Chromatids are pulled apart during anaphase, the phase of cell division that immediately follows metaphase. Once every chromosome is properly attached to the spindle fibers, an enzyme called separase cuts the protein glue holding sister chromatids together, and they are hauled toward opposite ends of the cell. The process looks simple under a microscope, but the molecular choreography behind it is remarkably precise, and the consequences of getting it wrong range from cell death to cancer.
What Happens During Anaphase
Anaphase itself breaks down into two overlapping stages. During the first stage, often called anaphase A, the fibers connecting each chromatid to a spindle pole shorten by losing subunits from their ends. This pulls the separated chromatids toward the poles of the cell.1PubMed Central. Anaphase A: Disassembling Microtubules Move Chromosomes toward Spindle Poles During the second stage, anaphase B, the spindle itself elongates, pushing the two poles farther apart and increasing the distance between the now-separated chromatids.2PubMed Central. Anaphase B: Long-standing models meet new concepts Both stages work together to make sure each new daughter cell gets a full, identical set of chromosomes.
Recent work in mammalian cells shows the physical separation is more nuanced than textbooks typically depict. Rather than all chromatids popping apart simultaneously, the process follows a three-stage pathway. First, sister chromatid arms separate in parallel along their lengths, pushed apart by forces within the chromatin itself. Then separation proceeds step by step from the centromere region toward the chromosome tips. Finally, the very ends of the chromosomes remain invisibly linked, likely by tangled DNA loops, until those links are resolved in the last moments of anaphase.3PubMed Central. Sister chromatids separate during anaphase in a three-stage program as directed by interaxis bridges An enzyme called topoisomerase IIα is needed to untangle those final DNA links near the centromere, which means separation depends on more than just mechanical pulling.
The Molecular Trigger That Starts It All
Throughout the earlier phases of division, sister chromatids are held together by a ring-shaped protein complex called cohesin. Cohesin essentially encircles both sister DNA molecules, keeping them physically paired so the cell can line them up properly at the center of the spindle. The moment anaphase begins, the cell activates an enzyme called separase, which cuts one of the key subunits of the cohesin ring.4PubMed Central. Secured cutting: controlling separase at the metaphase to anaphase transition. That cleavage destroys the ring structure, freeing the sister chromatids so the spindle can pull them apart.5PubMed. Dual inhibition of sister chromatid separation at metaphase
Separase is conserved across essentially all organisms with nuclei, from yeast to humans, which tells you how fundamental this step is to life. While separase cleavage is thought to remove all cohesin from chromosomes to initiate anaphase, the timing and completeness of that removal can have surprising consequences for how the DNA is organized after separation.6PubMed Central. Depletion or cleavage of cohesin during anaphase differentially affects chromatin structure and segregation The cell does not just sever a connection; it triggers a cascade that reorganizes how chromosomes are packaged as they move to their new homes.
Before separase can act, it has to be released from its own inhibitor, a protein called securin. Securin keeps separase locked in an inactive state throughout the earlier phases of division. Only when the cell’s internal signaling machinery tags securin for destruction does separase become free to do its job. This two-layered control system, securin holding back separase which in turn holds sister chromatids together, means the cell has multiple opportunities to prevent premature separation.7PubMed Central. Separase and Roads to Disengage Sister Chromatids during Anaphase
The Safety Checkpoint That Prevents Mistakes
Cells do not simply barrel from metaphase into anaphase on a timer. A surveillance system called the spindle assembly checkpoint monitors whether every single chromosome has properly attached to spindle fibers from both poles of the cell. If even one chromosome is unattached or incorrectly attached, the checkpoint sends a “wait” signal that blocks the destruction of securin, which means separase stays inactive and the chromatids remain glued together.8eLife. Spindle assembly checkpoint-dependent mitotic delay is required for cell division in absence of centrosomes Only when all chromosomes achieve the correct attachment does the checkpoint lift, securin gets destroyed, separase activates, and anaphase begins.
The checkpoint is remarkably sensitive. A cell with 46 chromosomes, all of which need proper two-sided attachment, will halt division if just one of those connections is missing or wrong. In experiments where centrosomes, the structures that help organize the spindle poles, are removed from cells, the checkpoint enforces a noticeably longer delay to give the spindle time to form properly before allowing anaphase to proceed. This demonstrates that the checkpoint is not a passive gate but an active response system that adjusts to the cell’s circumstances.
