Meiosis produces four haploid cells from a single parent cell. In humans, this means each new cell contains 23 chromosomes, exactly half the 46 found in the original. These cells go on to become sex cells (eggs and sperm in animals, spores in plants and fungi), making meiosis the foundation of sexual reproduction.
Four Cells From One
Meiosis involves two back-to-back cell divisions after just one round of DNA copying. The first division splits the parent cell’s paired chromosomes apart, producing two cells that each have one version of every chromosome. The second division then splits those cells again, separating the duplicated copies. The end result: four cells, each with a single set of chromosomes.
This halving is essential. When a sperm and egg fuse during fertilization, each contributes its single set of chromosomes, restoring the full count. Without meiosis cutting the number in half beforehand, chromosome numbers would double with every generation.
What Meiosis I and Meiosis II Each Produce
The two divisions do different jobs. Meiosis I is the “reduction division.” It takes a cell with paired chromosomes (46 in humans) and produces two cells, each with 23 unpaired chromosomes. These intermediate cells are already haploid in terms of chromosome number, but each chromosome still consists of two joined copies that need to be separated.
Meiosis II handles that separation. It works much like ordinary cell division, pulling the joined copies apart. Each of the two cells from meiosis I divides again, giving the final total of four haploid cells. Each one now carries 23 single chromosomes ready to participate in fertilization.
Sperm vs. Egg: Not All Four Cells Survive
In males, all four cells from a single round of meiosis become functional sperm. The process is prolific: each human testicle produces roughly 100 million sperm per day, and over a lifetime a man can generate trillions.
In females, the math works out differently. Meiosis still produces four haploid cells, but nearly all the cytoplasm (the nutrient-rich material the future embryo will need) gets funneled into just one of them. The other three, called polar bodies, are tiny and nonfunctional. So each round of oogenesis yields only one viable egg. Female meiosis also follows a dramatically different timeline. In humans, germ cells begin entering meiosis during fetal development, around 8 to 9 weeks after conception, then pause partway through the first division. Each egg completes meiosis only when it is fertilized, sometimes decades later.
Genetic Variation: Why No Two Cells Are Alike
Meiosis doesn’t just halve the chromosome number. It shuffles genetic information so thoroughly that virtually every cell it produces is unique. Two mechanisms drive this.
Independent assortment happens during meiosis I, when chromosome pairs line up randomly before being pulled to opposite sides of the cell. Each pair orients independently, so the mix of maternal and paternal chromosomes in the resulting cells is essentially random. In humans, with 23 chromosome pairs, this creates roughly 8.4 million possible combinations from independent assortment alone.
Crossing over multiplies that number enormously. During the earliest stage of meiosis I, paired chromosomes physically exchange segments of DNA. Most chromosome pairs swap material at one to four points along their length. These exchanges blend sequences from both parents onto a single chromosome, creating combinations that have never existed before. When you factor crossing over into the equation, the number of genetically distinct cells a person can produce is, for practical purposes, limitless.
Plants and Fungi Produce Spores, Not Gametes
In animals, meiosis directly produces sex cells. In plants and fungi, it doesn’t. Instead, meiosis in a plant’s spore-producing structures (the sporophyte) yields haploid spores. These spores aren’t eggs or sperm. They grow through regular cell division into a small, separate organism called a gametophyte, and the gametophyte then produces the actual gametes.
Mosses offer a clear example: meiosis inside the moss capsule releases haploid spores that drift away and germinate into the leafy green plant you’d recognize. That green plant is the gametophyte, and it produces eggs and sperm through ordinary cell division, not meiosis. Flowering plants follow the same general pattern, though their gametophyte stage is reduced to just a few cells tucked inside the flower.
What Happens When Meiosis Goes Wrong
Sometimes chromosomes fail to separate properly during meiosis, a glitch called nondisjunction. When this happens, the resulting cells end up with one too many or one too few chromosomes. If one of these abnormal cells is fertilized, the embryo will carry that imbalance in every cell of its body.
The most familiar example is Down syndrome. About 95% of cases result from a gamete carrying an extra copy of chromosome 21, giving the child three copies (47 total chromosomes) instead of the usual two. Nondisjunction can occur during either meiosis I or meiosis II, but the consequences differ slightly. An error in the first division produces a gamete with 24 chromosomes that includes both the maternal and paternal versions of the affected chromosome. An error in the second division also gives 24 chromosomes, but both extra copies come from the same parent.
Most other trisomies and nearly all monosomies (missing a chromosome) are so disruptive that the embryo doesn’t survive. Monosomy in humans is practically limited to the X chromosome, as in Turner syndrome, or occasionally chromosome 21. These errors highlight how precisely meiosis needs to work: the process tolerates almost no mistakes in dividing 46 chromosomes into sets of 23.