The Process of Nuclear Division: Step-by-Step Mitosis

Mitosis is the process by which a single cell duplicates its chromosomes and splits them into two identical sets, producing two genetically identical daughter cells. The whole sequence, from chromosome condensation through physical cell division, typically takes about an hour in human cells, though the timing varies by cell type. What looks simple from a distance is actually a tightly choreographed series of mechanical and chemical events, each dependent on the one before it, and each monitored by built-in quality-control systems that can halt the process if something goes wrong.

How a Cell Decides to Divide

Before mitosis visibly begins, the cell has already copied all of its DNA during an earlier phase called S phase. But having duplicated chromosomes does not automatically mean the cell will proceed to divide. The commitment to enter mitosis is controlled by a network of proteins that converge on one key enzyme complex: cyclin B bound to a partner called Cdk1. This complex is sometimes called mitosis-promoting factor, and its activation is what flips the switch from “preparing” to “dividing.”¹

The activation does not happen gradually or tentatively. Several positive feedback loops amplify cyclin B-Cdk1 activity once it crosses a threshold, making the decision essentially irreversible. Think of it like a spring-loaded trap: once triggered, the cell is fully committed.¹ Research has shown that this complex is inactive during the gap phase before mitosis and then switches on at a defined time before the nuclear envelope breaks down, kicking off the visible events of prophase.²

Prophase

Prophase is when the cell first shows visible signs that division is underway. The chromosomes, which have been loosely spread throughout the nucleus as long threads of DNA wrapped around proteins, begin to condense into compact, rod-like structures. This compaction is essential: trying to separate long, tangled DNA strands without condensing them first would be like trying to sort two intertwined balls of yarn by pulling them apart. A protein complex called condensin II drives this early condensation inside the still-intact nucleus.³

At the same time, the cell’s two centrosomes, small organelles that serve as the main organizing centers for the structural fibers (microtubules) that will pull chromosomes apart, begin migrating to opposite sides of the cell. A motor protein called Eg5, which belongs to the kinesin family, powers this separation by walking along microtubules and pushing the two centrosomes apart.⁴ By the end of prophase, the cell has two poles and a set of condensed chromosomes still enclosed within the nuclear envelope.

Prometaphase

The boundary between prophase and the next stage is one of the most dramatic moments in cell biology: the nuclear envelope breaks apart. This barrier, a double membrane reinforced by a mesh of structural proteins called lamins, does not simply tear. Instead, the same cyclin B-Cdk1 complex that triggered mitosis phosphorylates the lamins at specific sites, causing the mesh to depolymerize and the envelope to fragment into small vesicles.⁵ It is a controlled demolition, not a random collapse.

With the nuclear envelope gone, the microtubules radiating from the two centrosomes now have access to the chromosomes. Each chromosome has a specialized protein structure called a kinetochore, built on top of a unique stretch of DNA at the centromere region. The kinetochore is assembled from a core complex that includes the proteins CENP-A, CENP-C, and CENP-N, which together form a platform that microtubules can grab onto.⁶

The process of microtubules finding and attaching to kinetochores is often described as “search and capture.” Microtubules grow and shrink rapidly from the spindle poles, probing in different directions. This dynamic instability allows them to sweep through the cellular space until they encounter a kinetochore.⁷ Rotational diffusion of the microtubules also contributes, though modeling suggests it speeds up the capture process by only a modest amount compared to the effect of dynamic instability alone.⁸

The goal of prometaphase is to get each chromosome properly attached: one kinetochore connected to microtubules from one pole, and its sister kinetochore connected to microtubules from the opposite pole. This bi-oriented attachment is what ensures that when the chromosomes are eventually pulled apart, each daughter cell gets exactly one copy.

Metaphase

Once chromosomes achieve bi-orientation, they are pushed and pulled by opposing forces until they line up along the middle of the cell, forming what is called the metaphase plate. This alignment is not passive. Motor proteins on chromosome arms, including one called Xkid, generate “polar ejection forces” that push chromosomes away from the nearest pole, helping them settle at the equator.⁹ The tension between pulling from both poles and these ejection forces keeps the chromosomes neatly arrayed in a narrow band at the cell’s center.

Metaphase is also where the cell’s most important quality check takes place: the spindle assembly checkpoint. This surveillance system monitors whether every single kinetochore is properly attached to microtubules. As long as even one kinetochore remains unattached, the checkpoint blocks progression to the next stage by producing a signal that inhibits a protein complex called the anaphase-promoting complex (APC). The checkpoint works through a kinase called Mps1, which triggers a signaling cascade at unattached kinetochores.¹⁰ Downstream checkpoint proteins called Mad2 and BubR1 each bind to and inhibit Cdc20, the activating partner of the APC. BubR1 is roughly twelve times more potent than Mad2 at this inhibition, and the two proteins work together at normal cellular concentrations to shut the APC down completely.¹¹

Only when every chromosome has achieved proper bi-orientation and the checkpoint signal goes silent does the APC activate. The checkpoint exists because a single missegregated chromosome can have devastating consequences for the daughter cells, as we will see later.

