# Anaphase: Sister Chromatid Separation and Poleward Movement

Anaphase is the stage of mitosis when paired sister chromatids suddenly separate and move to opposite ends of the cell. This movement is powered by the mitotic spindle, a network of microtubules that attaches to each chromatid. Anaphase marks the point of no return in cell division, because once chromatids separate, each one becomes an independent chromosome destined for a different daughter cell. The process is fast, dramatic, and highly regulated to prevent errors in chromosome number.

## Key Takeaways

- Anaphase begins when the spindle assembly checkpoint is satisfied and the enzyme separase cleaves cohesin rings holding sister chromatids together.
- Anaphase A involves chromosomes moving toward spindle poles as kinetochore microtubules shorten and disassemble.
- Anaphase B involves spindle poles moving farther apart as interpolar microtubules elongate, causing the cell to stretch.
- In meiosis, anaphase 1 separates homologous chromosomes, while anaphase 2 separates sister chromatids.
- Failure of separation during anaphase causes nondisjunction, which leads to aneuploidy, a condition of abnormal chromosome number.

## The Trigger: Releasing the Spindle Checkpoint

Before anaphase can start, the cell must confirm that every chromosome is properly attached to spindle microtubules from opposite poles. This verification is performed by the spindle assembly checkpoint (SAC) [4]. The SAC is a surveillance system that delays anaphase until all kinetochores, the protein structures on each centromere, are correctly bound [4]. When even one kinetochore is unattached, the SAC sends an inhibitory signal that blocks the next step.

Once all chromosomes are properly aligned at the metaphase plate, the SAC is silenced. This silencing activates a cascade that leads to the destruction of securin, a protein that inhibits separase. With securin degraded, separase becomes active. Separase is a protease, an enzyme that cuts other proteins. Its target is cohesin, a ring-shaped protein complex that physically embraces sister chromatids [3]. Cohesin holds the two identical DNA molecules together from the time they are replicated in S phase until anaphase [3]. When separase cleaves cohesin, the ring opens and sister chromatids are freed. Each chromatid is now considered a full chromosome.

## Anaphase A: Chromosomes Move to the Poles

Anaphase A is the poleward movement of chromosomes. The motor for this movement is the kinetochore microtubule, a fiber that connects the kinetochore of each chromosome to one spindle pole. During anaphase A, these fibers shorten, pulling the attached chromosomes toward the pole [2]. The shortening occurs through disassembly of tubulin subunits at the kinetochore end of the microtubule [2].

This process is often described as a "reel-in" mechanism. The kinetochore maintains a firm grip on the shrinking microtubule tip, so the chromosome is dragged along as the fiber gets shorter. In some cells, microtubules also undergo a process called flux, where the entire fiber slides poleward while subunits are lost from the minus end near the pole [2]. Both mechanisms contribute to the same outcome: chromosomes are pulled closer to opposite poles. Anaphase A is the dominant form of movement in many cell types, especially in early anaphase.

## Anaphase B: Spindle Poles Separate

Anaphase B is the separation of the two spindle poles from each other. While anaphase A pulls chromosomes, anaphase B pushes the poles apart, which increases the distance between the two sets of chromosomes. This phase is driven by two main activities. First, interpolar microtubules, which extend from opposite poles and overlap in the middle of the cell, slide past each other. Motor proteins, such as kinesins, push these overlapping fibers apart, elongating the spindle. Second, the astral microtubules, which radiate from each pole toward the cell cortex, pull the poles outward. This combination of pushing and pulling causes the entire cell to elongate [2].

Anaphase B is more prominent in some organisms than in others. In yeast and certain animal cells, spindle elongation accounts for a large fraction of chromosome separation. In other cells, anaphase A does most of the work. Both processes often occur simultaneously, with anaphase A starting first and anaphase B continuing after chromosomes have reached the poles.

| Feature | Anaphase A | Anaphase B |
| --- | --- | --- |
| Main event | Chromosomes move to poles | Poles move apart |
| Microtubule type | Kinetochore microtubules | Interpolar and astral microtubules |
| Mechanism | Microtubule shortening and disassembly | Microtubule sliding and pole pulling |
| Result | Chromosomes approach poles | Cell elongates, poles separate |

## Anaphase in Meiosis: Anaphase 1 and Anaphase 2

Meiosis is the specialized division that produces gametes. It involves two rounds of chromosome segregation, so it has two anaphase stages. Anaphase 1 occurs in meiosis I, and anaphase 2 occurs in meiosis II. The key difference is what separates. In anaphase 1, homologous chromosomes separate. Homologous chromosomes are pairs, one from each parent, that carry the same genes. They are held together by chiasmata, which are crossovers between nonsister chromatids. During anaphase 1, cohesin along the chromosome arms is cleaved, allowing homologs to separate. However, cohesin at the centromere is protected, so sister chromatids stay together. Each pole receives a set of chromosomes that still have two chromatids.

In anaphase 2, the situation resembles mitosis. Sister chromatids separate as cohesin at the centromere is finally cleaved. Each chromatid becomes an independent chromosome and moves to a pole. The result is four haploid cells, each with one copy of each chromosome. For a detailed comparison of the two processes, see our [mitosis vs meiosis](/blog/guides/mitosis-vs-meiosis) guide.

| Stage | What separates | Cohesin cleavage | Result |
| --- | --- | --- | --- |
| Anaphase 1 | Homologous chromosomes | Arm cohesin cleaved, centromere cohesin protected | Sister chromatids remain together |
| Anaphase 2 | Sister chromatids | Centromere cohesin cleaved | Chromatids become separate chromosomes |

## Nondisjunction: When Anaphase Fails

Nondisjunction is the failure of chromosomes to separate properly during anaphase. It can happen in mitosis, anaphase 1, or anaphase 2. When nondisjunction occurs, one daughter cell receives both copies of a chromosome, while the other daughter cell receives none. This error produces aneuploidy, a condition where cells have an abnormal number of chromosomes [4]. The spindle assembly checkpoint normally prevents this, but the checkpoint is not perfect [4].

