Mitosis vs meiosis: how cells divide, compared

The difference between mitosis vs meiosis is what each division is for. Mitosis makes copies: one cell divides once into two cells with the same chromosomes as the original, which is how a body grows and repairs itself. Meiosis makes sex cells: one cell divides twice into four cells, each with half the usual number of chromosomes and a new combination of the parents' genes, which become sperm or, in women, one egg. In humans, mitosis keeps 46 chromosomes; meiosis halves them to 23.

In short

• Mitosis: one division, two identical daughter cells, chromosome number kept. Meiosis: two divisions, four different cells, chromosome number halved.
• Halving matters because fertilization adds two sets together; without it, the count would double every generation.
• Meiosis shuffles genes in two ways, crossing over and independent assortment, which is why siblings differ.

How do mitosis and meiosis compare, side by side?

Both start the same way: before dividing, the cell copies every chromosome, so each one becomes two identical sister chromatids joined at the middle. What happens next is where they part.

	Mitosis	Meiosis
Purpose	Growth, repair, replacing cells; asexual reproduction in some organisms	Making eggs and sperm (or spores, in plants and fungi) for sexual reproduction
Where in the human body	Almost every tissue	Only in the ovaries and testes
Number of divisions	One	Two, meiosis I and meiosis II
Daughter cells	2	4 (in women, one egg and small cells that are discarded)
Chromosomes in each human daughter cell	46, two sets (diploid)	23, one set (haploid)
Genetically identical to the parent cell?	Yes, barring copying errors	No; each one is unique
Do matching chromosomes pair up?	No	Yes, in meiosis I, and they swap pieces

The National Human Genome Research Institute's glossary puts it in a sentence each: mitosis produces "two identical nuclei," while meiosis "reduces the number of chromosomes in gametes" through "two rounds of division to yield four haploid daughter cells."

What happens in mitosis, stage by stage?

Mitosis is how one cell becomes two copies of itself. The chromosomes, already copied, go through four stages, and then the cell itself splits.

1. Prophase: the long, loose chromosomes coil up tight enough to see, each as an X of two sister chromatids. A scaffold of protein fibers, the spindle, starts to form.
2. Metaphase: with the envelope around the nucleus gone, spindle fibers attached to each chromosome's centromere pull the chromosomes into line in a single row across the middle of the cell.
3. Anaphase: the sister chromatids of every chromosome are pulled apart to opposite ends of the cell. From here, each one counts as a chromosome in its own right.
4. Telophase: a new nucleus forms around each set, and the chromosomes loosen again.
5. Cytokinesis: the cell pinches in two (or, in plants, builds a wall between the halves).

Many textbooks add another stage, prometaphase, between prophase and metaphase, for the moment the nuclear envelope breaks down and the fibers catch the chromosomes. The point of all the choreography is that each daughter cell gets exactly one copy of every chromosome, no more and no fewer. The cell checks this: it won't start anaphase until every chromosome is attached to fibers from both sides.

What happens in meiosis, and why twice?

Meiosis has the same named stages, run twice, but the first round does something mitosis never does.

Meiosis I separates partners. In prophase I, each chromosome finds its partner, the matching chromosome from the other parent, and the two lie side by side along their length. While paired, they swap matching stretches of DNA, a process called crossing over. In metaphase I the pairs, not single chromosomes, line up across the middle. In anaphase I the partners are pulled apart, one to each end. Each new cell now has 23 chromosomes, one from each pair, though each is still a doubled X.

Meiosis II separates sisters. It works like mitosis: the sister chromatids of each chromosome are pulled apart. The result is four cells, each with 23 single chromosomes.

The two rounds are needed because the chromosomes were copied once at the start. One division undoes the copying; the other halves the set. Fertilization then restores the number: 23 from the egg plus 23 from the sperm makes 46.

In women, meiosis has a long pause. Egg cells begin meiosis before birth and stop partway through prophase I. The Nature Education overview by Clare O'Connor notes that females are born with a finite number of oocytes arrested there, and each resumes only when it matures, which can be decades later.

How much variety does meiosis create, in numbers?

Meiosis shuffles genes in two ways, and the numbers explain why no two siblings, other than identical twins, are genetically the same.

Independent assortment

In metaphase I, each of the 23 pairs lines up with either the mother's or the father's chromosome facing a given end, independently of every other pair. Each pair is a coin toss.

1. With 1 pair, a sperm or egg can get one of 2 versions.
2. With 2 pairs, 2 × 2 = 4 combinations.
3. With 23 pairs, 2²³ = 8,388,608 combinations.

So each person can make about 8.4 million different eggs or sperm from the whole-chromosome shuffle alone. A child gets one egg and one sperm, so two parents could produce 8,388,608 × 8,388,608 = about 70 trillion (7.0 × 10¹³) different children, before counting crossing over.

Crossing over

Crossing over multiplies that further. Partner chromosomes usually swap pieces at one to a few points each, and each pair needs at least one crossover to separate properly, as a 2011 study of human recombination notes. Because the swap points vary each time, each chromosome a child receives is a new patchwork of the grandparents' versions, and the count of possible combinations becomes too large to be worth writing down.

This variety is what natural selection works with, as how evolution works explains: new combinations of existing alleles, made fresh every generation. Heredity with variation is one of the traits on biologists' checklist of what counts as alive, and meiosis is where much of the variation comes from.

What goes wrong, and what do people get wrong?

Errors in sharing chromosomes have different results in the two divisions. In mitosis, a mistake affects one cell and its descendants; it is one of the steps on the road to some cancers. In meiosis, a mistake affects a whole future person. If a pair or a set of sisters fails to separate, called nondisjunction, an egg or sperm ends up with 22 or 24 chromosomes. Most embryos that result don't develop. A few do: three copies of chromosome 21 gives Down syndrome, and such errors become more common in eggs as women age.

Three misconceptions are common.

• "Meiosis is just mitosis twice." Meiosis II resembles mitosis, but meiosis I is different in kind: it separates partner chromosomes, not copies, and it recombines them first.
• "Mitosis makes cells with half the DNA." No. Each daughter of mitosis has the full set; the DNA was doubled first.
• "Meiosis happens all over the body." It happens only in the cells that make eggs and sperm. Every other cell division in a human body is mitosis.

Check yourself

• Without looking back, give four differences between mitosis and meiosis, and say what each division is for.
• Explain why meiosis needs two divisions, not one.
• Work out how many chromosome combinations a species with 4 pairs can put into one egg, and how you got there.

Sources

• Mitosis — National Human Genome Research Institute
• Meiosis — National Human Genome Research Institute
• Meiosis, genetic recombination, and sexual reproduction — Clare O'Connor, Nature Education, 2008
• Meiosis — Wikipedia
• Variation in human recombination rates and its genetic determinants — Fledel-Alon and colleagues, PLOS ONE, 2011
• Mitosis — Wikipedia