Anaphase is the stage of cell division when duplicated chromosomes physically separate and move to opposite ends of the cell, setting the stage for two genetically identical daughter cells. It typically lasts only a few minutes, yet it requires an extraordinary coordination of molecular signals, mechanical forces, and quality-control systems. What looks through a microscope like a simple pulling-apart is one of the most tightly regulated events in all of biology, and when it goes wrong, the consequences range from failed cell division to cancer.
What Triggers Anaphase
A cell does not drift into anaphase. The transition is gated by a molecular checkpoint that essentially asks one question: is every chromosome properly attached to the spindle? Only when the answer is yes does a large enzyme complex called the anaphase-promoting complex (APC/C) become active. The APC/C is a kind of molecular tagging machine. It attaches small protein labels to key regulatory molecules, marking them for destruction. The timing and location of APC/C activity are themselves controlled by a web of activator proteins, inhibitor proteins, and enzymes that add or remove chemical tags, all working together to ensure that the switch flips at precisely the right moment.1Nature Reviews Molecular Cell Biology. Spatiotemporal regulation of the anaphase-promoting complex in mitosis
The most dramatic downstream effect of APC/C activation is the unleashing of an enzyme called separase. Until this point, each pair of sister chromatids has been physically glued together by ring-shaped protein complexes called cohesins. Separase is a protease that cuts a specific subunit of the cohesin ring, dissolving the glue and freeing the sister chromatids to move apart. Interestingly, separase requires the chromosomal DNA itself as a kind of cofactor. It can only cut cohesin that is actually wrapped around chromosomes, which prevents the enzyme from randomly destroying cohesin floating elsewhere in the cell.2PubMed Central. DNA-dependent cohesin cleavage by separase Structural studies have also shown that a chemical modification of cohesin by another enzyme, polo-like kinase 1, can enhance the speed of cleavage, adding yet another layer of fine-tuning.3PubMed Central. Structural basis of cohesin cleavage by separase
The Two Sub-Stages of Chromosome Movement
Once the chromatids are free, they need to get to opposite poles of the cell. This happens in two overlapping but mechanically distinct phases that researchers call anaphase A and anaphase B.
In anaphase A, the separated chromatids move toward the spindle poles. The engine behind this movement is the shortening of the microtubule fibers that connect each chromatid’s attachment point (the kinetochore) to a pole. Live-cell imaging has shown that these fibers shorten mainly by shedding protein subunits from the end attached to the kinetochore. In many cell types, a second mechanism called “flux” also contributes: the entire microtubule slides steadily poleward while losing subunits from the pole-facing end. The combination of these two processes reels the chromatids in.4PubMed Central. Anaphase A: Disassembling Microtubules Move Chromosomes toward Spindle Poles
In anaphase B, the poles themselves move farther apart, stretching the entire spindle and pushing the two chromosome groups even further from each other. This elongation relies on a different set of forces. Motor proteins in the middle of the spindle slide antiparallel microtubules outward, while other motors anchored at the cell’s edge pull on microtubules radiating from each pole. At the same time, new tubulin subunits are added to the plus ends of microtubules in the overlap zone, and subunits are removed from minus ends at the poles, so the spindle can grow in length even as individual fibers are turning over.5PubMed Central. Anaphase B The relative contribution of anaphase A versus B varies considerably among organisms. In some cells, nearly all the separation comes from chromosome-to-pole movement; in others, pole-to-pole elongation does most of the work.
Building the Midzone and Preparing for Cytokinesis
As the chromatids retreat toward opposite poles, the region between them does not simply empty out. Instead, a dense bundle of microtubules called the spindle midzone assembles in the space the chromosomes just vacated. The midzone serves as a signaling platform that tells the cell where to pinch in half during the final step of division, cytokinesis.
A key organizer of this structure is a protein called PRC1, which bundles overlapping microtubules into tight antiparallel arrays. In mouse egg cells, depleting PRC1 does not prevent the chromosomes from separating, but it completely blocks midzone and midbody formation. Without those structures, the cell cannot complete cytokinesis and fails to divide properly.6PubMed. PRC1 is a critical regulator for anaphase spindle midzone assembly and cytokinesis in mouse oocyte meiosis This illustrates an underappreciated point: anaphase is not just about separating chromosomes. It is also the window during which the cell lays down the infrastructure needed for everything that comes next.