In meiosis, nondisjunction leads to gametes with extra or missing chromosomes. If such a gamete participates in fertilization, the resulting embryo will have an abnormal chromosome number. Most aneuploid embryos do not survive, but some do. For example, trisomy 21, which causes Down syndrome, results from an extra copy of chromosome 21. Nondisjunction is more common in older mothers, and it is a major cause of miscarriage and developmental disorders. The consequences of anaphase errors are severe, which is why the cell invests so much energy in the spindle checkpoint.

## The Role of Cohesin and Separase in Detail

Cohesin is a multi-subunit protein complex that forms a large ring structure [3]. The ring is large enough to trap two sister chromatids inside. This physical entrapment is what holds the sisters together. Cohesin also has roles in DNA repair and gene expression, but its most famous function is sister chromatid cohesion [3]. The complex is loaded onto DNA during S phase, when DNA is replicated. Once loaded, it stays in place until anaphase.

Separase is the enzyme that triggers separation. It is kept inactive by securin, a chaperone that binds and inhibits it. When the spindle checkpoint is satisfied, the anaphase-promoting complex (APC) tags securin with ubiquitin, marking it for destruction. With securin gone, separase is free to cleave cohesin. The cleavage is site-specific, meaning separase cuts cohesin at a particular point in its structure. This opens the ring and releases the chromatids. The regulation of this process is critical, because premature cleavage would cause chromosome mis-segregation.

## How Anaphase Fits into the Cell Cycle

Anaphase is the shortest stage of mitosis, but it is also the most consequential. It follows metaphase, when chromosomes are aligned at the equator, and it precedes telophase, when the nuclear envelope reforms. The entire mitotic phase is part of the cell cycle, which includes interphase for growth and DNA replication [1]. During interphase, the cell prepares for division by duplicating its DNA and centrosomes [1]. The mitotic phase then distributes this duplicated material equally [1].

Anaphase is the physical separation of the genetic material. Without anaphase, the cell would have two copies of every chromosome in one nucleus. With anaphase, each daughter cell gets one complete set. The process is coordinated with cytokinesis, the division of the cytoplasm, which usually begins during late anaphase or telophase [1]. For a broader view, you can review the [phases of mitosis](/blog/guides/phases-of-mitosis) or start with our [Cell Biology 101](/blog/guides/cell-biology-101) guide. The movement of chromosomes during anaphase is also a key topic in [cell division](/blog/guides/cell-division) and relates to the structure of [sister chromatids](/blog/guides/sister-chromatid-vs-chromosome-key-differences).

## Frequently Asked Questions

### What happens in anaphase?

In anaphase, the cohesin proteins that hold sister chromatids together are cleaved by separase. Each chromatid becomes an independent chromosome. The chromosomes are then pulled to opposite poles of the cell. This movement happens through two mechanisms: anaphase A, where kinetochore microtubules shorten, and anaphase B, where spindle poles separate and the cell elongates.

### What happens in anaphase 1?

Anaphase 1 is the first division of meiosis. Homologous chromosomes, which are pairs of chromosomes from each parent, are separated. Cohesin along the chromosome arms is cleaved, but cohesin at the centromere is protected. This means sister chromatids remain attached. Each pole receives one homologous chromosome, which still consists of two chromatids.

### What is the difference between anaphase 1 and anaphase 2?

Anaphase 1 separates homologous chromosomes, while anaphase 2 separates sister chromatids. In anaphase 1, sister chromatids stay together because centromere cohesin is protected. In anaphase 2, that protection is removed, and centromere cohesin is cleaved. Anaphase 2 is similar to mitotic anaphase, while anaphase 1 is unique to meiosis.

### What triggers anaphase?

Anaphase is triggered by the silencing of the spindle assembly checkpoint. This checkpoint ensures all chromosomes are attached to spindle fibers. Once satisfied, the anaphase-promoting complex (APC) marks securin for destruction. Securin normally inhibits separase. When securin is degraded, separase becomes active and cleaves cohesin, releasing sister chromatids and starting anaphase.

### What happens if chromosomes fail to separate in anaphase?

If chromosomes fail to separate, the result is nondisjunction. One daughter cell receives both copies of a chromosome, and the other receives none. This causes aneuploidy, an abnormal chromosome number. In mitosis, this can lead to genetic instability in somatic cells. In meiosis, it produces gametes with extra or missing chromosomes, which can cause miscarriage or conditions like Down syndrome.

## References

1. Clark MA, Douglas M, Choi J (2018). [10.2 The Cell Cycle](https://openstax.org/books/biology-2e/pages/10-2-the-cell-cycle). *Biology 2e, OpenStax*.
2. Asbury CL (2017). [Anaphase A: Disassembling Microtubules Move Chromosomes toward Spindle Poles](https://doi.org/10.3390/biology6010015). *Biology 6:15*.
3. Nasmyth K, Haering CH (2009). [Cohesin: Its Roles and Mechanisms](https://doi.org/10.1146/annurev-genet-102108-134233). *Annual Review of Genetics 43:525-558*.
4. Musacchio A, Salmon ED (2007). [The spindle-assembly checkpoint in space and time](https://doi.org/10.1038/nrm2163). *Nature Reviews Molecular Cell Biology 8:379-393*.