During anaphase, the paired chromosomes in a dividing cell split apart and move to opposite ends of the cell. This is the moment when one cell’s genetic material physically becomes two separate sets, making it the most dramatic and mechanically complex stage of cell division. The entire process happens through a carefully coordinated sequence of molecular signals, protein activity, and physical forces.
The Signal That Starts It All
Anaphase begins abruptly. Before it starts, chromosomes sit lined up at the center of the cell (the metaphase plate), held together by ring-shaped protein complexes called cohesins. These molecular clamps keep each pair of sister chromatids joined at the center.
The trigger is a large protein machine called the anaphase-promoting complex. Once activated, it does two things simultaneously. First, it breaks down a protein called securin, which had been holding back an enzyme called separase. With securin gone, separase is free to cut open the cohesin rings. Second, the complex destroys a key molecule that keeps the cell in the division state, beginning the cell’s transition out of mitosis. The moment cohesin is cut, the sister chromatids release from each other and are now considered individual daughter chromosomes.
Anaphase A: Chromosomes Move Toward the Poles
Once separated, the daughter chromosomes don’t just float apart. They’re actively pulled toward opposite ends of the cell by the spindle fibers attached to them. This first movement is called anaphase A.
Each chromosome is connected to spindle fibers (microtubules) at a specialized attachment point called the kinetochore. During anaphase A, these kinetochore microtubules shorten, primarily by losing subunits at the point where they attach to the chromosome. Motor proteins at the kinetochore help reel the chromosome along the shrinking fiber, like pulling yourself along a rope that’s dissolving behind you. Some additional shortening happens at the spindle poles themselves. The net effect is that chromosomes steadily travel toward opposite ends of the cell.
Anaphase B: The Poles Push Apart
While chromosomes are being pulled poleward, a second process kicks in: the spindle itself stretches, pushing the two poles of the cell farther from each other. This is anaphase B, and it relies on a different set of forces than anaphase A. The two processes overlap in time but are mechanically independent.
Two forces drive anaphase B. In the middle of the spindle, bundles of overlapping microtubules from each pole interlock like fingers from two hands. Motor proteins walk along these overlapping fibers and slide them past each other, pushing the poles apart from the inside. At the same time, motor proteins anchored to the inner surface of the cell membrane grab onto microtubules radiating outward from each pole and pull the poles toward the edges of the cell. Together, these forces elongate the entire spindle and increase the distance between the two chromosome sets. The overlapping microtubules in the center actually grow longer during this process, maintaining the bridge between the separating halves of the cell.
Anaphase in Meiosis
Anaphase plays out differently depending on the type of cell division. In standard mitosis, sister chromatids separate, giving each daughter cell an identical copy of every chromosome. In meiosis, the cell division process that produces eggs and sperm, anaphase happens twice with different results each time.
During anaphase I, it’s not sister chromatids that separate but homologous chromosomes, the matched pairs you inherited from each parent. The sister chromatids stay joined and travel together to one pole. This is what halves the chromosome number. During anaphase II, which looks much more like mitotic anaphase, the sister chromatids finally separate. The distinction matters: anaphase I is what makes sexual reproduction possible by creating cells with half the normal chromosome count.
What Happens When Anaphase Goes Wrong
The precision of anaphase is critical. When chromosomes fail to separate correctly, a condition called nondisjunction, the resulting cells end up with the wrong number of chromosomes. This is called aneuploidy.
Aneuploidy is a hallmark of cancer cells and a leading cause of miscarriages. Around 31% of embryos analyzed after in vitro fertilization carry aneuploidies from errors during meiosis, and roughly 74% show mosaic aneuploidies from mistakes in the first few mitotic divisions after fertilization. Most whole-chromosome imbalances are lethal to a developing embryo, but certain trisomies involving smaller chromosomes are compatible with life. Trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome) are the most well-known examples. Smaller chromosomes tend to cause less cellular disruption when present in extra copies, which is why these particular trisomies can survive to birth while trisomies of larger chromosomes almost never do.
The risk of these errors increases with age, particularly in eggs. Older women have higher rates of aneuploidy involving small chromosomes, which is why the likelihood of conditions like Down syndrome rises with maternal age.