Meiosis is a type of cell division that produces sex cells, meaning sperm and eggs. Unlike ordinary cell division, which copies a cell into two identical halves, meiosis takes one cell and splits it twice to create four genetically unique cells, each carrying half the original number of chromosomes. This halving is essential: when a sperm and egg combine at fertilization, the full chromosome count is restored. In humans, that means each sex cell carries 23 chromosomes instead of the usual 46.
How Meiosis Differs From Regular Cell Division
Your body uses regular cell division (mitosis) for growth and repair. A skin cell divides into two identical skin cells, a liver cell into two identical liver cells. Mitosis produces two genetically identical daughter cells from one parent cell. Meiosis is fundamentally different in three ways: it divides twice instead of once, it produces four cells instead of two, and every one of those four cells is genetically unique.
That genetic uniqueness is the whole point. Meiosis exists to shuffle your DNA so that every sperm or egg you produce carries a slightly different combination of genes. This is why siblings from the same parents look related but never identical (unless they’re identical twins, which come from the same fertilized egg).
The First Division: Separating Chromosome Pairs
Meiosis happens in two rounds. The first round, called meiosis I, is the more complex one. It separates the paired chromosomes you inherited from each parent, so that each resulting cell gets one complete set instead of two.
The process begins with a long preparatory phase called prophase I, which can last days or even years depending on the organism and cell type. During this phase, matching chromosomes from your mother and father find each other and physically pair up. They’re held together by a protein structure called the synaptonemal complex, which acts like a zipper between them. While paired, the chromosomes swap segments of DNA with each other in a process called crossing over. Parts of your mother’s chromosome trade places with corresponding parts of your father’s chromosome, creating brand-new gene combinations that didn’t exist in either parent.
After this extended pairing phase, the paired chromosomes line up in the middle of the cell. Then, during anaphase I, the pairs are pulled apart. One member of each pair goes to one side of the cell, the other member to the opposite side. The cell then splits in two, leaving each new cell with 23 chromosomes (in humans), though each chromosome still consists of two joined copies called sister chromatids.
The Second Division: Splitting Sister Chromatids
Meiosis II follows almost immediately, with no new DNA copying beforehand. This round works more like regular cell division. The sister chromatids that stayed joined through the first division now line up and are pulled apart, one going to each side of the cell. Each of the two cells from meiosis I divides again, producing four total cells. Each of these final cells carries 23 single chromosomes, making them truly haploid and ready to combine with another sex cell at fertilization.
Two Sources of Genetic Diversity
Meiosis generates variety through two distinct mechanisms. The first is crossing over, which happens during that long prophase I stage. When matching chromosomes swap segments, they create chromosomes that are mosaics of both parents’ DNA. This means a single chromosome in your sperm or egg might carry your mother’s version of one gene right next to your father’s version of another.
The second mechanism is independent assortment. When the 23 chromosome pairs line up before being separated, each pair orients randomly. Your chromosome 1 from your mother might go to the same cell as chromosome 2 from your father, or they might end up in different cells. With 23 pairs sorting independently, there are over 8 million possible combinations before crossing over is even factored in. Together, these two mechanisms ensure that every sex cell is essentially one of a kind.
Meiosis in Sperm vs. Eggs
The basic steps are the same in both sexes, but the timing is dramatically different. In males, meiosis runs continuously from puberty onward, and sperm production from start to finish takes days to weeks. The process doesn’t pause.
In females, the timeline is far more drawn out. Egg cells begin meiosis before birth but then stall in the early stages of the first division. They remain frozen in this state for years, sometimes decades, until a hormone signal during the menstrual cycle restarts the process. A woman ovulating at age 35 is releasing an egg that has been paused in meiosis I since before she was born. This extraordinarily long pause is one reason why the risk of chromosomal errors increases with maternal age.
What Happens When Meiosis Goes Wrong
The most common meiotic error is called nondisjunction, where chromosomes fail to separate properly. Instead of each daughter cell getting one copy of a chromosome, one cell ends up with two copies and the other gets none. If a sex cell with an extra chromosome is fertilized, the resulting embryo has three copies of that chromosome instead of the normal two.
Most of these errors are incompatible with life, ending in early miscarriage. A few, however, produce viable pregnancies with developmental effects:
- Down syndrome: three copies of chromosome 21, the most common survivable trisomy
- Edwards syndrome: three copies of chromosome 18
- Patau syndrome: three copies of chromosome 13
- Klinefelter syndrome: a male with an extra X chromosome (47, XXY)
- Turner syndrome: a female with only one X chromosome (45, X), the only single-chromosome condition compatible with life
Nondisjunction can happen during either meiosis I (when paired chromosomes fail to separate) or meiosis II (when sister chromatids fail to separate). Errors in meiosis I tend to affect all four resulting sex cells, while errors in meiosis II leave two of the four cells normal. The risk of these errors rises with age, particularly in egg cells that have been stalled in meiosis I for decades.