Difference Between Anaphase 1 and 2: Meiosis Explained
Meiosis is the specialized cell division that produces gametes — sperm and egg cells — each with half the normal chromosome number. It runs through two consecutive divisions, meiosis I and meiosis II, and each has its own anaphase stage. The key difference between anaphase 1 and 2 is what gets separated: anaphase I pulls homologous chromosomes apart, while anaphase II pulls sister chromatids apart. That single distinction explains why meiosis I is called the “reductional” division (it halves chromosome number) and meiosis II is “equational” (it keeps chromosome number the same but separates identical copies).
What Happens During Anaphase I?
Anaphase I begins when the paired homologous chromosomes — the tetrads formed during prophase I — are pulled toward opposite poles of the cell. Each homolog still consists of two sister chromatids joined at a centromere, so the cell does not yet split chromatids apart. The spindle fibers attach to the kinetochores of each homolog and draw them in opposite directions.
Because homologous chromosomes were shuffled by crossing over during prophase I, the two homologs are no longer identical. Each carries a mix of maternal and paternal segments. When they separate in anaphase I, the resulting daughter cells get one homolog from each pair — but each chromosome still has two chromatids. This is the step that reduces the chromosome number from diploid (2n) to haploid (1n). For a human cell, that means going from 46 chromosomes to 23 chromosomes per daughter cell.
Sister chromatids stay glued together in anaphase I because the cohesin protein complex along the chromosome arms remains intact. Only the cohesin at the arms is cleaved to allow homologs to separate; the centromeric cohesin is protected. This is a crucial mechanical detail — it is why the two chromatids of a single chromosome do not drift apart during this phase. The molecular protection of centromeric cohesin is well established in cell biology (Nature Scitable).
What Happens During Anaphase II?
Anaphase II looks much more like mitosis. By this stage, the cell has already completed meiosis I and divided into two haploid cells. Each of those cells now has chromosomes that still consist of two sister chromatids — but there are no homologous pairs left to separate. Instead, the centromeres split, and the sister chromatids are pulled to opposite poles.
Because sister chromatids are (usually) genetically identical — apart from any crossing over that occurred earlier — anaphase II separates identical copies rather than different homologs. When the chromatids reach the poles, each becomes an individual chromosome. The cell then divides, producing four haploid daughter cells in total from the original diploid cell.
A Worked Example
Imagine a cell with two pairs of chromosomes (2n = 4). After meiosis I, you get two cells, each with 2 chromosomes — but each chromosome still has two chromatids. In anaphase II, those 2 chromosomes split their chromatids, sending one chromatid of each to each pole. The end result is four cells, each with 2 single-chromatid chromosomes. That is the whole point: one division halves the number, the next division separates the copies.
Side-by-Side Comparison
| Feature | Anaphase I | Anaphase II |
|---|---|---|
| What separates | Homologous chromosomes | Sister chromatids |
| Chromosome number in daughter cells | Reduced (2n → 1n) | Maintained (1n → 1n) |
| Centromere splitting | Does not occur | Occurs |
| Crossing over effects visible | Yes (homologs differ) | Yes, but sisters usually identical |
| Cohesin cleavage | Arm cohesin only | Centromeric cohesin cleaved |
| Cells at start of phase | One diploid cell | Two haploid cells |
| Cells after division | Two haploid cells | Four haploid cells |
This table captures the practical difference between anaphase 1 and 2 in a way that is easy to remember: anaphase I deals with chromosome pairs; anaphase II deals with chromatid copies.
Why the Distinction Matters
The separation of homologs in anaphase I is the physical basis of Mendel’s law of independent assortment — the random orientation of each tetrad determines which homolog ends up in which gamete. This is why siblings (except identical twins) are genetically different. Anaphase II, by contrast, is more like a cleanup step: it ensures each gamete gets a complete set of single-chromatid chromosomes.
Errors in either phase have different consequences. Nondisjunction in anaphase I produces gametes with an extra or missing whole chromosome. Nondisjunction in anaphase II produces a similar outcome but affects sister chromatids rather than homologs, which can lead to different patterns of chromosomal imbalance. Both are well documented in genetics textbooks and the NCBI Bookshelf.
Key Takeaways
- Anaphase I separates homologous chromosomes; anaphase II separates sister chromatids.
- Anaphase I is reductional (2n → 1n); anaphase II is equational (1n → 1n).
- Centromeres do not split in anaphase I but do split in anaphase II.
- Cohesin at chromosome arms is cleaved in anaphase I, while centromeric cohesin is cleaved in anaphase II.
- Crossing over makes homologs genetically different, but sister chromatids remain largely identical — so the two phases separate different kinds of genetic material.
- Errors in anaphase I and anaphase II both cause nondisjunction, but they affect homologs versus sister chromatids, respectively.
