Meiotic cell division

Almost every cell in your body carries its chromosomes in pairs. One chromosome of each pair came from your mother and the other from your father. A cell with full pairs like this is called diploid, and its chromosome number is written 2n. A human body cell has 46 chromosomes, which is 23 pairs. When a body cell divides by mitosis, it first copies its DNA and then gives each daughter cell exactly as many chromosomes as the mother cell had. That is right for skin, bone and muscle, but it would cause a problem in reproduction.

A new human life starts when a sperm from the father joins an egg from the mother. Suppose each of them carried all 46 chromosomes. The child's first cell would then have 92, and the next generation would have 184. The number would double in every generation. To stop this, the cells that make sperms and eggs go through a special division that cuts the chromosome number to half. This division is meiosis, or meiotic cell division.

Meiosis does not happen all over the body. It is confined to the mother cells of the major reproductive organs: the testes of adult males and the ovaries of adult females. In the testis it produces sperms, the male gametes. In the ovary it produces eggs, or ova, the female gametes. A skin cell or a liver cell never divides this way.

Meiosis differs from mitosis in two ways that matter. First, one mother cell gives four daughter cells, not two, and each of the four has only half the chromosome number of the mother cell. Second, the four daughter cells are not exact copies of the mother cell. There is a slight variation in their genetic makeup. The whole process is completed in two phases, one after the other.

Before meiosis begins, the DNA in each chromosome is copied, just as it is before mitosis. So each chromosome is now made of two identical copies held together, which gives it an X shape. Then something happens that never happens in mitosis. The two chromosomes of each pair, the one from the mother and the one from the father, come and lie close beside each other. These two partners are non-sister chromosomes. They touch at one or more points, break there, and swap matching pieces.

Because of crossing over, the genetic makeup of each chromosome changes. A chromosome that came entirely from the father now carries a small piece from the mother, and the other way round. After the exchange, the partner chromosomes separate. One member of each pair moves to one side and the other member to the other side, and the chromosomes get organised into two haploid nuclei.

Splitting one nucleus into two is called karyokinesis. Karyokinesis is followed by cytokinesis, in which the cytoplasm divides. So the first phase ends with two haploid cells.

In the second phase, each of the two haploid cells divides again by mitotic cell division. Nothing new is copied this time. The two joined copies of every chromosome simply pull apart, one going into each new nucleus. This is karyokinesis again, and cytokinesis follows it, so each haploid cell becomes two. Two cells that each become two give four cells at the end of meiotic cell division.

All four cells are haploid, and no two of them are exactly alike. The difference comes from crossing over in the first phase, which mixed pieces of the parents' chromosomes in a different way on each chromosome.

Following the chromosome count through one human mother cell makes the job of each phase clear:

Mitosis is called equational division because the daughter cells keep the same chromosome number as the mother cell. Meiosis does the opposite. The chromosome number is reduced to half in the daughter cells, so meiosis is also called reductional cell division. In a human, 46 becomes 23. In an animal whose body cells have 8 chromosomes, each gamete gets 4.

Meiosis makes gametes, and gametes are needed for sexual reproduction. During sexual reproduction, the male gamete fuses with the female gamete. Each brings n chromosomes, so the cell they form has the full 2n again. This diploid cell is the zygote. Adding the two haploid sets:

\[ n + n = 2n \]

For a human, the sperm brings 23 chromosomes and the egg brings 23:

\[ 23 + 23 = 46 \]

The zygote then divides again and again by mitosis. Every one of those divisions keeps the number at 46, and the cells they produce form the whole body of the organism. So the two kinds of division work as a team: meiosis halves the number to make gametes, the fusion of two gametes restores it, and mitosis builds the body.

Brothers and sisters share the same parents, yet their faces, body structure and behaviour are never exactly the same. Meiosis explains much of this. Crossing over mixes the parents' chromosomes differently every time, and each chromosome pair separates independently of the others. So every sperm a father makes, and every egg a mother makes, has a slightly different genetic makeup. Each child grows from a different sperm and a different egg, so each child receives a different combination.

Only identical twins, who come from one zygote that splits into two, share the same genetic makeup. Behaviour also depends on the surroundings a child grows up in, but the difference begins in the gametes.

• Meiotic cell division plays a main role in the sexual reproduction of organisms. Without it there would be no haploid gametes, and the chromosome number would double in every generation.
• It helps in evolution by bringing variation. Crossing over produces new combinations of characteristics, and over many generations these differences are what evolution works on.
• It helps to repair chromosomal disorders.

Taken together, meiosis keeps the chromosome number of a species the same from one generation to the next, while making sure that no two offspring are exactly alike.

Placing the two divisions side by side shows what makes meiosis special: