Meiosis

Both meiosis and mitosis are ways a cell divides, and it is easy to blur them together — but they do fundamentally different jobs. Mitosis is a single division that produces two daughter cells that are identical to the parent and to each other, and still diploid (2n). It is used for growth and repair. Meiosis is two divisions in a row that produce four daughter cells that are genetically varied and haploid (n). It is used to make gametes for sexual reproduction.

The second big idea — the one graders love to test — is that meiosis halves the chromosome number. A diploid cell carrying two of each chromosome (2n) ends up as haploid cells carrying one of each (n). This reduction is not an afterthought; it is the whole point. Because gametes are halved, fertilization can fuse two of them and land back at the diploid number instead of doubling the chromosome count every generation.

Hold onto two contrasts and the rest of the topic follows: how many divisions (meiosis two, mitosis one) and what happens to the chromosome count (meiosis halves it 2n → n; mitosis keeps it 2n). If you can answer those two questions for a given process, you will not confuse the two.

Meiosis is one round of DNA replication followed by two divisions. Watching what separates in each division is what tells you when the chromosome number is actually reduced.

1. Replicate once. Before meiosis begins, the cell copies its DNA a single time, so every chromosome now consists of two identical sister chromatids. Crucially, replication happens only once, even though the cell will divide twice — that mismatch is exactly what ends up halving the count.
2. Meiosis I — separate the homologs (reductional division). Homologous chromosomes pair up and then the two members of each pair are pulled into separate cells. Because whole homologs — not sister chromatids — are split apart, each resulting cell now has only one chromosome from each pair. This is where diploid (2n) becomes haploid (n): meiosis I is the reductional division.
3. After meiosis I. There are now two haploid cells, but each chromosome still has its two sister chromatids attached. The number has already been halved — the cells are haploid — even though a second division is still to come.
4. Meiosis II — separate the sister chromatids. In each of the two cells, the sister chromatids of every chromosome are finally pulled apart, just as they are in mitosis. This division does not reduce the chromosome number again; it simply distributes the chromatids so each cell holds one copy.
5. The result. Four haploid (n) cells, each genetically varied. From one diploid parent cell, meiosis produces four gametes with half the chromosome number, ready for fertilization to restore the diploid count.

Notice the through-line: replicate once, divide twice. The halving is locked in at meiosis I when homologs separate; meiosis II is the mitosis-like clean-up that splits sister chromatids without changing the count.

Quick reference card. For each term, read what it is and how it distinguishes meiosis from mitosis — divisions and chromosome count are the whole game.

It is tempting to picture meiosis as just a slower version of mitosis that happens to make gametes. But the defining feature is the reduction: a diploid cell (2n) becomes haploid (n). Missing that halving — or misplacing when it happens — is where most points are lost.

The reduction is the point. If gametes kept the full diploid set, then every fertilization would double the chromosome number: 2n + 2n = 4n, then 8n, and so on generation after generation. Halving in meiosis (n + n = 2n at fertilization) is what keeps the chromosome number stable across generations. So meiosis absolutely does change the chromosome number — that is not a side effect, it is the reason meiosis exists.

Reduction happens at meiosis I, not meiosis II. The number is halved the moment homologous chromosomes separate in meiosis I, because each new cell then carries only one member of each pair. After meiosis I the cells are already haploid, even though each chromosome still has two sister chromatids. Meiosis II merely separates those sister chromatids — it looks just like mitosis and does not reduce the count again.

Four cells, all varied. Because meiosis runs two divisions from one replication, one diploid cell yields four haploid cells. And unlike the identical products of mitosis, these four are genetically varied. The end state — four, haploid, varied — is the fingerprint of meiosis, and it is the opposite of mitosis at every point.

Keep the two questions straight. How many divisions? (Meiosis two, mitosis one.) What happens to the chromosome count? (Meiosis halves it at meiosis I; mitosis keeps it.) Answer those and you will never mistake one process for the other, nor claim that meiosis leaves the number unchanged.