Gametes (sperm and egg cells) are produced by meiosis, not mitosis. This is the core distinction between the two types of cell division: mitosis copies a cell to make two identical daughter cells, while meiosis reduces the chromosome count in half, producing four genetically unique cells. In humans, that means gametes carry 23 chromosomes instead of the 46 found in every other cell in the body.
That said, the full picture has a few wrinkles worth understanding, including a role for mitosis in the early stages and an interesting exception in plants.
Why Meiosis and Not Mitosis
The entire point of gametes is to combine with another gamete at fertilization. If egg and sperm cells each carried a full set of 46 chromosomes, the resulting embryo would have 92, and the number would double with every generation. Meiosis solves this by cutting the chromosome number in half. One round of DNA copying is followed by two rounds of division, yielding four haploid cells from a single diploid starting cell.
Mitosis, by contrast, produces two cells that are genetically identical to the parent cell. It’s how your body grows, replaces skin cells, heals wounds, and maintains tissues. The daughter cells keep the full set of 46 chromosomes. That’s useful for building a body, but it can’t produce a gamete that’s ready for fertilization.
How Meiosis Works in Two Divisions
Meiosis begins the same way mitosis does: the cell copies all of its DNA during a preparatory phase called interphase. From there, the process splits into two back-to-back divisions with no additional DNA copying in between.
In the first division (meiosis I), matching chromosomes from each parent pair up side by side. They can swap segments of DNA with each other, a process called crossing over. Then the pairs are pulled apart, sending one full chromosome from each pair into each of the two new cells. These cells now have 23 chromosomes, but each chromosome still consists of two joined copies (sister chromatids), so the total amount of DNA is still higher than it will be in the final gamete.
The second division (meiosis II) looks a lot like a standard mitotic division. The sister chromatids separate, and each one goes to a different cell. The result: four cells, each with 23 single chromosomes and a truly haploid amount of DNA.
Meiosis Creates Genetic Variation
If gametes were made by mitosis, every sperm cell from one man would be genetically identical, and every egg from one woman would be too. Meiosis prevents that through two built-in shuffling mechanisms.
The first is crossing over. When matched chromosomes pair up during meiosis I, they physically exchange stretches of DNA. A gene variant you inherited from your mother can end up on the same chromosome as a variant from your father that was originally on a different copy. This recombination creates chromosomes that are genuinely new combinations, not exact copies of either parent’s originals.
The second mechanism is independent assortment. Humans have 23 pairs of chromosomes, and during meiosis I, each pair sorts into the two daughter cells randomly. Whether you send your mother’s copy of chromosome 3 to one cell has no effect on which copy of chromosome 7 ends up there. This alone allows for over 8 million possible chromosome combinations per gamete, before crossing over adds even more variety.
Where Mitosis Still Plays a Role
Although gametes themselves come from meiosis, the precursor cells that eventually enter meiosis are maintained and multiplied through mitosis. In males, stem cells called spermatogonia sit along the outer wall of the seminiferous tubules in the testes. These cells divide by mitosis repeatedly, producing both more stem cells (to keep the supply going) and committed cells that will eventually become primary spermatocytes. It’s only at that spermatocyte stage that meiosis begins.
In females, a similar mitotic expansion happens, but the timing is dramatically different. Precursor cells called oogonia multiply by mitosis during fetal development, peaking at roughly 7 million germ cells by the seventh month of gestation. Most of these die off. The survivors enter the first stage of meiosis before birth and then pause. They stay frozen in that early meiotic phase, sometimes for decades, until hormonal signals at ovulation restart the process.
So mitosis builds and maintains the pool of germ cells. Meiosis is the step that actually transforms them into gametes.
Spermatogenesis vs. Oogenesis
In males, the process from stem cell to mature sperm takes about 65 days. Each primary spermatocyte that enters meiosis produces four functional spermatids, which then reshape themselves into streamlined sperm cells. This process runs continuously from puberty onward.
In females, the timeline stretches across a lifetime. Primary oocytes begin meiosis during fetal development and arrest at an early prophase stage called the dictyate state. They can remain paused for 12 to nearly 50 years. When a hormonal surge triggers ovulation, one oocyte resumes meiosis and completes the first division. The second meiotic division only finishes if the egg is fertilized. And unlike sperm production, the divisions are unequal: most of the cell’s cytoplasm goes to one large egg cell, while the leftover material forms tiny polar bodies that disintegrate.
The Plant Exception
In animals, the answer is straightforward: gametes come from meiosis. In plants, the story has an extra step. Plants alternate between two life stages. The diploid stage (the sporophyte, which is the leafy plant you see) produces spores through meiosis. Those spores grow by mitosis into a tiny haploid stage called the gametophyte. The gametophyte then produces egg or sperm cells through mitosis, since its cells are already haploid.
In flowering plants, the gametophyte has been reduced to just a few cells, so it’s easy to overlook. But technically, the sperm and egg in a flower are produced by mitotic divisions within a haploid structure. Meiosis still happens earlier in the process to create that haploid stage, so both types of division are involved. If your biology class focuses on animals, though, the answer is simply meiosis.
When Meiosis Goes Wrong
Because meiosis involves such precise chromosome sorting, errors are possible. The most common mistake is called nondisjunction, where a pair of chromosomes fails to separate properly. This produces a gamete with one too many or one too few chromosomes. If that gamete is fertilized, the resulting embryo has an abnormal chromosome count.
Most chromosomal imbalances are fatal to the embryo early in development. A few are survivable. Down syndrome results from three copies of chromosome 21 and is the most common viable autosomal trisomy. Turner syndrome, where a female has only one X chromosome instead of two, is the only survivable monosomy. Other conditions, like Klinefelter syndrome (an extra X in males) or Edwards syndrome (trisomy 18), each carry distinct developmental effects. The risk of nondisjunction increases with the age of the egg cell, which is part of why chromosomal conditions become more likely with advanced maternal age: those oocytes have been paused in meiosis for decades.