Set-up: copy first, then pair
Before meiosis begins, the cell copies all its DNA, just as it would before ordinary division. Each chromosome now exists as two identical sister chromatids joined at the centromere. So a diploid human cell entering meiosis has 46 chromosomes, but 92 chromatids in total.
Then comes the move unique to meiosis: each chromosome finds its homolog and they line up side by side. This precise pairing is called synapsis. A synapsed pair of homologs — four chromatids together — is a tetrad (also called a bivalent). Holding the four chromatids snug against one another is what makes the next step possible.
Crossing over: swapping pieces
While the homologs are pressed together in a tetrad, they swap matching segments. A chromatid from one homolog breaks, a chromatid from the other breaks at the same spot, and the broken ends rejoin across the pair. This exchange is crossing over. The visible X-shaped point where it has happened is a chiasma (plural chiasmata).
Two divisions, two different jobs
- Meiosis I — separate the homologs. The tetrads line up, and each pair is pulled apart so one homolog goes to each new cell. The number of chromosomes is now halved (this is the reductional division), but each chromosome still has its two sister chromatids.
- Meiosis II — separate the sisters. Without copying the DNA again, each cell divides a second time. Now the two sister chromatids of each chromosome are pulled apart, just like an ordinary division. Each chromosome becomes a single chromatid.
- Result — four haploid cells. One diploid cell has become four haploid cells, each with a single set of chromosomes. In males these mature into four sperm; in females, typically one egg (the others become small polar bodies).
Notice the rhythm: one DNA copy, two divisions. That is exactly why the chromosome number ends up halved. The first division removes the homolog pairing; the second division removes the sister-chromatid doubling.