Defects in this checkpoint are linked to cancer. When the checkpoint is weakened or deregulated, cells can enter anaphase before chromosomes are properly attached, leading to daughter cells with the wrong number of chromosomes. This condition, called aneuploidy, is one of the hallmarks of tumor cells. Because the checkpoint is so central to accurate division, researchers have explored its components as potential targets for cancer therapy, particularly in breast cancer where checkpoint defects are frequently reported.9PubMed. Targeting the spindle assembly checkpoint for breast cancer treatment
How Much Force Does It Take to Move a Chromosome?
The forces involved in pulling chromatids apart are tiny by everyday standards but massive relative to the size of the objects being moved. The spindle generates forces on the pico-Newton scale through a combination of dynamic microtubule behavior and motor proteins.10PubMed Central. Mitotic force generators and chromosome segregation Under normal conditions, a chromatid gliding through the cell’s interior faces so little resistance that only a minuscule force is needed to keep it moving. But the spindle can produce far more force than what is needed for routine movement.
Classic experiments using tiny calibrated glass needles to push back against moving chromosomes found that the spindle’s maximum force output was roughly 10,000 times greater than what is needed just to overcome the viscous drag of the cell’s interior.11PubMed Central. Measurements of the force produced by the mitotic spindle in anaphase Chromosomes kept moving at their normal speed until the opposing force became substantial, then velocity dropped steeply. The enormous surplus of force capacity means the spindle has a wide safety margin. Even if something partially obstructs a chromosome’s path, the machinery can usually overpower it.
Where does this force come from mechanically? The shrinking end of a microtubule, as it loses subunits, can physically pull along the structure attached to its tip, a bit like reeling in a fishing line by shortening the rod. But research suggests that this shrinking alone is not enough; electrostatic attractions between the chromosome’s attachment site and the microtubule surface also contribute to keeping the chromosome moving in the right direction.12PubMed Central. A driving and coupling “Pac-Man” mechanism for chromosome poleward translocation in anaphase A Specialized motor proteins belonging to the kinesin family also play a role by actively promoting microtubule shrinkage at both ends, ensuring that chromatids are driven steadily poleward.13PubMed Central. The Drosophila kinesin-13, KLP59D, impacts Pacman- and Flux-based chromosome movement
Anaphase in Meiosis Is a Two-Round Affair
Everything described so far applies to mitosis, the kind of cell division that produces genetically identical daughter cells for growth and repair. Meiosis, the division that produces eggs and sperm, puts a twist on the story by splitting into two rounds, and the chromatid-pulling step happens differently in each.
In the first round, meiosis I, what gets pulled apart is not sister chromatids but homologous chromosome pairs. The spindle fibers attach to kinetochores in a way that tugs one whole chromosome (still consisting of two joined sister chromatids) toward each pole, separating the pair of homologs.14PubMed Central. Meiosis I chromosome segregation is established through regulation of microtubule–kinetochore interactions Cohesin at the centromere region is specifically protected from separase during this round, which is what keeps sister chromatids together even as the homologs separate.
The second round, meiosis II, resembles mitosis. The protective signal at the centromere is removed, allowing separase to cleave the remaining cohesin, and sister chromatids are finally pulled apart toward opposite poles.15PubMed Central. Sister chromatid segregation in meiosis II: deprotection through phosphorylation This two-step removal of cohesin, first from chromosome arms in meiosis I and then from centromeres in meiosis II, is what allows the cell to halve its chromosome number across two successive divisions. Getting this sequence wrong can lead to eggs or sperm with too many or too few chromosomes, which is a common cause of miscarriages and conditions like Down syndrome.
What Happens When Chromatids Do Not Separate Properly
When sister chromatids fail to come apart during anaphase, a situation called nondisjunction, the consequences can be severe. In yeast studies designed to block separation of a single chromosome, the cell does not simply pause and fix the problem. Instead, it barrels ahead with division, and the physical force of the cell pinching in two actually severs the stuck chromosome at or near a repetitive gene region. One broken end winds up in each daughter cell, creating a type of DNA damage that is not detected until the daughter cells reach the next round of their cell cycle.16PubMed Central. Nondisjunction of a Single Chromosome Leads to Breakage and Activation of DNA Damage Checkpoint in G2
This finding highlights something important: the force of cell division can physically break DNA. Chromosomes are not indestructible cables. When they are caught in the wrong place at the wrong time, the cell’s machinery will damage them rather than halt indefinitely. The resulting broken DNA can trigger repair pathways, but those repair pathways are error-prone. Misrepaired breaks can lead to chromosome rearrangements, loss of genetic material, or fusion of chromosome fragments, all of which are commonly seen in cancer cells.
