Meiosis: one cell becomes four sex cells

How to use this simulation

1. Start at interphase and play: the cell grows, and in S phase a second chromatid appears on every chromosome.
2. Pause at prophase I: two homologous chromosomes pair up tightly, and past the halfway mark one pair shows a colour swap — that is crossing over.
3. Watch metaphase I: pairs (bivalents) line up on the plate, not single chromosomes. This is the single biggest difference from mitosis.
4. Step through anaphase I and telophase I: there are now two cells, and the “chromosome number” reading drops from 2n to n.
5. Run meiosis II: both cells split sister chromatids apart, just like an ordinary mitosis, ending in four cells.
6. Tick “Compare side by side with mitosis” and run the whole thing: the same starting cell would give two identical diploid cells by mitosis, against four different haploid cells here.

You look like your parents, but never identical to them

Siblings resemble each other, but no two are ever perfectly identical — height, eye colour, blood group all differ. Each parent hands down half of their chromosomes, but which half is different every single time. That shuffling has one source: the cell division called meiosis.

Every ordinary human cell carries 46 chromosomes, 23 pairs. A sperm or an egg cell carries only 23. Put the two together and the child’s cells are back to 46. Something has to do that halving, and it also has to make sure no two sex cells carry exactly the same set — that is exactly what this page’s simulation lets you watch happen.

A fair question: cells already divide by mitosis, so why not use that to make sperm and eggs? The problem is that mitosis keeps the chromosome number the same. If both parents made sex cells by mitosis, every generation’s chromosome count would double. Meiosis exists to solve exactly that problem.

Starting from zero: what meiosis is and why it exists

Meiosis is a type of cell division in which one diploid (2n) parent cell divides twice in succession to produce four haploid (n) cells. In the simulation's cell 2n = 4, meaning two pairs of homologous chromosomes; by the end each cell holds n = 2 chromosomes, one from each pair.

Sex cells (gametes) such as sperm and eggs are built by meiosis, which is why it is the basis of gametogenesis. In plants the same process builds pollen and the egg cell inside the ovule. Without meiosis, sexual reproduction could not keep the chromosome number constant across generations.

Meiosis comes in two named rounds: meiosis I and meiosis II. Meiosis I is called the reductional division, because it is here that the chromosome number drops from 2n to n. Meiosis II looks like an ordinary mitosis, because the chromosome number does not change there — only the two sister chromatids of each chromosome separate. Across both rounds, DNA is copied once (in S phase) while the nucleus divides twice — that one sentence is the whole definition.

Not every cell in the body does meiosis. In humans it happens only in the testes (making sperm) and the ovaries (making eggs); in plants, in the anther and the ovule. Every other cell, such as skin or liver cells, grows and repairs itself through mitosis.

Key terms in meiosis

Get the vocabulary straight before the stages — definition questions in exams come straight from this table.

Meiosis I: the four stages of the reductional division

Meiosis I has one job: pull the two homologous chromosomes of each pair apart. Sister chromatids do not separate in this round — they stay attached the whole time.

Meiosis II: the four stages of the equational division

Both cells from meiosis I now run meiosis II independently and at the same time. It is called the equational division because the chromosome number does not change — it looks and behaves exactly like an ordinary mitosis, just inside a cell whose chromosome number is n, not 2n.

Crossing over and independent assortment: two engines of variation

Meiosis creates genetic variety through two separate events. The first is crossing over: in prophase I, non-sister chromatids swap segments and create chromatids with new gene combinations. The second is independent assortment: at metaphase I, which chromosome of a bivalent faces which pole is decided randomly and separately for every pair.

For a cell with n pairs of chromosomes, independent assortment alone gives 2ⁿ possible different chromosome combinations. In the simulation's cell n = 2, so that is 2^2 = 4. In humans n = 23, so the figure is 2^23 = 8,388,608 — before crossing over even adds its own variety. That is why no two siblings ever look exactly alike.

Mitosis vs meiosis

Both are cell division and both copy DNA once, but the outcomes are completely different. If you have already looked at this site’s mitosis animation, match the table below against what you saw there.

