Meiosis occurs exclusively in germ cells, the specialized cells in reproductive organs that give rise to sperm and eggs. Every other cell in your body divides by mitosis. Only germ cells undergo this unique two-step division that cuts the chromosome count in half, going from 46 chromosomes down to 23.
Germ Cells: Where Meiosis Happens
In humans, meiosis takes place in two locations: the testes in males and the ovaries in females. The cells that enter meiosis have specific names depending on sex. In males, cells called spermatogonia divide to produce primary spermatocytes, which are the cells that actually enter meiosis. In females, the equivalent cells are called oogonia, which develop into primary oocytes before beginning meiotic division.
These germ cells don’t originate in the reproductive organs. During embryonic development, a small population of precursor cells called primordial germ cells forms separately from the rest of the body. In a mouse embryo (a well-studied model), these cells are specified around day 6.5 of development and then physically migrate through the developing body, traveling through the gut region before reaching the developing gonads around day 9.5. Once they arrive, they settle in and eventually become the cells capable of meiosis.
How Meiosis Differs in Males and Females
The timing of meiosis is dramatically different between the sexes. In males, meiosis doesn’t begin until puberty and then continues throughout life. The entire process from stem cell to mature sperm takes about 65 days, and millions of sperm are produced continuously. Each round of meiosis produces four functional sperm cells.
In females, meiosis starts before birth. Oogonia enter the early stages of meiosis during fetal development, then pause. They remain frozen in this arrested state for years, sometimes decades, only resuming when an egg is released during ovulation. Unlike sperm production, each round of meiosis in females produces only one functional egg (the remaining genetic material is discarded into smaller cells called polar bodies). This long pause has significant consequences for egg quality over time.
What Meiosis Actually Does to These Cells
The whole point of meiosis is to produce cells with exactly half the normal chromosome count. Human body cells are diploid, meaning they carry 46 chromosomes (23 pairs). Gametes (sperm and eggs) need to be haploid, carrying just 23 chromosomes, so that when they fuse at fertilization the resulting embryo has the correct 46.
Meiosis achieves this through two consecutive divisions after only one round of DNA copying. During the first division (meiosis I), paired chromosomes line up and then separate, so each daughter cell gets one chromosome from each pair. The second division (meiosis II) works more like ordinary cell division: the two halves of each chromosome split apart. The end result is four haploid cells, each with a single copy of every chromosome.
A critical feature of meiosis I is that paired chromosomes physically exchange segments of DNA before separating. This shuffling means every sperm or egg carries a unique combination of genetic material, which is why siblings from the same parents look different from one another.
When Meiosis Goes Wrong
Because eggs remain paused in meiosis for so long, they become more error-prone with age. The most common meiotic error is nondisjunction, where chromosomes fail to separate properly, leaving a gamete with too many or too few chromosomes. Studies of over 1,300 human eggs found that about 8.5% of eggs from women under 24 had chromosome errors, compared to nearly 40% of eggs from women between 40 and 44.
The most familiar consequence of nondisjunction is Down syndrome, caused by an extra copy of chromosome 21. Research on trisomy 21 cases has shown that the link to maternal age is specific to errors in the egg, not in sperm or in cell divisions after fertilization. Women aged 40 and older were roughly 8.5 times more likely than women aged 20 to 24 to have an egg with a meiosis I error, and over 15 times more likely to have a meiosis II error. Sperm production, by contrast, restarts fresh every 65 days, so age-related meiotic errors in males are far less common.
Meiosis in Plants and Other Organisms
Meiosis isn’t limited to animals. In flowering plants, it occurs in specialized cells within the reproductive structures of the flower. Inside the anther (the pollen-producing part), diploid cells called microspore mother cells undergo meiosis to produce four microspores, each of which develops into a pollen grain. In the ovule, a megaspore mother cell divides by meiosis to produce megaspores, one of which develops into the egg-containing structure.
Even single-celled organisms use meiosis. Budding yeast, for example, can exist in either a haploid or diploid state. When nutrients run low, particularly nitrogen, diploid yeast cells are triggered to undergo meiosis. This produces four haploid spores packaged together in a protective sac. When conditions improve, these spores germinate into individual haploid cells that can grow and eventually fuse with another cell to become diploid again. The trigger is environmental stress rather than a developmental program, but the core mechanics of meiosis, two divisions producing four haploid cells, are the same across all eukaryotic life.