When Separation Goes Wrong
Even with all these safeguards, anaphase errors happen, and they can have serious consequences. The most common visible defect is a lagging chromosome: one or more chromatids that linger near the center of the cell while their siblings have already reached the poles. Most lagging chromosomes trace back to a specific attachment problem called merotely, where a single kinetochore is connected to microtubules from both poles instead of just one.7PubMed Central. Chromosome missegregation in human cells arises through specific types of kinetochore-microtubule attachment errors Merotelic attachments are not caught by the spindle checkpoint because the kinetochore is technically under tension, so they slip through into anaphase.
Research into what perpetuates these errors has pointed to an enzyme called Aurora B, which normally corrects improper attachments before anaphase begins. When Aurora B activity is compromised, merotelic connections persist and chromosomes lag.8PLoS ONE. Chromosomal Instability by Inefficient Mps1 Auto-Activation Due to a Weakened Mitotic Checkpoint and Lagging Chromosomes Fortunately, cells have a backup. A surveillance mechanism centered in the spindle midzone uses an Aurora B activity gradient to stabilize kinetochore-microtubule connections on lagging chromosomes, giving the cell a second chance to correct the error even after anaphase has started. This same mechanism delays the reassembly of the nuclear envelope around stragglers, buying time for them to rejoin the correct chromosome mass and avoid being trapped in a micronucleus.9PubMed Central. An anaphase surveillance mechanism prevents micronuclei formation from frequent chromosome segregation errors
Ultrafine Bridges Between Separating Chromatids
Lagging chromosomes are visible under a standard microscope, but there is a subtler category of anaphase defect that escaped notice until researchers developed specialized staining techniques. Ultrafine anaphase bridges (UFBs) are impossibly thin threads of DNA that stretch between the two separating chromosome masses. They arise when topological entanglements between the DNA of sister chromatids are not fully resolved before anaphase begins.10PubMed. Detection of Ultrafine Anaphase Bridges UFBs are invisible to conventional chromosome stains because they contain so little chromatin, but they are surprisingly common.
Left unresolved, UFBs can cause chromosomes to missegregate. The cell’s primary tool for dealing with them is a DNA-processing enzyme called PICH, which localizes to UFBs and recruits additional proteins that unwind and disentangle the strands. PICH also plays broader roles in organizing chromosome architecture during mitosis, so its loss leads to widespread segregation problems.11PubMed Central. Regulation of mitotic chromosome architecture and resolution of ultrafine anaphase bridges by PICH The existence of UFBs is a reminder that DNA is a physical polymer with real topological constraints, not just an information-carrying molecule. The spindle must contend with tangled, intertwined strands that resist being pulled apart, and the cell deploys specialized enzymatic machinery to deal with this mechanical reality.
Anaphase in Meiosis
Anaphase looks broadly similar whether it occurs in ordinary cell division (mitosis) or in the specialized divisions that produce eggs and sperm (meiosis), but the underlying logic differs in important ways. In mitosis, anaphase separates identical sister chromatids. In the first meiotic division, anaphase separates homologous chromosomes, which are held together not by cohesin alone but also by physical crossover points called chiasmata. To accomplish this, the cell must orient each pair of homologs so that both sisters of one homolog face the same pole, exactly the opposite of what happens in mitosis. The cell also selectively protects cohesin near the centromere during meiosis I so that sisters stay together until the second division.12Cell. Un ménage à quatre: the molecular biology of chromosome segregation in meiosis
These extra requirements make meiotic anaphase more error-prone than its mitotic counterpart. In mammalian eggs in particular, the spindle is assembled without centrosomes, the organelles that normally serve as the main microtubule-organizing centers. Instead, the oocyte builds its spindle from scratch using a self-organizing process.13PubMed Central. Acentrosomal spindle assembly and chromosome segregation during oocyte meiosis Recent work has identified a liquid-like structure called the LISD (liquid-like meiotic spindle domain) that permeates the spindle poles in mammalian oocytes. The LISD concentrates microtubule-regulating factors and allows them to move rapidly within the spindle volume. Disrupting this structure leads to severe spindle defects, suggesting it serves as a reservoir that keeps the right proteins in the right place during acentrosomal spindle assembly.14PubMed Central. A liquid-like spindle domain promotes acentrosomal spindle assembly in mammalian oocytes The fragility of this system helps explain why chromosome segregation errors during oocyte meiosis are the leading cause of aneuploidy in human embryos.
