Mitosis & Meiosis
Chromosome, chromatid and DNA counts at every stage of mitosis and meiosis, for any 2n.
in each body cell
in each gamete · 23 bivalents in prophase I
2^23 gametes from independent assortment alone
| Stage | Chromosomes | Chromatids | DNA | Cells | What happens |
|---|---|---|---|---|---|
| G₁ | 46 | 46 | 2C | 1 | The cell grows. Each chromosome is a single chromatid. |
| S phase | 46 | 92 | 4C | 1 | DNA replicates. Each chromosome gains a sister chromatid. |
| Prophase | 46 | 92 | 4C | 1 | Chromosomes condense; the nuclear envelope breaks down. |
| Metaphase | 46 | 92 | 4C | 1 | Chromosomes line up single file on the metaphase plate. |
| Anaphase | 92 | 92 | 4C | 1 | Sister chromatids separate — each becomes its own chromosome, so the count doubles while the DNA does not. |
| Telophase | 46 | 46 | 2C | 2 | Two nuclei re-form, each around a full set. |
| After cytokinesis | 46 | 46 | 2C | 2 | Two daughter cells, each genetically identical to the parent. |
Watch anaphase: the chromosome count jumps from 46 to 92 without the DNA changing, because separating sister chromatids turns each one into a chromosome in its own right. Nothing has been copied — it has only been renamed.
Mitosis copies a cell; meiosis halves one. The counts are where marks go, because three things change at different moments — for a human cell, 46 chromosomes become 92 chromatids after replication, then 92 chromosomes in anaphase, with no new DNA made.
Three quantities, changing at three different times
The confusion is almost always about counting, not about the biology. DNA replication in S phase doubles the DNA and doubles the chromatids, but leaves the chromosome number alone — each chromosome simply now has two sister chromatids joined at a centromere. That is why a human cell entering mitosis has 46 chromosomes and 92 chromatids.
Then in anaphase the centromeres split and the sisters are pulled apart. At that instant the chromosome count doubles to 92, because a chromatid that is no longer attached to a partner counts as a chromosome in its own right. No DNA has been made; the same material has only been renamed. Cytokinesis then halves everything between two cells, returning each to 46.
Meiosis separates the wrong things first
The single most useful fact about meiosis is that anaphase I does not do what mitotic anaphase does. It pulls whole homologues apart while leaving sister chromatids firmly together — the opposite order. That is precisely why the chromosome number halves: each daughter cell receives one member of each pair rather than one chromatid from each chromosome. Anaphase II then does the mitotic job, separating sisters, in cells that are already haploid.
counting centromeres is the reliable way to get the chromosome number at any stage
- 1 Count centromeres, not arms. One centromere is one chromosome, whether it carries one chromatid or two. This single rule settles almost every counting question.
- 2 Ask whether replication has happened. After S phase every chromosome has two chromatids, so chromatids are double the chromosome count. Before it, they are equal.
- 3 Check whether centromeres have split. In anaphase of mitosis, and anaphase II of meiosis, they do — so the chromosome count doubles while the DNA stays put.
- 4 Count the cells, not just the chromosomes. After cytokinesis the material is shared out. Two cells of 46 in mitosis; eventually four cells of 23 in meiosis.
- 5 Sanity-check the ploidy. Mitosis ends diploid, meiosis ends haploid. If your count says otherwise, a separation step has been misread.
A human cell through both processes
2n = 46. Notice the DNA never increases except in S phase.
| Stage | Chromosomes | Chromatids | DNA | Cells |
|---|---|---|---|---|
| G₁ | 46 | 46 | 2C | 1 |
| After S phase | 46 | 92 | 4C | 1 |
| Mitosis: metaphase | 46 | 92 | 4C | 1 |
| Mitosis: anaphase | 92 | 92 | 4C | 1 |
| Mitosis: two daughters | 46 each | 46 each | 2C each | 2 |
| Meiosis: metaphase I | 46 | 92 | 4C | 1 |
| Meiosis: after telophase I | 23 each | 46 each | 2C each | 2 |
| Meiosis: four gametes | 23 each | 23 each | 1C each | 4 |
Where the variation comes from
Meiosis produces four cells that all differ, and two mechanisms do the work. The first is independent assortment: in metaphase I each bivalent lines up without regard to the others, so with 23 pairs there are 2²³ — about 8.4 million — possible combinations before anything else happens. The second is crossing over in prophase I, where homologues physically exchange segments, so even a single chromosome handed on is a mixture of the two a person inherited.
Together they mean the figure the tool reports is a floor rather than a total. Crossing over multiplies it by an amount that depends on how many chiasmata form, and then fertilisation squares the whole thing when two gametes combine. The 8.4 million is the part that can be counted exactly; the rest is why siblings are not clones.
One boundary worth stating. The DNA figures use C-values, where 1C is the amount in a gamete, because that is how the counts are usually examined. They describe amount, not sequence: two cells can both hold 2C and carry quite different alleles, which is exactly what meiosis is for. And the stages here are the textbook discrete steps; a real cell moves through them continuously, and prometaphase in particular is often folded into prophase or metaphase depending on the syllabus.