Mitosis and Meiosis Explorer

Mitosis makes two identical cells; meiosis makes four different ones

Mitosis divides one cell into two genetically identical daughter cells, each with the parent’s chromosome number, the full diploid number 2n in a body cell. Meiosis divides a diploid cell twice to make four genetically different cells, each with the haploid number, n: in a human, 46 chromosomes in each daughter of mitosis and 23 in each product of meiosis. The pairs line up independently at metaphase I, so one person can make 2^n kinds of gamete before crossing over is counted.

Every count on this page follows two rules: chromosomes = centromeres and chromatids = DNA molecules, each chromatid being one DNA double helix. Copying the DNA in S phase doubles the chromatids and leaves the chromosome number alone. Anaphase doubles the chromosome number, because once the sisters separate each chromatid counts as a chromosome in its own right. Cytokinesis then shares everything between two cells.

How to use it

Choose mitosis or meiosis and a diploid number, then press Play to watch the division stage by stage, or step with Previous and Next, the arrow keys or the rows of the stage table. The readouts give what each cell holds at the stage on screen, and the table lists every stage at once for the 2n you chose, so a question such as how many chromatids a cell has at metaphase II can be read straight off it. The graph plots the DNA per cell through the whole division, with the chromosome number drawn against it.

In meiosis the colours carry the genetics: red chromosomes came from the mother and blue from the father. Shuffle picks another way for the pairs to line up at metaphase I, which changes the four cells made at the end, and crossing over leaves some chromatids with a tip of the other colour. A population of cells dividing by mitosis grows at the rate the cell doubling time calculator works out.

Worked example: a human cell, 2n = 46

Two rows catch most people out. A cell that has finished meiosis I has 23 chromosomes but 46 chromatids, and it is haploid even though it holds as much DNA as a body cell in G1. In anaphase II it has 46 chromosomes again, but they are the separated halves of 23, with no homologous pairs among them.

The same 23 pairs set the variety. Each pair can face either pole at metaphase I, so one person can make 2^23 = 8,388,608 kinds of gamete before crossing over is counted, and two parents can combine them in 4^23 ways, about 7.04 × 10¹³. Set 2n to 46 and the tool reproduces every number in the table.

Where the variety comes from

Independent assortment. At metaphase I each homologous pair lines up with one member facing each pole, and which way round it lands is independent of every other pair. Walter Sutton pointed out in 1903 that this alone lets one individual make 2^n kinds of gamete, and tabulated the numbers from 2 to 36 chromosomes. He illustrated them with the lowest estimate of the human count then published, 16; the count of 46 was established by Tjio and Levan in 1956.

The tool counts arrangements the careful way. With n pairs there are 2^n ways to orient them, but an arrangement and its mirror image send the same two sets of chromosomes to the two poles, so there are 2^(n − 1) distinct arrangements, each making two kinds of cell. For a human cell that is 4,194,304 arrangements and 8,388,608 kinds of gamete.

Mendel’s ratios follow from the same two facts, and the Punnett square calculator works them out: each gamete gets one chromosome of every pair, so one allele of each gene, and genes on different chromosomes assort independently because their pairs do.

Crossing over. In prophase I a maternal and a paternal chromatid of each pair break at the same point and rejoin the other way round, so a chromosome can leave carrying genes from both parents. In most species every pair needs at least one crossover, because the chiasma it forms is what holds the homologues together until anaphase I. Watch the colours: each side of a bivalent stays one colour until the homologues are pulled apart, because cohesin still holds each exchanged tip beside its old sister, and only then do the recombinant chromatids show a tip of the other colour.

The stages in order

Interphase comes first: G1, then S, when the DNA is copied, then G2. Mitosis itself runs prophase, prometaphase, metaphase, anaphase and telophase, and cytokinesis then divides the cell. Some courses list only four stages and count prometaphase as part of prophase. The stages, the counting rules and the captions here follow OpenStax Biology 2e, sections 10.2 and 11.1.

1. Prophase: chromosomes condense and the spindle starts to form.
2. Prometaphase: the nuclear envelope breaks down and kinetochores attach to the spindle.
3. Metaphase: chromosomes line up on the metaphase plate.
4. Anaphase: sister chromatids separate and move to opposite poles.
5. Telophase: nuclear envelopes reform around the two sets and the chromosomes decondense.

Meiosis passes through the same stages twice, as meiosis I and meiosis II, with no S phase in between, and the tool folds prometaphase into the stages either side of it. Prophase I is long and is usually split into leptotene, zygotene, pachytene, diplotene and diakinesis. The three prophase I steps here are pairing, which covers leptotene and zygotene, crossing over, which is pachytene, and chiasmata, which covers diplotene and diakinesis.

What the drawing leaves out

• Real timing. Each stage gets a few seconds so it can be watched, and none of the durations are to scale. A human egg, for instance, stops in prophase I before birth and can stay there for decades.
• Real sizes. Chromosomes are drawn to fit the cell, each pair a different length with its centromere in a different place so that the pairs can be told apart. They are not to scale for any species. When there are too many to fit end to end across the cell, the metaphase plate is drawn as a band several chromosomes deep, standing for a plate seen edge on.
• Variable crossovers. Every pair crosses over exactly once, on its long arm, between its two inner chromatids. Real pairs can have more than one crossover, at varying places.
• Egg formation. The four equal cells at the end match sperm formation, which happens in the testes the male reproductive organs explorer shows in 3D. In egg formation the divisions are unequal, giving one egg and small polar bodies, and meiosis II is completed only if the egg is fertilised.
• Plant meiosis. Meiosis is drawn in an animal cell. In a plant it makes haploid spores rather than gametes, and the gametes come later from the gametophytes those spores grow into.
• Mistakes. Nondisjunction, when homologues or sister chromatids fail to separate, leaves cells with an extra or a missing chromosome, and is not modelled.

Common mistakes

• Counting chromatids as chromosomes. A replicated chromosome is one chromosome with two chromatids. S phase doubles the DNA, not the chromosome number.
• Halving the chromosome number in meiosis II. It halves in meiosis I, when the homologues separate. Meiosis II halves the DNA in each cell and leaves the chromosome number at n.
• Calling the cells after meiosis I diploid. They hold 2C of DNA, but only one chromosome of each pair, so they are haploid.
• Putting DNA replication in prophase. It happens in S phase, before division starts, and not at all between meiosis I and meiosis II.
• Treating homologous chromosomes as copies. Homologues carry the same genes in the same places, one from each parent, in versions that can differ. Sister chromatids are the copies.
• Crossing over between sisters. The exchange that makes new combinations is between non-sister chromatids, one maternal and one paternal.