During anaphase, the copied chromosomes in a dividing cell split apart and move to opposite ends of the cell. This is the critical moment of cell division: the point where one set of genetic material physically becomes two. The entire process takes only a few minutes, but it involves a tightly coordinated sequence of molecular events, from breaking the molecular glue that holds chromosomes together to physically dragging them apart along protein tracks.
The Signal That Starts Anaphase
Cells don’t enter anaphase until a quality control system gives the green light. This system, called the spindle assembly checkpoint, monitors whether every chromosome is properly attached to the network of protein fibers (called the spindle) that will pull them apart. Each chromosome has a small docking site called a kinetochore, and until every kinetochore is firmly connected to fibers from both sides of the cell, the checkpoint keeps anaphase on hold.
Once all chromosomes are correctly attached, the checkpoint releases its hold on a large molecular machine called the anaphase-promoting complex. This complex works like a tagging system: it marks two key proteins for destruction. The first is securin, which normally acts as a leash on the enzyme that will cut chromosomes apart. The second is cyclin B, which keeps the cell locked in its dividing state. As soon as these two proteins are broken down, anaphase begins almost instantly. The balance between checkpoint activation and the destruction machinery is so finely tuned that small shifts in the abundance of checkpoint proteins can tip the cell into anaphase.
How Chromosomes Split Apart
Up until anaphase, each chromosome exists as two identical copies (sister chromatids) physically glued together by ring-shaped protein complexes called cohesins. With securin destroyed, the enzyme separase is unleashed. Separase cuts a specific component of the cohesin ring, causing it to fall apart. This single molecular cut is enough to release the sister chromatids from each other. Experiments have shown that artificially cutting the same cohesin component with an unrelated enzyme triggers chromatid separation even without separase, confirming that the physical act of cleaving cohesin is what drives the split.
Anaphase A: Chromosomes Move to the Poles
Anaphase unfolds in two overlapping stages. In Anaphase A, the now-separated chromatids travel toward opposite ends of the cell. This movement is powered by the shortening of the spindle fibers attached to each chromatid’s kinetochore. As these fibers disassemble from their ends, the chromatids are reeled inward toward the poles. Motor proteins from the kinesin and dynein families help drive this movement, walking along the fibers and converting chemical energy into physical force.
Under a microscope, the migrating chromatids take on distinctive shapes. Because the spindle fibers pull from the centromere (the narrow pinch point on each chromosome), the chromosome arms trail behind. Chromosomes with a centromere in the middle look V-shaped as they move. Those with a centromere closer to one end appear L-shaped. Chromosomes with the centromere at the very tip look like single rods being dragged by their ends.
Anaphase B: The Cell Stretches
In Anaphase B, the spindle poles themselves move farther apart, stretching the entire cell. This happens through elongation of the spindle fibers that overlap in the cell’s center. Motor proteins push these overlapping fibers apart like two people pushing off each other on ice, driving the poles toward opposite sides of the cell. At the same time, other motor proteins anchored at the cell’s outer edges pull the poles outward. Together, Anaphase A and B ensure that the two chromosome sets end up as far apart as possible before the cell finishes dividing.
Cytokinesis Begins During Anaphase
The physical splitting of the cell doesn’t wait for anaphase to finish. As chromatids separate, a belt of contractile proteins begins assembling around the cell’s equator. This contractile ring, built from the same type of protein that powers muscle contraction, starts tightening to form a visible groove called the cleavage furrow. The spindle fibers help position this ring precisely between the two chromosome sets, and they continue stabilizing the advancing furrow through anaphase and into the next stage, telophase. In plant cells, which have rigid walls, the process looks different: a structure called the phragmoplast begins assembling a new cell wall (the cell plate) between the two chromosome sets starting in late anaphase.
Anaphase in Meiosis
Anaphase plays out differently in meiosis, the type of cell division that produces eggs and sperm. Meiosis involves two rounds of division, and anaphase occurs in each.
In Anaphase I, it is not sister chromatids that separate but homologous chromosomes, the matched pairs you inherited from each parent. The sister chromatids of each chromosome stay glued together and travel as a unit to one pole. This is what halves the chromosome number from 46 to 23.
In Anaphase II, the process looks much more like regular mitotic anaphase. The sister chromatids finally separate and move to opposite poles, just as they would in a normal dividing body cell. The result is four cells, each with 23 single chromosomes.
What Happens When Anaphase Goes Wrong
If chromosomes fail to separate correctly during anaphase, a condition called nondisjunction, the resulting cells end up with the wrong number of chromosomes. This is called aneuploidy. A cell might get an extra chromosome (trisomy, with 47 total) or lose one (monosomy, with 45 total).
When nondisjunction happens during meiosis, the consequences can be severe because every cell in the resulting embryo will carry the error. Most aneuploidies are incompatible with life and result in early miscarriage. A few produce viable but affected individuals:
- Trisomy 21 (Down syndrome) is the most common survivable autosomal trisomy, with life expectancy around 60 years. It causes intellectual disability, characteristic facial features, and increased risk of heart defects and leukemia.
- Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome) are far more severe, with life expectancy rarely exceeding one year.
- Turner syndrome (45, X) is the only survivable monosomy. It affects females, causing short stature, heart defects, and ovarian dysfunction. It is the most common cause of absent menstrual periods in young women who have never menstruated.
- Klinefelter syndrome (47, XXY) affects males and can cause tall stature, breast tissue development, and developmental delays.
Nondisjunction during mitosis, ordinary cell division in the body, produces a different pattern. Only the descendants of the affected cell carry the abnormal chromosome count, creating a mosaic of normal and abnormal cells. This type of error can contribute to certain cancers, including retinoblastoma, a childhood eye cancer.