During anaphase, the sister chromatids that were joined together are pulled apart and moved to opposite ends of the cell. This is the critical moment in cell division when one copy of each chromosome travels to each future daughter cell. The entire process takes roughly 10 minutes in human cells and involves a precise chain of molecular signals, enzyme activity, and physical force.
What Triggers Anaphase to Begin
Anaphase doesn’t start until every chromosome is properly attached to the spindle, the network of protein fibers that will pull them apart. A surveillance system called the spindle assembly checkpoint monitors these attachments. As long as even one chromosome remains unattached, a group of checkpoint proteins binds to and blocks a key activator molecule, preventing the cell from moving forward. This is the cell’s quality control step, and it’s remarkably strict.
Once all chromosomes are correctly attached, the checkpoint releases its hold. A large molecular machine called the anaphase-promoting complex then tags specific proteins for destruction. Among these targets is securin, a protein that acts as a lock on the enzyme separase. When securin is destroyed, separase is freed and immediately cuts the cohesin rings that physically hold sister chromatids together. Cohesin works like a protein bracelet encircling both chromatids, and separase snips it open. The cutting is enhanced by a prior chemical modification (phosphorylation) that makes the cohesin subunit easier to cleave. Once that ring opens, the chromatids are free to move apart.
Two Types of Movement: Anaphase A and B
Chromosome separation actually happens through two overlapping but distinct mechanisms, often called anaphase A and anaphase B.
In anaphase A, the chromosomes move toward the spindle poles (the two ends of the cell). This happens primarily because the protein fibers connecting each chromosome to its nearest pole shorten. These fibers, called kinetochore microtubules, depolymerize, essentially disassembling from their ends, and the chromosome is reeled in toward the pole as the fiber shrinks. Recent research suggests this depolymerization may also serve a second purpose: pulling the poles inward slightly, which helps regulate how far the spindle stretches overall.
In anaphase B, the spindle itself elongates, pushing the two poles farther apart. This widens the gap between the separating chromosome sets. The force comes from two sources. From inside the spindle, motor proteins walk along overlapping fibers in the middle zone, sliding them apart like extending a telescope. Multiple motor proteins contribute to this sliding, with some crosslinking antiparallel fibers and walking along them simultaneously. From outside the spindle, motor proteins anchored at the cell’s outer edge can pull on fibers radiating from the poles, tugging the poles outward. Interestingly, experiments in human cells have shown that spindle elongation can still occur even without the outer pulling force, suggesting the internal sliding mechanism is sufficient on its own.
How This Prepares the Cell to Divide
While chromosomes are still moving apart, the cell is already setting up for physical division. Between the two separating chromosome groups, the spindle fibers become bundled into a structure called the central spindle. This structure acts as a signaling platform. A signaling protein called Rho travels to the middle of the cell, arriving at what will become the division site before any visible pinching begins. Rho activates the assembly of a contractile ring made of protein filaments just beneath the cell membrane. This ring will eventually tighten like a drawstring to split the cell in two during cytokinesis, the final stage of division.
The positioning is not random. The central spindle physically determines where the ring forms, ensuring the cell divides right down the middle, with one complete set of chromosomes on each side.
Anaphase in Meiosis
Cells that produce eggs and sperm go through two rounds of division called meiosis, and anaphase plays a different role in each.
During anaphase I, homologous chromosomes (the matched pairs you inherited from each parent) are pulled apart. The sister chromatids stay joined. This is what reduces the chromosome number by half. During anaphase II, the sister chromatids are separated, just as they are in regular mitosis. The mechanical process is similar in both cases, but the outcome is different: anaphase I creates cells with half the chromosome number, while anaphase II produces cells with single copies of each chromatid.
What Goes Wrong When Anaphase Fails
Errors during anaphase can have serious consequences. The most common problem is nondisjunction, where chromatids fail to separate properly and both copies end up in the same daughter cell. This produces aneuploidy, meaning one cell has too many chromosomes and the other has too few. In human reproduction, aneuploidy is the leading cause of miscarriage and is responsible for conditions like Down syndrome (an extra copy of chromosome 21).
Chromosomes that lag behind during separation can also get caught in the middle of the dividing cell. These lagging chromosomes can physically block cell division, leading to cells with double the normal chromosome count and multiple organizing centers for future divisions. This state is a hallmark of early cancer development. If a chromosome stretches between the two poles rather than cleanly separating, it can form a bridge that eventually snaps. The broken ends then enter a destructive cycle of breakage, fusion, and re-breakage that scrambles genetic information and has been linked to tumor diversity and the amplification of cancer-promoting genes.
Even a single chromosome failing to separate properly can trigger DNA damage responses in the resulting daughter cells, reducing their fitness and potentially introducing permanent rearrangements in their genetic material. The precision of anaphase is not just a cellular housekeeping detail. It is one of the most consequential moments in the life of a cell.