During anaphase I, homologous chromosomes separate and move to opposite poles of the cell. Sister chromatids do not separate at this stage. They remain joined at their centromeres and won’t come apart until anaphase II. This distinction is the single most important difference between the first and second meiotic divisions.
How Homologous Chromosomes Pull Apart
Before anaphase I begins, homologous chromosomes (one inherited from each parent) are paired together in structures called bivalents. Each bivalent consists of four chromatids total: two sister chromatids from the maternal copy and two from the paternal copy. These homologs are physically linked by crossover points called chiasmata, where segments of DNA were swapped earlier in meiosis.
At the start of anaphase I, the protein glue holding the chromosome arms together is destroyed. Ring-shaped protein complexes called cohesins run along the arms of the paired chromosomes, and an enzyme called separase cuts them apart. Once the arm cohesion dissolves, the crossover connections resolve, and spindle fibers pull each homolog toward opposite ends of the cell. The microtubules contract and disassemble during this process, dragging the chromosomes with them.
Each homolog still looks like an X shape throughout this process, because the two sister chromatids within it stay connected at the centromere.
Why Sister Chromatids Stay Together
The cell uses a clever two-step system. While cohesins along the chromosome arms get cut during anaphase I, the cohesins at the centromere are specifically protected by a protein called shugoshin (Japanese for “guardian spirit”). Shugoshin sits at the centromere and shields the cohesin proteins there from being cleaved by separase. This ensures that sister chromatids remain paired through the end of the first division.
This protection is essential. If centromeric cohesion were lost too early, sister chromatids would fly apart during meiosis I, and the cell would have no way to properly distribute chromosomes during meiosis II. The centromeric cohesion that persists after anaphase I is what allows the second division to separate sister chromatids correctly, much like a normal cell division.
What Triggers Anaphase I
The transition from metaphase I to anaphase I is tightly controlled. A large protein complex called the anaphase-promoting complex acts as the trigger. It tags a safety protein called securin for destruction. Securin normally keeps separase inactive, so once securin is degraded, separase is free to cut the cohesin along chromosome arms. This cascade happens rapidly and irreversibly, committing the cell to chromosome separation.
Before this trigger fires, the cell runs a quality check. During metaphase I, the two sister chromatids of each homolog orient their attachment points toward the same pole, while the homologs as a pair attach to opposite poles. If a homolog accidentally attaches to the wrong pole, there’s no tension on the connection, and a correction system (driven by a protein called Aurora B) releases the faulty attachment so the spindle can try again. Only when all bivalents are properly oriented does the cell proceed into anaphase I.
Chromosome Count After Separation
Because homologs separate but sister chromatids do not, each daughter cell produced after meiosis I contains half the original chromosome number. In humans, that means going from 46 chromosomes to 23. Each of those 23 chromosomes still consists of two joined sister chromatids, so while the chromosome count has been halved, each chromosome carries double the DNA of a single chromatid. The second meiotic division then splits the sister chromatids apart, producing cells with 23 single-chromatid chromosomes.
How This Creates Genetic Variation
The separation during anaphase I is not just a mechanical event. It’s one of the two major sources of genetic diversity in sexual reproduction. During metaphase I, each pair of homologs lines up at the cell’s equator with a random orientation. The maternal homolog might face one pole and the paternal the other, or vice versa, and this orientation is independent for every chromosome pair. In humans, with 23 pairs, this random arrangement produces roughly 8 million possible combinations of maternal and paternal chromosomes in a single gamete.
On top of that, the crossovers that occurred earlier in meiosis mean the homologs being separated are no longer purely maternal or paternal. They’re mosaics, carrying shuffled segments from both parents. So the chromosomes that anaphase I pulls apart are already genetically unique before they even reach opposite poles.
Anaphase I vs. Anaphase II
The simplest way to keep these straight: anaphase I separates homologous chromosomes, while anaphase II separates sister chromatids. In anaphase I, the cell goes from having paired homologs to having individual chromosomes (each still made of two chromatids). In anaphase II, the cell splits each of those chromosomes into single chromatids. Anaphase I reduces chromosome number by half. Anaphase II does not change the chromosome count, it simply splits each chromosome into its two identical copies.
Another key difference is how the spindle fibers attach. In anaphase I, the two sister chromatids of each homolog function as a single unit, with their attachment points directed toward the same pole. In anaphase II, the sister chromatids finally orient toward opposite poles, just like in a standard cell division.