meiosis

Here is a puzzle every sexually reproducing species must solve: if a child gets a full set of chromosomes from each parent, the count would double every generation and quickly spiral out of control. The answer is to make special sex cells that carry only half a set, so that when egg and sperm combine, the full count is restored. Meiosis is the special kind of cell division that produces sex cells (eggs and sperm) with half the usual number of chromosomes.

Meiosis takes a cell with the full, paired set of chromosomes and, through two consecutive divisions, produces cells with a single, unpaired set. The DNA is copied just once, but the cell divides twice. In meiosis I, the matching pairs of chromosomes (homologs, one inherited from each parent) are separated — this is the 'reduction division' that halves the chromosome number. In meiosis II, which looks much like mitosis, the sister chromatids of each chromosome are finally split apart. The result is four cells, each with half the original chromosome number.

Meiosis matters for two reasons: it keeps the chromosome number stable across generations, and it is a major engine of genetic variety. During meiosis I, homologous chromosomes swap segments (crossing over) and the pairs line up and separate randomly, so each sex cell ends up with a unique shuffle of the parents' genes. This is why siblings differ. A common confusion is treating meiosis as 'just like mitosis but halved' — the crucial, unique step is separating the homologous pairs in meiosis I, something that never happens in mitosis.