Mitosis vs Meiosis: the Phases, Crossing Over and Nondisjunction

Two kinds of division

Mitosis splits one cell into two that are genetically identical to it, each with the full set of chromosomes. It is how a fertilized egg becomes a body and how skin, gut lining and blood are replaced. Meiosis splits one cell into four, each with half the chromosomes, to make eggs and sperm (or spores in plants and fungi). When two such cells fuse at fertilization, the full number is restored.

Body cells are diploid (2n): they hold two sets of chromosomes, one from each parent. Humans have 2n = 46, that is 23 pairs. Each pair are homologous chromosomes: same genes, same order, possibly different versions (alleles). Sex cells are haploid (n = 23).

Before either division: replication

In the S phase of interphase each chromosome is copied. The two copies, sister chromatids, stay joined at the centromere, which is why a chromosome in mitosis looks like an X. The number of chromosomes is unchanged (46), but the DNA content doubles, from 2C to 4C, where C is the DNA in one set.

Mitosis, phase by phase

1. Prophase: the long, thin chromatin coils up into compact chromosomes; the spindle starts to form between the centrosomes.
2. Prometaphase: the nuclear envelope breaks up and spindle fibers attach to the kinetochores at each centromere, one sister to each pole.
3. Metaphase: the chromosomes line up on the equator, the metaphase plate. A checkpoint waits until every one is attached from both sides.
4. Anaphase: the sisters separate and are pulled to opposite poles. Each is now a chromosome in its own right.
5. Telophase and cytokinesis: nuclear envelopes form round each group, the chromosomes uncoil, and the cell pinches in two.

Result: two diploid cells, 46 chromosomes each, one chromatid per chromosome, 2C of DNA.

Meiosis, phase by phase

Meiosis I separates the homologs:

1. Prophase I: homologous chromosomes pair up along their length (synapsis) into four-chromatid bundles. Non-sister chromatids exchange matching segments: crossing over, visible as chiasmata. This is the longest phase; in human eggs it is paused for decades.
2. Metaphase I: the pairs line up on the equator, each pair facing the poles at random: independent assortment.
3. Anaphase I: the homologs go to opposite poles; the sisters stay together.
4. Telophase I: two cells, each with 23 chromosomes of two chromatids each (2C).

Meiosis II is like mitosis without a new round of copying: the chromosomes line up in each cell, the sisters separate, and four haploid cells result, each with 23 single-chromatid chromosomes (1C).

Counting through a division (n = 23)

• After S phase: 46 chromosomes, 92 chromatids, 4C.
• Mitosis, each daughter: 46 chromosomes, 46 chromatids, 2C.
• After meiosis I, each cell: 23 chromosomes, 46 chromatids, 2C.
• After meiosis II, each cell: 23 chromosomes, 23 chromatids, 1C.

Count chromosomes by centromeres, not by chromatids: an X-shaped replicated chromosome is still one chromosome.

Why every sex cell is different

Independent assortment alone gives 2ⁿ combinations of whole chromosomes: 2²³ = 8,388,608 in humans. Crossing over, one to three exchanges per chromosome pair, mixes alleles within chromosomes as well, so the number of possible sex cells is effectively unlimited. Add random fertilization and no two children of the same parents (apart from identical twins) share a genome.

Nondisjunction

If a pair of homologs (in meiosis I) or two sisters (in meiosis II or mitosis) fail to separate, one daughter gets both and the other gets neither. A sex cell with an extra chromosome produces a trisomy at fertilization, one lacking a chromosome a monosomy. Trisomy 21 (Down syndrome) is the most common survivable one; its risk rises steeply with the mother's age, because eggs stay paused in meiosis I for decades. Nondisjunction in mitosis produces a mosaic, and is common in cancer cells.

Using the simulation

The Mitosis and Meiosis Simulator plays a division with red chromosomes from one parent and blue from the other. Things to try:

• Run mitosis with two pairs, then meiosis with the same, and compare the end: two cells of four chromosomes against four cells of two.
• In meiosis, watch prophase I: the tip of one red and one blue chromatid swap colors at the crossover point. Move the slider and the swapped piece changes size.
• Press Reset several times and note in metaphase I which side each red chromosome faces: it changes at random.
• Tick nondisjunction and read the chromosome counts in the final cells.

What the model assumes

• Up to four pairs of chromosomes, of decreasing length, standing in for the real number.
• One crossover per pair, at the same point along every chromosome, between the two inner chromatids. Real crossovers vary in number and position, and can involve any pair of non-sister chromatids.
• Fixed phase timings. Real mitosis takes about an hour of a cell cycle that lasts about a day, with interphase taking almost all of it; prophase I of meiosis can last days in sperm and decades in eggs.
• Nondisjunction of the first pair only, in the first division.
• Schematic cells: centrosomes, spindles and nuclei are simplified; there are no other organelles, and the four products of meiosis are drawn equal.

Edge cases

• One pair (2n = 2): only two ways to assort; the products of meiosis differ only by crossing over.
• Crossing over at the very end of a chromosome swaps almost nothing; near the centromere it swaps almost a whole arm.
• Nondisjunction in mitosis gives 2n + 1 and 2n − 1 cells; in meiosis I, two cells with n + 1 and two with n − 1.
• DNA content stays 4C through the whole of mitosis until cytokinesis, even after the sisters have separated in anaphase.

Where the model stops being right

• Unequal division in eggs. In female meiosis the divisions are lopsided: one large egg and two or three tiny polar bodies that die. Only male meiosis gives four equal sperm.
• Sex chromosomes. X and Y pair only at small regions at their tips, and the simulation's homologs are all autosomes.
• Plant cells build a new wall from the middle out (a cell plate) instead of pinching in, and many plants are polyploid, with more than two sets.
• Errors and checkpoints. Real cells have spindle checkpoints, cohesin proteins that hold sisters together and cut at anaphase, and repair pathways; when these fail, cells can end up with whole extra sets or broken chromosomes, which this model does not show.
• Bacteria do not do either; they copy a single circular chromosome and split by binary fission, as in the Bacterial Growth Simulator.

Related tools

Follow alleles from parents to offspring with the Punnett Square Calculator and its guide, How to Use a Punnett Square; see how allele frequencies change over generations in the Natural Selection and Population Genetics Simulator; and look at the molecule being copied in the DNA and RNA Folding Viewer.