Side by side
| Mitosis | Meiosis | |
|---|---|---|
| Nuclear divisions | One division, following one round of DNA replication | Two divisions, meiosis I and meiosis II, following one round of DNA replication |
| Nuclei produced | Two, usually partitioned into two new cells | Four, usually partitioned into four new cells |
| Ploidy of the product | The same number of chromosome sets as the original nucleus, so a diploid cell yields diploid nuclei | One chromosome set only, which is half the sets of the diploid cell it started from |
| Genetic outcome | The resulting nuclei are genetically identical to the original nucleus | The four nuclei are not genetically identical, because there is at least one crossover per chromosome |
| Homolog pairing | Homologous chromosomes never pair, and no synaptonemal complex forms | Homologous pairs meet in meiosis I, bind through the synaptonemal complex, and develop chiasmata |
| What lines up at the metaphase plate | Duplicated chromosomes, with sister kinetochores attached to opposite poles | Tetrads of paired homologs in metaphase I, then duplicated chromosomes in metaphase II |
| Where the reduction happens | Nowhere; there is no reduction in ploidy level during mitosis | Meiosis I is the reductional division; meiosis II is not, and is analogous to mitosis |
| Biological role | Replicates chromosomes and produces two identical nuclei ahead of cell division | Forms the egg and sperm cells of sexually reproducing organisms |
One division versus two, from one copy of the DNA
Both processes start the same way. The cell replicates its DNA once, in S phase, so every chromosome arrives at division as two sister chromatids. What differs is how many times the nucleus then divides. Mitosis divides once and stops, so each product keeps the full chromosome complement it started with. Meiosis divides twice on that single replication, which is the whole trick: two divisions spread across one set of copies leaves each of the four products with half the chromosome sets. That is why a diploid cell entering meiosis produces haploid gametes, and why fertilization can restore the diploid number instead of doubling it every generation.
Meiosis I is where the real difference lives
Students often try to memorize eight parallel stage lists and drown. The shortcut is that meiosis II is essentially a mitotic division: duplicated chromosomes line up, kinetochores split, sister chromatids separate. Everything genuinely new happens in meiosis I. Homologous chromosomes physically pair, bind through the synaptonemal complex, develop chiasmata and cross over between nonsister chromatids. At metaphase I whole tetrads line up, and at anaphase I the homologs, not the sisters, move apart. That single event drops the ploidy level from two sets to one, which is why meiosis I is called the reductional division and meiosis II is not.
Where this leads next
Two questions follow naturally. The first is what the stages of mitosis actually look like one by one, since meiosis borrows their names and adds a roman numeral; that is the stages of mitosis page. The second is what haploid and diploid mean in numbers rather than in words, because the phrase halves the chromosome number only makes sense once you can say that a human somatic cell carries 23 chromosome pairs while a gamete carries one set of 23. Both divisions also sit inside the larger cell cycle, where interphase, not division, takes up most of a cell's life.
Common mistakes
- Saying meiosis has two rounds of DNA replication. There is one replication, in S phase, followed by two divisions; that mismatch is what halves the chromosome sets.
- Calling meiosis II a reduction division. It separates sister chromatids and leaves one chromosome set in each product, exactly as it found it.
- Claiming meiosis produces four identical cells because the parent cell was one cell. Crossover in prophase I means at least one exchange per chromosome, so the four products differ.
- Confusing homologous chromosomes with sister chromatids. Homologs pair and separate in meiosis I; sisters separate in mitosis and in meiosis II.
Related concepts
- Haploid vs diploid: A haploid cell contains a single set of chromosomes.
- Stages of mitosis: OpenStax divides mitosis, or karyokinesis, into five stages: prophase, prometaphase, metaphase, anaphase and telophase.
- Cell cycle: The eukaryotic cell cycle has four phases: G₁, S and G₂, which together make up interphase, followed by M phase, the mitotic phase.