Meiosis is a type of cell division that produces sex cells, specifically eggs and sperm. It takes one cell containing 46 chromosomes and, through two rounds of division, creates four cells with just 23 chromosomes each. This halving is essential: when a 23-chromosome egg is fertilized by a 23-chromosome sperm, the resulting embryo has exactly 46, restoring the full set.
Why Meiosis Matters for Reproduction
Every cell in your body (besides your sex cells) carries 46 chromosomes arranged in 23 pairs. One chromosome in each pair came from your mother, the other from your father. If eggs and sperm also carried 46 chromosomes, a fertilized egg would end up with 92, and the number would double with every generation. Meiosis solves this by cutting the chromosome count in half before fertilization.
Meiosis only happens in your reproductive organs. In males, it occurs in the testes to produce sperm. In females, it occurs in the ovaries to produce eggs. Every other type of cell division in your body uses a simpler process called mitosis.
Meiosis I: Separating Chromosome Pairs
Before meiosis begins, the cell copies all of its DNA so that each chromosome exists as two identical halves (called sister chromatids) joined at a center point. The first division then separates matched chromosome pairs, so each daughter cell gets one version of every chromosome rather than two.
Prophase I
This is the longest and most complex phase in all of meiosis. Matching chromosomes (one from each parent) physically line up side by side and zip together through a protein structure that holds them in alignment. While they’re locked together, something remarkable happens: segments of DNA break and swap between the maternal and paternal chromosomes. This swapping, called crossing over, creates chromosomes that are a patchwork of both parents’ DNA rather than a copy of either one alone. The visible connection points where the swap occurred are called chiasmata, and at least one forms per chromosome pair. These connections physically hold the pair together until it’s time to separate.
Metaphase I and Anaphase I
The paired chromosomes line up along the middle of the cell. Each pair orients randomly, meaning the maternal chromosome can face either side of the cell, with the paternal chromosome facing the other. This random orientation is called independent assortment, and it’s a second major source of genetic variety. In humans, with 23 chromosome pairs, this random shuffling alone can produce roughly 8 million different combinations in a single person’s sex cells.
During anaphase I, the cell pulls the two chromosomes in each pair apart, sending one to each side. The sister chromatids stay joined. The cell then divides, producing two cells that each contain 23 chromosomes (still in their doubled, sister-chromatid form).
Meiosis II: Splitting Sister Chromatids
The second division works much like ordinary cell division. The 23 doubled chromosomes in each cell line up, the sister chromatids are pulled apart, and the cell splits. No new DNA is copied between the two divisions. The result is four cells, each carrying a single copy of each chromosome: 23 total, all genetically unique.
In males, all four cells develop into functional sperm. In females, the divisions are unequal in size. Typically only one of the four cells becomes a mature egg, while the other three (called polar bodies) are much smaller and eventually break down.
How Meiosis Creates Genetic Diversity
Two mechanisms during meiosis ensure that no two sex cells are genetically identical. The first is crossing over during prophase I, which physically mixes stretches of DNA between chromosomes inherited from each parent. Because different segments swap each time, every egg or sperm carries a unique mosaic of genetic information.
The second is independent assortment at metaphase I, where each pair of chromosomes lines up randomly. Combined with crossing over, these two processes generate an effectively limitless number of genetically distinct sex cells. This is why siblings who share the same two parents can look and behave so differently from each other.
How Meiosis Differs From Mitosis
Mitosis is the cell division your body uses for growth and repair. It produces two cells that are genetically identical to the original. Meiosis, by contrast, produces four cells that are genetically unique and contain half the original chromosome count. Mitosis involves one round of division; meiosis involves two. And while mitosis happens throughout the body in many cell types, meiosis is restricted entirely to the cells that will become eggs or sperm.
What Happens When Meiosis Goes Wrong
Sometimes chromosomes fail to separate properly during one of the two divisions. This error, called nondisjunction, means one daughter cell ends up with an extra chromosome while another is missing one. If a sex cell with the wrong number of chromosomes is involved in fertilization, the embryo will have an abnormal chromosome count, a condition called aneuploidy.
Most aneuploidies are so severe that the embryo cannot survive. A few, however, result in viable births with recognizable patterns of developmental differences:
- Down syndrome (trisomy 21): Three copies of chromosome 21 instead of two. Associated with characteristic facial features, intellectual disability, and increased risk of congenital heart disease. Life expectancy is around 60 years.
- Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13): Extra copies of chromosomes 18 or 13, respectively. Both are far more severe, and most affected infants do not survive past their first year.
- Klinefelter syndrome (47, XXY): Males born with an extra X chromosome, often leading to reduced fertility and sometimes subtle learning differences.
- Turner syndrome (45, X): Females born with only one X chromosome instead of two. This is the only single-chromosome loss compatible with life, and it typically causes short stature, heart differences, and affects ovarian development.
Nondisjunction can happen during either meiosis I or meiosis II. The risk increases with parental age, particularly maternal age, because egg cells begin meiosis before birth and remain paused in prophase I for decades before completing the process at ovulation.