Sister chromatids separate during anaphase, the fourth stage of mitosis. The trigger is an enzyme called separase, which cuts through the protein rings holding the two copies of each chromosome together at the boundary between metaphase and anaphase. Though anaphase itself lasts only a few minutes in most cells, the preparation for this moment involves a sophisticated surveillance system and multiple physical forces working in concert.
What Holds Chromatids Together and What Cuts Them Apart
After DNA replication, the two identical copies of each chromosome are linked by ring-shaped protein complexes called cohesins. These molecular handcuffs encircle both DNA strands, keeping the sister chromatids paired from the time they are made through the early stages of mitosis. Without them, chromatids would drift apart long before the cell was ready to divide.
The chromatids stay joined until the metaphase-to-anaphase transition. At that point, separase activates and cleaves a key subunit of the cohesin ring known as SCC1.1PubMed Central. DNA-dependent cohesin cleavage by separase Once this subunit is cut, the ring falls open and the sisters are free to move apart.2PubMed. Dual inhibition of sister chromatid separation at metaphase This isn’t a gentle untangling; it’s a decisive, irreversible cleavage event.
Research in human cells confirmed how essential this step is. Scientists identified two separase cleavage sites on SCC1 and engineered mutant versions that could not be cut. Cells expressing these uncleavable mutants could not separate their chromatids and failed to complete division.3PubMed. Cohesin cleavage by separase required for anaphase and cytokinesis in human cells So cohesin cleavage isn’t just one of several ways cells can pull chromatids apart. It is the mechanism.
Before the cell reaches anaphase, most cohesin is actually removed from chromosome arms during earlier mitotic stages through a separase-independent pathway. A guardian protein called shugoshin protects the remaining cohesin at centromeres, the regions where chromosomes are connected to the spindle apparatus.4PubMed Central. Shugoshin protects cohesin complexes at centromeres This ensures that chromatids stay linked at their centers right up until separase delivers the final cut, even though the arms have already been loosened.
The Safety Check That Prevents Premature Separation
The cell doesn’t just activate separase on a timer. It uses a surveillance system called the spindle assembly checkpoint to verify that every single chromosome is properly attached to spindle fibers from opposite poles before green-lighting separation. Even one improperly attached chromosome is enough to stall the whole process.
As long as any kinetochore — the protein structure at a chromosome’s centromere that grabs spindle microtubules — is unattached or incorrectly attached, the checkpoint produces a “wait” signal. This signal blocks the activation of a molecular complex called the APC/C, which would otherwise tag securin, the inhibitor of separase, for destruction.5PubMed Central. Mutual regulation between the spindle checkpoint and APC/C With securin intact, separase stays locked in its inactive form and anaphase cannot begin.6Computational and Structural Biotechnology Journal. Spindle assembly checkpoint is sufficient for complete Cdc20 sequestering in mitotic control
The question of exactly what satisfies the checkpoint has a surprisingly long and contentious history. Some experiments showed that microtubule attachment alone was enough to silence the signal, while others, particularly in meiotic cells, showed that tension across paired kinetochores was the critical requirement. Work in maize resolved this by showing both sides were right, in different contexts: during mitosis, attachment was what mattered, while during meiosis in the same organism, tension was the key trigger.7Cell. Waiting for Anaphase: Mad2 and the Spindle Assembly Checkpoint
Once all kinetochores are properly attached and under tension, the checkpoint is silenced rapidly. Kinetochore stretching promotes the recruitment of a phosphatase called PP1, which flips the checkpoint off and enables a fast transition into anaphase.8Current Biology. Kinetochore Stretching Governs Spindle Assembly Checkpoint Silencing and Mitotic Fidelity The result is an all-or-nothing switch: securin is destroyed across the cell at once, separase activates everywhere simultaneously, and every chromosome separates in concert.
How Chromosomes Physically Move to Opposite Poles
Cutting the cohesin rings frees the chromatids, but they still need to travel to opposite ends of the cell. Two overlapping force-generating mechanisms handle this, and researchers call them anaphase A and anaphase B.
In anaphase A, the microtubules connecting each kinetochore to its spindle pole shorten by depolymerizing. As the tubule shrinks, it reels the chromatid toward the pole.9PubMed. Microtubule depolymerization at kinetochores restricts anaphase spindle elongation This is the most intuitive part of the process: the rope gets shorter, and the cargo moves.
In anaphase B, the spindle itself elongates. Antiparallel microtubules in the spindle’s central zone slide apart, pushing the two poles farther from each other. Motors anchored at the cell’s outer surface also pull on microtubules radiating from each pole, adding an outward tug.10PubMed Central. Anaphase B The combined effect is that the two clusters of separated chromatids end up at a comfortable distance from each other, ready for the cell to divide between them.
How much each mechanism contributes varies by organism and cell type. In some cells, nearly all the chromatid-to-pole movement comes from microtubule shortening. In others, spindle elongation does the heavy lifting. Most animal cells use both simultaneously, with anaphase A initiating first and anaphase B ramping up shortly after.
