Meiosis is a type of cell division that produces sex cells, specifically sperm and eggs. Unlike ordinary cell division, which copies a cell exactly, meiosis cuts the number of chromosomes in half so that when a sperm and egg combine at fertilization, the resulting embryo ends up with the correct total. The word comes from Greek, meaning “diminution,” and the process turns one cell with 46 chromosomes into four cells with 23 chromosomes each.
How Meiosis Works
Every cell in your body (besides your sex cells) carries two sets of chromosomes: one set from your mother and one from your father, totaling 46. If sperm and eggs also carried 46 chromosomes, a fertilized egg would end up with 92, doubling with every generation. Meiosis solves this by halving the chromosome count through two rounds of division after only one round of DNA copying.
In the first division, the matching pairs of chromosomes (one maternal, one paternal) line up together and then separate into two new cells. Each of those cells still has two copies of each chromosome at this point, similar to ordinary cell division. The second division then splits those copies apart without copying the DNA again, producing four cells that each contain just one set of 23 chromosomes. These are the haploid cells that become functional sperm or eggs.
How Meiosis Differs From Mitosis
Mitosis is the everyday cell division that grows and repairs your body. It produces two daughter cells that are genetically identical to the parent cell, each with the full 46 chromosomes. Skin cells, blood cells, and gut lining cells all multiply this way. Meiosis, by contrast, only happens in reproductive organs and produces four genetically unique cells, each with half the chromosomes.
The critical difference is that second round of division. Mitosis divides once and produces two identical cells. Meiosis divides twice and produces four cells that are neither identical to each other nor to the original cell. This is why children look similar to their parents but never identical (unless they’re identical twins, which results from a different process entirely).
Why Meiosis Creates Genetic Diversity
Meiosis doesn’t just halve the chromosome number. It actively shuffles genetic material in two major ways, making every sperm or egg genetically one of a kind.
The first is crossing over. During the early stages of the first division, matched chromosome pairs physically exchange segments of DNA. The cell deliberately breaks its own DNA strands, and the broken ends from one chromosome connect with the matching region on the partner chromosome. The result is hybrid chromosomes that carry a patchwork of maternal and paternal genetic information rather than a pure copy of either.
The second is independent assortment. When the 23 chromosome pairs line up before the first division, each pair sorts independently. The maternal copy might go left while the paternal copy goes right, and this happens randomly for every pair. With 23 pairs, that creates over 8 million possible chromosome combinations in a single sperm or egg cell. When a sperm with over 8 million possible arrangements fertilizes an egg with its own 8 million possible arrangements, the number of unique outcomes is staggering. Add crossing over on top of that, and the genetic possibilities become essentially infinite.
Meiosis in Males vs. Females
The basic mechanics are the same in both sexes, but the timing and output differ dramatically. In males, meiosis doesn’t begin until puberty. From that point on, spermatogonia (precursor cells in the testes) continuously enter meiosis, and each one produces four functional sperm cells of roughly equal size. A reserve pool of stem cells keeps replenishing the supply throughout life.
In females, the process starts before birth. Fetal ovaries begin meiosis during development, but the cells pause partway through the first division and stay frozen there for years, sometimes decades. Human females are born with hundreds of thousands of these paused egg cells. Each month after puberty, hormonal signals restart meiosis in a small number of them. Unlike sperm production, the cell divisions in egg formation are lopsided: most of the cell’s bulk stays with the future egg, and the leftover material forms tiny, non-functional structures called polar bodies. So while meiosis technically produces four cells, only one becomes a viable egg.
When Meiosis Goes Wrong
The precision required during meiosis is remarkable, and errors are more common than most people realize. The most significant type of error is called nondisjunction, where chromosomes fail to separate properly during one of the two divisions. Instead of each new cell getting one copy, one cell ends up with an extra chromosome and another ends up missing one.
If a sperm or egg with the wrong chromosome count is involved in fertilization, the embryo will have either three copies of a particular chromosome (trisomy) or just one (monosomy). Most of these errors are incompatible with life and result in early miscarriage. A few are survivable:
- Down syndrome: trisomy of chromosome 21, the most common viable chromosomal abnormality
- Edwards syndrome: trisomy of chromosome 18
- Patau syndrome: trisomy of chromosome 13
The error rate is surprisingly high in human eggs. Roughly 30% of fertilized eggs carry the wrong number of chromosomes, and that figure more than doubles in women around age 38. Research examining eggs directly found that about 43% showed chromosomal abnormalities, with the rate climbing above 50% in women over 40. This is a major reason why fertility declines with age and why miscarriage rates increase. The eggs have been paused in mid-division since before birth, and the cellular machinery that holds chromosomes in place deteriorates over time.
Sperm are less prone to these errors, partly because they’re produced fresh from stem cells rather than stored for decades.
Quality Control During Division
Cells have built-in safety mechanisms to catch errors before they become permanent. During both rounds of meiotic division, the cell pauses at a checkpoint before pulling chromosomes apart. Specialized proteins monitor whether every chromosome is properly attached to the cell’s internal scaffolding (the spindle) and whether the attachments are pulling in opposite directions, creating tension. If a chromosome isn’t correctly anchored, the checkpoint blocks the cell from proceeding. Sensor proteins detect the unattached or improperly attached chromosome and halt the process until the problem is corrected.
These checkpoints work well, but they’re not perfect. The fact that a significant percentage of human eggs still end up with the wrong chromosome count shows that the system has limits, particularly as the cellular components age. This is one reason why chromosomal testing during pregnancy or fertility treatment has become more common for older prospective parents.