Meiosis: Making Eggs and Sperm
Your body cells carry 46 chromosomes — 23 pairs, one of each from your mother and one from your father. But an egg or a sperm can only carry 23, or every generation would double the count. Meiosis is the two-round division that solves this: it copies the DNA once, then divides twice, and along the way it shuffles your parents’ genes so that no two gametes are ever the same. Press play and watch one cell become four — and watch why you are not a clone of either parent.
Try this: switch to Crossing over and watch coloured tips swap between the maternal and paternal chromosomes — then flip on ⚠ Nondisjunction and watch one pair refuse to separate, leaving gametes with 24 and 22 chromosomes.
Live meiosis readout
46
2C
1
Crossovers this run: 0
What's happening
The staging (3 pairs, timing, cell shapes) is an illustrative model. The numbers are real: human body cells hold 46 chromosomes, gametes 23, and independent assortment alone gives 223 ≈ 8,388,608 combinations.
The Science in Plain Language
Two divisions, one job: cut the chromosomes in half
Almost every cell in your body is diploid: it carries 46 chromosomes, arranged as 23 matching pairs. One member of each pair came from your mother, the other from your father. Eggs and sperm cannot follow that rule — if a 46-chromosome egg met a 46-chromosome sperm, the child would have 92, and the number would keep doubling forever. So gametes must be haploid: 23 chromosomes, one from each pair. Meiosis is the special division that gets there. It copies the DNA once and then divides twice, so a single starting cell yields four cells, each with half the chromosomes. Fertilisation then adds the missing half back, restoring 46.
Meiosis vs mitosis: what is actually different
Both divisions start the same way — the DNA is copied so every chromosome becomes two sister chromatids. From there they split apart. Mitosis is one division that makes two cells identical to the parent (46 chromosomes each), for growth and repair; the chromosomes never pair up and nothing is shuffled. Meiosis is two divisions that make four cells with half the chromosomes (23 each), only for making gametes; the homologues pair, cross over and are then separated. The single most important difference is that first meiotic division: in mitosis the sister chromatids separate straight away, but in meiosis I the whole homologous pairs separate first, and the sisters are not pulled apart until meiosis II. That one extra separation is what turns diploid into haploid.
Meiosis I is the reduction division
This is the step that actually halves the count, and it is what makes meiosis different from ordinary mitosis. First the DNA is copied, so every chromosome becomes two identical sister chromatids joined at a centromere (DNA content rises from 2C to 4C, but the chromosome number is still 46). Then, in prophase I, the homologous chromosomes find each other and pair up along their whole length — something mitosis never does. In anaphase I, the spindle pulls whole homologues (still two chromatids each) to opposite poles. Because each pole receives one of each pair rather than one of each chromosome, the daughter cells are already haploid: 23 chromosomes, each still doubled. A protein called shugoshin guards the glue (cohesin, specifically REC8) at the centromeres so the sisters stay together for now — they will be separated later, in meiosis II.
Crossing over: your chromosomes are remixed, not copied
While the homologues are paired in prophase I, they do something remarkable. An enzyme called SPO11 deliberately cuts the DNA, and the cell repairs the break using the partner chromosome as a template. The result is a physical swap of matching segments at points called chiasmata — this is crossing over, or recombination. A chromosome you pass on is therefore a mosaic: part maternal, part paternal, stitched together in a new order. It is not enough to be optional, either — each pair needs at least one crossover (an “obligate chiasma”) to line up and separate correctly, and human chromosomes typically make on the order of one to three each. The visualization shows this as coloured tips swapping between a blue and a pink chromosome.
Independent assortment: about 8.4 million ways to pack a gamete
In metaphase I the pairs line up in the middle of the cell, and here is the second engine of variety: each pair orients at random, with no memory of which parent’s copies the other pairs chose. That means any combination of your two grandparental sets can end up together. With 23 pairs, the number of possible arrangements is 223 = 8,388,608 — over eight million distinct gametes from independent assortment alone. Layer crossing over on top and the practical number is effectively unlimited. This is the real reason siblings differ so much despite sharing the same two parents.
Meiosis II is the “mitosis-like” division
After a brief pause, each of the two haploid cells divides again — but this time there is no pairing and no crossing over. The chromosomes simply line up and the sister chromatids separate, exactly as they would in mitosis. DNA content drops from 2C to 1C: a single copy of each chromosome. The final tally is four haploid cells, each with 23 single chromosomes, and each genetically unique thanks to the shuffling that happened in meiosis I.
Sperm make four; eggs make one (and toss the rest)
The two sexes run the same machinery but keep different products. In spermatogenesis, all four haploid cells mature into sperm, and the whole process runs continuously — a single sperm takes roughly 64–74 days to make, and a man produces them by the hundreds of millions daily. In oogenesis, the divisions are deeply lopsided: one cell keeps nearly all the cytoplasm and becomes the egg, while the leftover chromosomes are discarded in tiny polar bodies. A female is born with all her egg cells already started and then frozen in prophase I — some stay arrested for 40+ years before ovulation finishes the job. Of the one-to-two million she is born with, only around 400 are ever ovulated.
When it goes wrong: nondisjunction and trisomy
Meiosis is astonishingly precise, but not perfect. If a pair fails to separate in anaphase I (or sisters fail in anaphase II), the error is called nondisjunction. One gamete ends up with an extra chromosome (24 in our model) and the other is short one (22). Fertilising the 24-chromosome gamete produces a trisomy — three copies instead of two. The best known is Down syndrome (trisomy 21); trisomy 18 (Edwards) and 13 (Patau) also occur, along with sex-chromosome variations such as 45,X (Turner) and 47,XXY (Klinefelter). Most other aneuploid conceptions do not survive to birth. The risk of trisomy 21 rises sharply with maternal age — commonly cited figures run from roughly 1 in 1,300–1,500 around age 20 to about 1 in 100 by age 40, because the decades-long arrest in prophase I lets the chromosome glue weaken over time. Toggle Nondisjunction in the diagram to watch exactly this failure play out.
An honest correction: “half from each parent” is only half the story
It is true that you inherit 23 chromosomes from each parent. But two common beliefs need fixing. First, you do not pass on your parents’ chromosomes intact — crossing over means each chromosome you hand to your own child is a fresh maternal-plus-paternal mosaic, which is why no two of your gametes are alike. Second, not everything is shuffled: your mitochondrial DNA comes almost entirely from your mother’s egg and is passed down essentially unchanged, outside the meiotic reshuffle. So “you’re a 50/50 blend” is a useful shorthand for nuclear genes, but the real picture is a remix — with one small, strictly maternal exception.