During anaphase, sister chromatids separate at the centromere and move to opposite poles of the cell. This is the defining event of anaphase in mitosis, but several other coordinated processes happen simultaneously: spindle fibers shorten, the cell begins to elongate, and the machinery for cell division starts assembling at the midline. The whole phase lasts only a few minutes, yet errors here can produce cells with the wrong number of chromosomes.
How Sister Chromatids Separate
Throughout earlier phases of mitosis, sister chromatids are held together by ring-shaped protein complexes called cohesins. These molecular rings physically encircle the two copies of each chromosome, keeping them paired. When the cell is ready to enter anaphase, an enzyme called separase activates and cuts a key component of the cohesin ring. This cleavage irreversibly opens the ring and frees each chromatid to move independently.
Separase doesn’t just switch on by itself. For most of mitosis, it’s locked in an inactive state by two different inhibitors. Both of those inhibitors are tagged for destruction by a large protein complex called the anaphase-promoting complex (APC/C). The APC/C marks them with a molecular “dispose” signal, and the cell’s recycling machinery breaks them down. Anaphase begins once roughly half the inhibitor protein (securin) has been degraded. That’s enough to unleash separase and trigger chromatid separation.
Anaphase A: Chromatids Move to the Poles
Anaphase unfolds in two overlapping sub-phases. During anaphase A, the separated chromatids travel toward opposite ends of the cell. This movement depends on the kinetochore fibers, the bundles of microtubules that connect each chromatid’s attachment point to a spindle pole. As these fibers shorten, they reel chromosomes poleward.
The shortening happens through disassembly of the microtubule itself. Protein subunits are lost from the ends of the fiber, progressively shrinking it. In many cell types, there’s also a process called “flux,” where the entire microtubule slides toward the pole while losing subunits at its far end, like a conveyor belt that’s also getting shorter. Motor proteins at the kinetochore help maintain the grip on shrinking microtubules so that chromosomes don’t detach mid-journey.
Anaphase B: The Spindle Elongates
While chromatids are being pulled inward during anaphase A, the spindle poles themselves move apart during anaphase B, stretching the cell. This elongation is driven by a different set of microtubules. Overlapping microtubules from opposite poles slide against each other in the cell’s center, pushing the poles further apart. Motor proteins anchored at the cell’s outer edge also pull on microtubules radiating from each pole, tugging the poles outward.
Recent research has refined the picture of how these two sub-phases interact. Studies in yeast found that it’s actually the sliding of central spindle microtubules, not just kinetochore fiber shortening, that drives chromosome separation. Increased microtubule disassembly at kinetochores slowed pole separation without changing how quickly chromosomes moved apart. In other words, the antiparallel sliding force in the cell’s middle zone does much of the heavy lifting for getting chromosomes to their final destinations.
Early Signs of Cell Division
Even before anaphase ends, the cell begins preparing to physically split in two. During late anaphase, signaling proteins from the spindle midzone (the region between the separating chromosome groups) direct the placement of the future cleavage furrow at the cell’s equator. One of the earliest markers to arrive at this site is a protein called INCENP, which accumulates at the equatorial cortex before any visible pinching of the cell membrane and before the motor protein myosin concentrates there. By the time the chromatids have moved more than about 4 micrometers apart, furrowing is underway and myosin joins INCENP at the cleavage site. This handoff from anaphase into cytokinesis is seamless: the same spindle structures that separated the chromosomes now guide where the cell will divide.
Anaphase in Meiosis
Meiosis involves two rounds of division, and anaphase looks different in each. During anaphase I, it’s homologous chromosomes (matched pairs, one from each parent) that separate and move to opposite poles. The sister chromatids within each chromosome stay joined. During anaphase II, the process closely resembles mitotic anaphase: sister chromatids separate and move apart. The result of meiosis I is two cells, each with one complete set of chromosomes still made of paired chromatids. After anaphase II completes, four cells exist, each with a single copy of every chromosome.
What Happens When Anaphase Goes Wrong
The most consequential error during anaphase is nondisjunction, when chromosomes fail to separate properly. Instead of one copy going to each pole, both copies get pulled to the same side. In mitosis, this produces two abnormal daughter cells: one with 47 chromosomes and one with 45 (instead of the normal 46). These cells are called aneuploid.
Nondisjunction can result from inactivation of separase (meaning cohesins never get cut), problems with the enzymes that untangle intertwined DNA, or defects in the protein complexes that compact chromosomes before division. When nondisjunction occurs during meiosis I, all four resulting sex cells end up with abnormal chromosome counts: two with an extra chromosome and two missing one. When it occurs during meiosis II, two of the four cells are normal and two are aneuploid. Aneuploidy from meiotic errors is the cause of conditions like Down syndrome (three copies of chromosome 21) and Turner syndrome (a single X chromosome).
The cell has a built-in safeguard called the spindle assembly checkpoint that tries to prevent these mistakes. This checkpoint holds the cell at the boundary before anaphase until every chromosome is properly attached to spindle fibers from both poles. Only when all attachments are verified does the checkpoint release its hold, allowing the APC/C to degrade securin and launch the separation cascade. When this checkpoint fails or is overridden, nondisjunction becomes far more likely.