Why meiosis matters

In short: mitosis keeps a body alive; meiosis keeps a species alive.

• It produces sex cells, which is what makes sexual reproduction possible at all.
• It keeps the chromosome number constant across generations; without it, fertilisation would double the chromosome count every generation.
• It generates genetic variation through crossing over and independent assortment, which improves a species’ chances of surviving change.
• It speeds up evolution by spreading harmful mutations thin and bringing beneficial mutations together into new combinations.

Try this in the simulation

Predict each result before you run it, then check.

• Run from interphase to telophase I at 0.5× speed and note the exact moment the “2n = 4” reading flips to “n = 2”.
• Pause in prophase I and drag the timeline slider slowly: watch for the exact moment the colour swap (crossing over) appears.
• Jump between metaphase I and metaphase II: one shows pairs lined up, the other single chromosomes — the difference should be obvious at a glance.
• Turn on “Compare side by side with mitosis” and run the whole timeline: match the two identical diploid cells on the right against the four different haploid cells on the left.
• Watch the “Total cells” reading: check that it moves from 1 to 2 and then from 2 to 4 exactly at the ends of telophase I and telophase II.

Solved problems, step by step

Meiosis problems are mostly counting problems: chromosomes, chromatids, bivalents and possible combinations.

Mistakes almost everyone makes

Avoiding these keeps exam marks from slipping away on meiosis questions.

• Thinking single chromosomes line up at metaphase I. Pairs (bivalents) line up there; single chromosomes line up only at metaphase II.
• Confusing anaphase I with anaphase II. Anaphase I separates homologous chromosomes (centromere intact); anaphase II separates sister chromatids (centromere splits).
• Treating crossing over and independent assortment as the same event. Crossing over swaps segments within one bivalent; independent assortment is the random choice of which bivalent faces which pole.
• Assuming all four cells from meiosis are always the same size. That is true for sperm, but egg formation divides cytoplasm unequally, giving one large egg and several small polar bodies.
• Believing new DNA is copied between meiosis I and meiosis II. DNA is copied exactly once in meiosis, during S phase, before meiosis I even begins.

Meiosis in real life

Meiosis is not only a textbook chapter; it is the reason every new generation looks a little different from the last.

• Human reproduction: sperm and egg cells are made by meiosis in the testes and ovaries, and their fusion at fertilisation starts a new individual.
• New crop varieties: plant breeders rely on the crossing over and independent assortment that happen during meiosis to combine useful traits when crossing varieties.
• Genetic screening: when chromosomes fail to separate correctly in meiosis (nondisjunction), conditions such as Down syndrome can result; prenatal chromosome tests build on this knowledge.
• Conservation breeding: programmes for endangered species plan matings to preserve genetic variety, using exactly the logic behind independent assortment.
• Genetic counselling: advice about a couple’s chance of a particular inherited trait in their children rests on meiosis and Mendel’s rules together.

Exam corner

Meiosis is a standard topic in cell-biology units at secondary and higher-secondary level. Expect definition questions on synapsis, crossing over and haploid/diploid, short-answer questions comparing mitosis and meiosis, and numerical questions on bivalents, chromatid counts or possible combinations.

Revision: last-minute summary

The night before an exam, this list plus the mitosis-vs-meiosis table is all you need to revisit.

• Meiosis: one diploid (2n) cell divides twice to make four haploid (n) sex cells; in the simulation 4 → 2.
• Meiosis I = reductional division: synapsis and crossing over in prophase I, pairs line up in metaphase I, homologous chromosomes separate in anaphase I.
• Meiosis II = equational division: single chromosomes line up (metaphase II), sister chromatids separate (anaphase II).
• Crossing over and independent assortment together create variation; for n pairs, possible combinations = 2ⁿ.
• Mitosis: one division, two identical diploid cells. Meiosis: two divisions, four different haploid cells.
• DNA is copied only once in meiosis, before meiosis I.

Frequently asked questions