In human cells, chromosomes that lag behind during anaphase because of attachment errors can end up enclosed in small, separate compartments called micronuclei rather than being incorporated into the main nucleus. The DNA trapped in micronuclei is prone to further damage and can undergo a catastrophic fragmentation event called chromothripsis, where a chromosome is shattered and reassembled in a scrambled order. Chromothripsis has been identified as a driver of certain aggressive cancers, linking a single failed anaphase event to massive genomic instability.
Separation Starts Before Anaphase
While the dramatic pulling apart of chromatids is the hallmark of anaphase, the physical process of separating sister chromatids actually begins much earlier than most people realize. Live imaging of marked chromosome regions in cells shows that sister chromatids start cycling between a joined and partially separated state during the G2 phase, the gap period before mitosis even begins. This partial resolution increases in frequency as cells approach division, with full separation of the marked region completing during prophase, the earliest phase of mitosis.17PubMed Central. Live imaging of marked chromosome regions reveals their dynamic resolution and compaction in mitosis
These observations reveal that the sister chromatids are not rigidly locked together right up until anaphase. They flicker between resolved and unresolved states in the hours before division, with the interchange happening over intervals as short as a few minutes. The cohesin rings keep them generally paired, but the DNA between cohesion points can breathe apart and come back together, almost like a zipper being partially opened and closed. Full commitment to separation, the irreversible pulling apart, still waits for anaphase and the activation of separase. But the groundwork is laid well before that.
Environmental Chemicals That Disrupt the Process
The spindle fibers that pull chromatids apart are built from a protein called tubulin, and anything that interferes with tubulin assembly or stability can derail anaphase. Certain environmental pollutants are known to do exactly this. Hexavalent chromium, a toxic form of the metal found in industrial waste and contaminated groundwater, has been shown to disrupt microtubule arrays at every stage of mitosis, including anaphase, in plant cells. Depending on the species, the microtubules were either broken down and replaced by abnormal tubulin structures, or bundled together in unusual ways. In both cases, the result was the same: chromosomes could not properly gather at the cell equator during metaphase or separate during anaphase.18PubMed. Aberration of mitosis by hexavalent chromium in some Fabaceae members is mediated by species-specific microtubule disruption
This principle extends well beyond chromium. Many cancer chemotherapy drugs work by deliberately targeting microtubules. Drugs like taxol stabilize microtubules so they cannot shorten, trapping cells in mitosis because the spindle cannot function properly. Vinca alkaloids do the opposite, preventing microtubules from assembling in the first place. Either way, the cell is unable to complete anaphase, and a stuck mitosis typically leads to cell death. The same vulnerability that makes dividing cells sensitive to environmental toxins is exploited therapeutically to kill rapidly dividing tumor cells. It is a narrow window: too little disruption and the cancer cells power through, too much and healthy dividing cells in the gut and bone marrow are destroyed as collateral damage.
Outside the clinic, substances like colchicine, extracted from autumn crocus plants, have been used in agriculture to intentionally block chromosome separation. When cells are treated with colchicine, they enter mitosis and duplicate their chromosomes but cannot pull them apart, resulting in cells with doubled chromosome sets. Plant breeders use this trick to create polyploid varieties, organisms with extra sets of chromosomes that often grow larger fruits or more vigorous stems. The entire seedless watermelon industry depends on a version of this technique: crossing a normal plant with a colchicine-treated polyploid produces offspring that cannot complete meiosis properly, leading to sterile, seedless fruit.
Why Anaphase Errors Accumulate with Age
In women, egg cells begin meiosis before birth and then pause, sometimes for decades, before completing division. The cohesin complexes that hold sister chromatids together during that long pause gradually deteriorate over time. Because cells do not replenish cohesin once the original meiotic cohesion is established, older eggs have progressively weaker chromosome connections. This age-related cohesin loss is a leading explanation for why chromosome separation errors during meiosis, and consequently conditions like trisomy 21, become dramatically more common as maternal age increases.
The problem is not limited to eggs. Somatic cells in aging tissues also show increased rates of chromosome missegregation during mitosis. Weakened checkpoint function, accumulated damage to spindle-organizing structures, and gradual loss of cohesin maintenance all contribute. Aneuploidy in non-reproductive tissues has been linked to age-related conditions including neurodegeneration and immune system decline, though untangling cause from effect in aging biology remains an active area of research. The fidelity of anaphase, it turns out, is not just a concern for embryos and tumors but a factor that quietly degrades across a lifetime.