During anaphase I of meiosis, homologous chromosomes separate and move to opposite poles of the cell. Unlike regular cell division, where individual chromatids split apart, anaphase I pulls entire paired chromosomes away from each other, with each chromosome still consisting of two sister chromatids joined at the centromere. This distinction is the defining feature of meiosis I and the reason it reduces chromosome number by half.
How Homologous Chromosomes Separate
Before anaphase I begins, homologous chromosome pairs are lined up along the middle of the cell. Each pair consists of one chromosome inherited from the mother and one from the father, and each of those chromosomes is made up of two identical sister chromatids glued together by proteins called cohesins. Spindle fibers (microtubules) extend from opposite ends of the cell and attach to each chromosome’s kinetochore, a protein structure at the centromere.
The key difference from regular cell division is how kinetochores attach. In mitosis, the two sister chromatids of a single chromosome connect to opposite poles so they’ll be pulled apart. In meiosis I, both sister chromatids of each chromosome attach to microtubules from the same pole. This means the sister chromatids travel together as a unit. The tension that matters here is between the two homologous chromosomes, not between sister chromatids.
When anaphase I begins, an enzyme called separase cleaves the cohesin proteins along the arms of the sister chromatids. This releases the physical connections (called chiasmata) that had been holding homologous chromosomes together since they swapped DNA segments during crossing over. With those links dissolved, the spindle fibers pull one complete homolog toward each pole of the cell.
Why Sister Chromatids Stay Together
While the cohesin along chromosome arms gets destroyed at anaphase I, the cohesin at the centromere is specifically protected. A protein called shugoshin (Japanese for “guardian spirit”) shields the centromeric cohesin from being cut by separase. This protection is what keeps sister chromatids attached to each other through the rest of meiosis I. They won’t finally separate until anaphase II, when shugoshin’s protection is removed and the centromeric cohesin is cleaved.
Meiosis also uses a slightly different version of cohesin than regular cell division. This meiosis-specific cohesin, called Rec8, is what allows the system to selectively destroy arm connections while preserving centromere connections. Without this two-step process, the cell couldn’t reduce chromosome number in the first division and then separate sister chromatids in the second.
Independent Assortment and Genetic Variation
Anaphase I is where independent assortment happens. Each homologous pair separates independently of every other pair, meaning the maternal chromosome from pair 1 might go to the same pole as the paternal chromosome from pair 2, or the opposite. The orientation is random for each pair.
In humans, with 23 pairs of chromosomes, this creates 2^23 possible combinations of maternal and paternal chromosomes in a single gamete. That’s roughly 8.4 million different outcomes, and that’s before accounting for the genetic reshuffling that already happened during crossing over. This random sorting is one of the main reasons siblings from the same parents can look so different from each other.
What Happens When Separation Fails
Sometimes homologous chromosomes fail to separate during anaphase I, a mistake called nondisjunction. Instead of one homolog going to each pole, both get pulled to the same side. This produces two abnormal daughter cells: one with an extra chromosome and one missing a chromosome. Since both of these cells go on to complete meiosis II, the final result is four gametes, two with an extra chromosome and two with a missing one.
If a gamete with an extra chromosome is fertilized, the resulting embryo has three copies of that chromosome instead of two, a condition called trisomy. The most well-known example is Down syndrome, caused by three copies of chromosome 21. Other trisomies include Edwards syndrome (chromosome 18) and Patau syndrome (chromosome 13), both of which are far more severe. A gamete missing a chromosome leads to monosomy, which is usually fatal for autosomal chromosomes, though monosomy of the X chromosome (Turner syndrome) is survivable.
Nondisjunction during anaphase I is particularly consequential because it affects all four resulting gametes, not just two. Errors at anaphase II, by comparison, only affect two of the four gametes.
Anaphase I vs. Anaphase II
The simplest way to keep these straight: anaphase I separates homologous chromosomes, while anaphase II separates sister chromatids. After anaphase I, each cell has one version of each chromosome, but that chromosome is still doubled (two sister chromatids connected at the centromere). After anaphase II, centromeres finally split and each cell gets a single chromatid, now called a chromosome.
Anaphase II looks much more like what happens in regular mitosis. Sister kinetochores attach to opposite poles, centromeric cohesin is degraded (no longer protected by shugoshin), and individual chromatids are pulled apart. Anaphase I is the uniquely meiotic event, the step that actually halves the chromosome number from 46 to 23 in human cells.