Meiosis I

A body cell has two copies of each chromosome, one from each parent. If gametes kept that number, offspring would double their chromosome count every generation. Meiosis I prevents that by splitting homologous pairs apart, so each resulting cell gets just one version of each chromosome. That's called a reductional division: the chromosome number is cut in half here, before the chromatids ever separate.

In prophase I, homologous chromosomes find each other and zip together in a process called synapsis, forming a four-stranded structure called a tetrad (one duplicated chromosome from mom, one from dad). While paired, they often swap segments at crossover points, called chiasmata, mixing maternal and paternal DNA. In metaphase I, those tetrads, not single chromosomes, line up along the metaphase plate, with each homolog of a pair facing an opposite pole. In anaphase I, the homologs are pulled apart to opposite ends of the cell, but each chromosome's sister chromatids stay glued together at the centromere. Compare that to mitosis, where individual duplicated chromosomes line up alone and sister chromatids separate, keeping the chromosome number constant.

Meiosis I sets up meiosis II, which looks much more like mitosis: it takes the two haploid cells produced here and splits their sister chromatids apart, with no further pairing or crossing over. Because synapsis, crossing over, and homolog separation only happen in meiosis I, this is where most of the genetic shuffling that makes gametes unique actually occurs.