fly embryos demonstrated that the rate of spindle elongation is controlled by suppressing a process called poleward flux, in which microtubule subunits continuously move toward the poles and are removed there. Blocking flux switches the motor-driven sliding of midzone microtubules from recycling into outright elongation.13PubMed Central. Model for anaphase B: role of three mitotic motors in a switch from poleward flux to spindle elongation
Positioning the Division Plane
After the chromosomes have separated, the cell still has to physically divide in two, a process called cytokinesis. Microtubules play a direct role in telling the cell where to split. Between the two separating chromosome masses, a dense bundle of antiparallel microtubules called the central spindle forms during late anaphase. This structure acts as a signaling platform: a protein complex called centralspindlin travels along central spindle microtubules and deposits a signaling molecule at their plus ends near the cell’s equator.14PubMed Central. Centralspindlin-mediated transport of RhoGEF positions the cleavage plane for cytokinesis That signaling molecule activates a contractile ring of actin and myosin in the cell membrane directly above the central spindle, pinching the cell into two daughters. Without the central spindle, the cell cannot reliably position its cleavage plane.15PubMed Central. Mechanistic insights into central spindle assembly mediated by the centralspindlin complex
Breaking Down the Nuclear Envelope
Before microtubules can reach chromosomes in animal cells, the membrane surrounding the nucleus has to be dismantled. Microtubules actively contribute to this breakdown. Live-cell imaging revealed that early spindle microtubules push into and fold the nuclear envelope up to an hour before it fully ruptures, creating mechanical tension in the protein mesh lining the inner surface of the nuclear membrane. The first hole in the envelope appears at the site of greatest tension, before the mesh proteins have even begun to disassemble chemically.16PubMed. Nuclear envelope breakdown proceeds by microtubule-induced tearing of the lamina Meanwhile, the motor protein dynein, which travels along microtubules, grabs pieces of the nuclear membrane and hauls them toward the spindle poles, physically stripping the envelope away from the chromosomes.17PubMed. Cytoplasmic dynein as a facilitator of nuclear envelope breakdown So microtubules are not passive bystanders waiting for the envelope to dissolve; they actively rip it apart.
How Other Organisms Do It Differently
The centrosome-driven spindle assembly described above is the standard story for animal cells, but it is far from the only way life organizes microtubules during division. Plant cells lack centrosomes entirely. Instead, they nucleate microtubules on the surface of the nuclear envelope, then organize them into a bipolar arrangement called the prophase spindle before the envelope breaks down. After the envelope opens, kinetochore fibers form and merge into a spindle with convergent poles rather than the sharply focused poles of animal cells.18PubMed. Spindle Assembly and Mitosis in Plants This flexibility shows that the basic tasks microtubules perform in mitosis are universal, but the organizational details are remarkably adaptable.
Fungi take yet another approach. Many yeasts undergo “closed mitosis,” meaning the nuclear envelope never breaks down at all. The spindle forms entirely inside the intact nucleus, and the spindle pole bodies, structures embedded in the nuclear envelope that function like centrosomes, stretch the nucleus between them until it divides. In fission yeast, experiments that physically severed spindle pole bodies from the spindle showed that midzone sliding forces alone could continue to push the spindle outward, deforming the nuclear envelope into elongated protrusions.19PLoS Biology. The Spindle Pole Bodies Facilitate Nuclear Envelope Division during Closed Mitosis in Fission Yeast These variations across kingdoms illustrate that evolution has tinkered extensively with the logistics of microtubule-based division while preserving the core principle: microtubules separate chromosomes.
The Tubulin Code
Not all microtubules in the spindle are identical. Cells chemically modify their tubulin subunits after assembly, adding or removing small chemical groups. These post-translational modifications create what researchers have termed a “tubulin code,” a set of chemical flags on the microtubule surface that motor proteins and other associated molecules can read. Different modifications can make a microtubule more or less stable, alter how strongly motors grip it, or change which cargo gets delivered along it.20PubMed Central. Dissecting the role of the tubulin code in mitosis During mitosis, specific modification patterns distinguish kinetochore microtubules from the more dynamic astral microtubules at the cell periphery, helping the cell fine-tune the behavior of different microtubule populations within the same spindle. As we will see in the next section, these modifications also turn out to be relevant to cancer drug resistance.
Why Cancer Drugs Target Microtubules
Given how central microtubules are to cell division, it is no surprise that some of the oldest and most widely used chemotherapy drugs work by disrupting them. Vinca alkaloids, first approved more than fifty years ago, prevent tubulin from assembling into microtubules. Taxanes, such as paclitaxel, do the opposite: they lock microtubules in place and prevent them from disassembling. Either way, the cell cannot complete mitosis. The spindle assembly checkpoint senses the attachment failures, stalls the cell in mitosis, and eventually the cell dies. Yet researchers have recognized that these drugs affect far more than just dividing cells. Because microtubules also serve as the transport network for cargo delivery and cell signaling throughout the cell cycle, disrupting microtubule dynamics can trigger cell death even in cells that are not actively dividing.21PubMed Central. Microtubule destabilising agents: far more than just antimitotic anticancer drugs
Drug resistance remains a persistent problem. One route to resistance involves mutations in tubulin itself. Studies have shown that specific mutations in the gene for beta-tubulin can counteract the stabilizing effect of taxanes, and that resistance scales with how much mutant tubulin the cell produces. Those same mutations tend to make cells more sensitive to microtubule-destabilizing drugs, which fits with a model where the mutations shift microtubule stability in one direction, partially neutralizing a drug that pushes it the other way.22PubMed Central. Human mutations that confer paclitaxel resistance
A more recently uncovered resistance mechanism involves the tubulin code. In triple-negative breast cancer cells resistant to paclitaxel, researchers found elevated levels of a microtubule-depolymerizing motor called KIF2C alongside increased tubulin polyglutamylation, one of the chemical modifications in the tubulin code. KIF2C preferentially dismantles polyglutamylated microtubules, even in the presence of paclitaxel, effectively counteracting the drug’s stabilizing action. A newly developed chemical inhibitor of KIF2C, combined with paclitaxel, significantly reduced tumor growth in resistant mouse models and restored sensitivity to several different microtubule-targeting drugs.23Developmental Cell. KIF2C mediates paclitaxel resistance and cross-resistance to microtubule-targeting agents via tubulin polyglutamylation in breast cancer
Because resistance to one class of microtubule drug often brings cross-resistance to others, researchers have also explored targeting proteins that depend on microtubules rather than tubulin itself. Inhibitors of the motor protein Eg5, which is essential for separating spindle poles early in mitosis, cause cells to arrest with a single-pole spindle and die. Testing these inhibitors in taxane-resistant cancer cells showed that they retained their activity, since resistance mutations in tubulin do not protect a cell from losing a completely different spindle component.24Journal of Biological Chemistry. Mitotic Kinesin Inhibitors Induce Mitotic Arrest and Cell Death in Taxol-resistant and -sensitive Cancer Cells This strategy of targeting spindle motors rather than tubulin itself represents an active area of drug development.
Microtubules as Mechanical Machines
It is easy to think of microtubules as static scaffolding, but the mitotic spindle is fundamentally a mechanical device. Microtubules must bear pushing and pulling forces, resist buckling under compression, and flex around obstacles. Physical measurements using tiny cantilevers pressed against individual microtubules showed that they behave like miniature elastic tubes: they deform linearly and reversibly under forces up to about 0.3 nanonewtons, absorbing roughly a 15 percent compression before suffering irreversible damage.25Biophysical Journal. Elastic Response, Buckling, and Instability of Microtubules under Radial Indentation Those properties matter inside the spindle, where microtubules collectively generate and transmit the piconewton-scale forces that shuffle chromosomes around. Ongoing work combining mechanical perturbation experiments with computational modeling continues to revise how researchers think about spindle mechanics, including the question of how much force individual microtubules contribute versus how much comes from the collective behavior of thousands of them working as a network.