What Happens Immediately After Anaphase
Anaphase transitions seamlessly into telophase and cytokinesis, but the events that begin during late anaphase are themselves worth understanding. One of the most important is the reassembly of the nuclear envelope. Once the chromosomes are safely at the poles and beginning to decondense, membrane fragments derived from the endoplasmic reticulum start wrapping around each chromosome mass, re-forming the double membrane of the nucleus. Nuclear pore complexes are assembled and inserted into the nascent membrane, and the chromatin gradually relaxes back into its extended interphase state. All of these processes must be coordinated with each other and with the declining activity of the main mitotic kinase, Cdk1.
The decline in Cdk1 activity during late anaphase is itself a tightly regulated event. A family of phosphatases, exemplified by Cdc14 in yeast, becomes active as Cdk1 drops and systematically reverses the chemical modifications that Cdk1 imposed to push the cell into mitosis. This reversal drives the cell out of the mitotic state and back toward interphase.15PubMed. Phospho-regulation of the Cdc14/Clp1 phosphatase delays late mitotic events in S. pombe The feedback is elegant: as Cdk1 falls, the phosphatase becomes more active, which further accelerates Cdk1 inactivation, creating a sharp, irreversible switch from mitosis to interphase.
Dividing More Than Just Chromosomes
Chromosomes get most of the attention during anaphase, but the cell also has to partition its organelles. Mitochondria, for example, must be distributed roughly equally between the two daughter cells to ensure both have enough energy-producing capacity. In mouse embryos, mitochondria form a ring-like arrangement during division. As the cell constricts during the transition from anaphase to telophase, the cytokinetic furrow effectively bisects this mitochondrial ring, parceling out roughly equal shares of mitochondria to each daughter cell.16PubMed Central. Redistribution of fragmented mitochondria ensures symmetric organelle partitioning and faithful chromosome segregation in mitotic mouse zygotes Other organelles like the endoplasmic reticulum and Golgi apparatus also fragment or redistribute during mitosis and are rebuilt in the daughter cells, though the mechanisms differ from organelle to organelle.
Why Cancer Researchers Care About Anaphase
The connection between anaphase errors and cancer is not abstract. Many tumors exhibit chromosomal instability, meaning their cells routinely gain or lose chromosomes during division. Lagging chromosomes, unresolved UFBs, and micronuclei formation are all hallmarks of this instability. Because anaphase is the moment when segregation fidelity is most directly tested, the proteins that drive and regulate it have become attractive drug targets.
One example is a motor protein called KIF11 (also known as kinesin spindle protein), which is essential for building the bipolar spindle in the first place. KIF11 is overexpressed in several cancer types, where it contributes to uncontrolled cell proliferation. Drugs that inhibit KIF11 prevent spindle formation and block cells from completing anaphase, effectively halting division.17PubMed Central. Kinesin Spindle Protein (KIF11) in Mitosis and Cancer This strategy is conceptually similar to how existing chemotherapy drugs like taxanes work: by disrupting the mitotic spindle, they trap dividing cancer cells at or near anaphase and trigger cell death. The challenge, as always, is selectivity. Normal cells divide too, and any drug that broadly disrupts anaphase will also harm healthy tissues. Research into more targeted approaches, such as exploiting the specific molecular vulnerabilities of cancer cells’ error-correction machinery, is an active and growing field.
How Anaphase Is Studied
Much of what we know about anaphase comes from live-cell fluorescence microscopy, where researchers tag spindle components or chromosomes with fluorescent markers and watch the process unfold in real time. This approach has been indispensable for measuring the speed of chromosome movement, the dynamics of microtubule turnover, and the timing of checkpoint satisfaction. High-resolution imaging has also made it possible to detect structures like ultrafine bridges that are invisible to conventional staining.
Model organisms have been equally important. Yeast cells divide quickly and are genetically tractable, making them ideal for dissecting the signaling pathways that control anaphase onset and mitotic exit. Frog egg extracts, which can assemble spindles in a test tube, have allowed biochemists to reconstitute and manipulate individual steps of the process. Mammalian cell lines, especially human cancer lines with known chromosomal instability, have been crucial for understanding how anaphase errors lead to the kinds of genome changes seen in tumors. Each system has its strengths, and the current picture of anaphase is a composite drawn from all of them.
Biophysical approaches have added another dimension. By modeling the mechanical forces on DNA during anaphase, researchers have shown that the intertwining of sister chromatid DNA creates real physical resistance to separation. Resolving those intertwinings depends on a coupling between the pulling force of the spindle and the enzymatic action of topoisomerases, the enzymes that cut and re-seal DNA strands to relieve tangles.18Biophysical Journal. Biophysical Journal When topoisomerase activity is blocked experimentally, chromatids cannot separate cleanly, confirming that anaphase is not just a matter of pulling harder but of enzymatically clearing the way.