Anaphase

With the checkpoint satisfied, the APC activates and triggers one of the sharpest transitions in cell biology. The APC targets an inhibitor protein called securin for destruction, which frees an enzyme called separase. Separase then cuts a specific subunit of the cohesin complex, the molecular glue that has been holding sister chromatids together since they were made. A key detail: separase requires chromosomal DNA itself as a cofactor, which means it can only cut cohesin that is actually bound to chromosomes, leaving any free-floating cohesin alone.¹²

Once the cohesin is cut, the sister chromatids separate and move toward opposite poles. This movement has two distinct components, sometimes called anaphase A and anaphase B. In anaphase A, the chromosomes are reeled toward the poles as the microtubules connecting them shorten. In anaphase B, the poles themselves move farther apart, elongating the entire spindle.¹³

The shortening in anaphase A comes from two sources. Roughly 60 to 70 percent of the chromosome-to-pole movement in human cells is driven by microtubule disassembly at the plus end (the end attached to the kinetochore), while the remaining 30 to 40 percent comes from a process called poleward flux, where the entire microtubule slides toward and is chewed up at the pole end.¹⁴ The combined effect moves chromosomes at a speed of a few micrometers per minute, which does not sound fast until you consider that the entire cell is only tens of micrometers across.

Cytokinesis and Organelle Inheritance

Mitosis technically refers to the division of the nucleus. The physical splitting of the cell into two daughter cells is a separate process called cytokinesis, though it overlaps with late anaphase. In animal cells, a ring of contractile proteins, mainly actin and myosin, assembles just beneath the cell membrane at the cell equator. This ring pinches inward like a drawstring bag, creating a cleavage furrow that deepens until the two halves separate.

An underappreciated part of cell division is how each daughter cell ends up with a workable set of organelles. The endoplasmic reticulum and the Golgi complex, for example, fragment into small vesicles during mitosis, probably because the normal balance between membrane budding and fusion shifts. By anaphase, these vesicle fragments are so widely dispersed that when the cell pinches in half, each daughter gets a roughly equal share without needing any dedicated sorting machinery.¹⁵ Mitochondria, which exist in large numbers, are similarly distributed by their sheer abundance and dispersal throughout the cytoplasm. The system is remarkably robust: rather than carefully counting out organelles, the cell just makes sure they are spread around and trusts the split to be fair enough.

Why the Cell Rounds Up

If you have ever watched time-lapse microscopy of dividing cells, you have probably noticed that cells become noticeably rounder as they enter mitosis. This is not a passive side effect. The cell actively remodels its outer skeleton, reorganizing a layer of actin and myosin just beneath the membrane and taking up water through osmotic swelling. These changes stiffen the cell and push it into a spherical shape, which gives the mitotic spindle enough room to assemble correctly.¹⁶

This rounding turns out to matter a great deal. When cells are physically confined and unable to round up, they develop severe problems with spindle assembly and frequently fail to divide properly.¹⁶ This is relevant in dense tissues, tumors, and laboratory settings where cells are grown on rigid surfaces. The geometry of the cell is not just a consequence of division; it is an active requirement for it.

When Mitosis Goes Wrong

Despite the spindle assembly checkpoint, errors do slip through. One common defect is called merotelic attachment, where a single kinetochore gets connected to microtubules from both poles instead of just one. These chromosomes often appear as “lagging chromosomes” that trail behind during anaphase while their sisters move to the poles.¹⁷ Merotelic attachments are particularly dangerous because the kinetochore is technically attached to the spindle, so the checkpoint does not detect the problem.

A lagging chromosome that fails to join either main nucleus can end up enclosed in a small, separate membrane bubble called a micronucleus. Chromosomes trapped in micronuclei are poorly replicated, prone to DNA damage, and tend to missegregate again in the next division. This creates a self-reinforcing cycle of errors: a chromosome that was missegregated once is more likely to missegregate again, contributing to the kind of chromosomal instability that is a hallmark of many cancers.¹⁸

How Cancer Drugs Exploit Mitosis

The dependence of mitosis on microtubule dynamics makes the process a target for chemotherapy. Taxanes, a class of drugs that includes paclitaxel, work by binding to microtubules and stabilizing them so they cannot shrink. This locks the spindle in place and prevents normal chromosome segregation, triggering prolonged mitotic arrest and, in many cases, cell death through apoptosis.¹⁹

The story is more nuanced than “drug freezes the spindle and the cell dies,” though. Research has shown that the concentrations of paclitaxel actually achieved inside tumors are often too low to completely halt mitosis. Instead of frozen spindles, the drug at lower concentrations causes abnormal multipolar divisions, where the cell attempts to split into three or more parts instead of two.²⁰ These scrambled divisions are just as lethal to the cancer cells, but through a different mechanism than the textbook explanation of mitotic arrest. This distinction matters for understanding why some tumors develop resistance: if the primary killing mechanism in real tumors is multipolar division rather than mitotic arrest, then resistance mechanisms that prevent arrest might not actually help the cancer cell survive.

Not All Organisms Divide the Same Way

The mitosis described so far, with the nuclear envelope breaking down and an extranuclear spindle pulling chromosomes apart, is what happens in animal cells. But this “open” mitosis is not universal. Many organisms, including most fungi and many single-celled organisms, divide by “closed” mitosis, where the nuclear envelope stays intact and the spindle forms entirely inside the nucleus. Evolutionary reconstruction suggests that the last common ancestor of all organisms with nucleated cells actually used closed mitosis with intranuclear spindles, meaning open mitosis evolved later, not the other way around.²¹

Plants have their own twist. Land plant cells lack centrosomes entirely, yet still build perfectly functional mitotic spindles. Different plant lineages use different workarounds. Liverworts concentrate a protein called gamma-tubulin into distinct organizing points that radiate microtubules. Mosses organize gamma-tubulin in clumps around the nuclear envelope. Hornworts use an unusual arrangement of two perpendicular microtubule systems to initiate spindle formation.²² Plants also replace the animal-style cleavage furrow with a completely different structure called a phragmoplast, which builds a new cell wall between the daughter cells from the inside out. The core logic of mitosis, duplicate the chromosomes, separate them faithfully, divide the cell, is ancient and conserved, but evolution has found remarkably varied mechanical solutions for carrying it out.