Separation Unfolds in Stages, Not a Single Snap
Anaphase is sometimes depicted as a single dramatic moment — the chromosomes split and fly apart. Detailed imaging tells a more nuanced story. Researchers found that the arms of sister chromatids begin pulling apart in a broad, parallel movement about a minute before the centromeres visibly separate and begin their poleward journey.11PubMed Central. Sister chromatids separate during anaphase in a three-stage program as directed by interaxis bridges
After that initial arm separation, centromere splitting begins, followed by a zipper-like peeling of the arms from the centromere outward toward their tips. So the process is sequential: first a global loosening of the arms, then centromere splitting, then directional separation moving distally along the chromosome. The geometry of the chromosome arms and the distribution of residual physical connections between them shape when and where each part comes apart. This three-stage choreography is something textbook diagrams rarely capture.
DNA Tangles Are Cleared Before and During Separation
Cohesin isn’t the only thing keeping sister chromatids physically connected. During DNA replication, the two new DNA molecules can become intertwined, creating topological links called catenanes. Most of these tangles are resolved during and after replication by enzymes called topoisomerases, which cut one DNA strand, pass the other through, and reseal the break.
Some catenanes persist into mitosis, particularly around centromeric regions where DNA is tightly packed. If these tangles aren’t cleared before anaphase, chromatids can form thin bridges as they try to move apart, risking DNA breakage or uneven distribution. The condensin complex, which compacts chromosomes during mitosis, cooperates with topoisomerase II to clear these remaining links.12PubMed Central. Condensin aids sister chromatid decatenation by topoisomerase II This is a behind-the-scenes step that rarely gets mentioned alongside cohesin cleavage but is just as necessary for clean separation.
How Meiosis Handles Chromatid Separation Differently
In mitosis, sister chromatids separate during one anaphase. Meiosis, the specialized division that produces eggs and sperm, takes a fundamentally different approach. It splits the genome across two successive rounds of division, and sister chromatid separation only occurs in the second one.
During meiosis I, homologous chromosomes — the maternal and paternal copies — separate, but sister chromatids remain glued together. This works because centromeric cohesin is protected by shugoshin and an associated phosphatase during the first division. Arm cohesin is removed to let homologs pull apart, but the centromeric connection persists so sisters stay paired.13PubMed Central. Sister chromatid segregation in meiosis II: deprotection through phosphorylation
For meiosis II, that protection must be actively removed. The centromeric cohesin is deprotected through phosphorylation, making it accessible to separase, which then cleaves it so sister chromatids can finally move apart.13PubMed Central. Sister chromatid segregation in meiosis II: deprotection through phosphorylation This two-step removal of cohesin is what allows the distinctive meiotic segregation pattern: homologs first, then sisters.
The added complexity of meiosis helps explain why meiotic errors are more common. The cell has to maintain cohesin protection across the gap between the two divisions, and if that protection degrades prematurely — something more likely as eggs age — chromatids can separate at the wrong time, producing cells with too many or too few chromosomes.
Cancer Drugs That Exploit the Separation Machinery
Because cells are especially vulnerable during mitosis, several classes of cancer drugs target the division machinery. Taxanes like docetaxel stabilize microtubules, preventing the normal dynamics needed for spindle function. This activates the spindle assembly checkpoint and traps cancer cells in a prolonged mitotic arrest.
When cells are stuck at metaphase this way, securin and cyclin B accumulate because the APC/C remains blocked and cannot tag them for destruction. Research in lung cancer cell models found that combining a taxane with an HSP90 inhibitor amplified this accumulation, including a phosphorylated form of securin associated with sustained checkpoint activation.14PLOS ONE. HSP90 Inhibition Enhances Antimitotic Drug-Induced Mitotic Arrest and Cell Death in Preclinical Models of Non-Small Cell Lung Cancer The combination increased both mitotic arrest and cell death compared to either drug alone.
The logic is worth appreciating: the very checkpoint that evolved to protect genomic integrity becomes a weapon against rapidly dividing cancer cells. If you can lock cells in the pre-anaphase state for long enough, they activate their own death pathways. Some newer experimental approaches try the opposite strategy, deliberately weakening the checkpoint to push cancer cells through a sloppy division that produces so many chromosome errors the cells cannot survive. Both strategies depend on understanding the same anaphase machinery in detail.
Watching Cohesin Come and Go in Real Time
For decades, understanding cohesin behavior required fixing cells at specific moments and staining them, which gave useful snapshots but couldn’t capture the full cycle. A newer approach uses a fluorescence technique that makes cohesin molecules glow only when they are fully assembled on chromosomes, letting researchers watch the entire process of cohesin loading, maintenance, and removal in living cells.15Life Science Alliance. BiFCo: visualizing cohesin assembly/disassembly cycle in living cells
Live-imaging studies using this technique confirm that cohesin removal from mitotic chromosomes at the metaphase-to-anaphase transition happens rapidly and is tightly controlled by mitotic regulators, including the separase-dependent cleavage pathway.15Life Science Alliance. BiFCo: visualizing cohesin assembly/disassembly cycle in living cells Being able to watch this in real time has helped clarify questions about when and where cohesin is released versus cleaved, and opens the door to studying how different mutations or drugs alter the process in individual cells rather than in averaged populations. For a field that long relied on fixed snapshots, the shift to live visualization has been a quiet but meaningful advance.