Meiosis

Meiosis (may-OH-sis) is a specialized form of cell division that takes place only in the reproductive cells. The goal of meiosis is to produce sex cells (sperm and egg) that have only one set of twenty-three chromosomes. When a sex cell unites with another sex cell, the zygote (fertilized egg) will have the proper total of forty-six chromosomes.

It is important to distinguish meiosis from mitosis (my-TOH-sis). Although both are a form of cell division, mitosis produces two identical cells, while meiosis produces four different cells. Mitosis makes new, identical cells so that an organism will be able to replace damaged and dead cells and be able to grow. Nearly all the cell division in an organism can be described as mitosis. Meiosis only occurs in an organism's sex cells and is structured so that it deliberately produces different rather than identical cells. Without meiosis producing differing cells, there would be no variation in offspring, who also would have twice the number of chromosomes than they should have.

If mitosis occurred in reproductive cells the way it does in all other cells of the body, the new cell produced would have twice the number of chromosomes that it should have. For example, the exact amount of chromosomes needed to be human is forty-six. Without meiosis cutting the number of chromosomes in half, that number would be ninety-two chromosomes after fertilization has taken place. Meiosis splits in half the number of chromosomes in sperm and egg cells, so when the cells unite, the zygote will get half the number of chromosomes from each parent.

Besides halving the number of chromosomes, meiosis also performs another very important function. It allows genetic material to be "shuffled" as the chromosomes cross over each other and swap genes before the cell divides. This is a random exchange of genetic material that guarantees that an entirely new individual will be produced after fertilization. Because of this shuffle of genetic instructions, each reproductive cell is given its own unique set of instructions. This assures that no two sperm or egg cells have the same exact combination of genes. This also partly explains why brothers and sisters (except identical twins) of the same parents have different characteristics.

Eventually, when two of these unique sex cells are joined as sperm and egg and form a new individual (thus further mixing the genetic instructions), an entirely unique organism is created unlike any other existing organism. The variations or differences caused by meiosis are very important to evolution, since the process of natural selection (the process of survival and reproduction of organisms that are best suited to their environment) needs genetic variety from which to "select." If there were no differences, there would be no evolution.

In humans, meiosis occurs in the gametes or sex cells (sperm and egg). In males, the process of gamete production is known as spermato-genesis. During this process, each dividing cell in the testes produces four functional sperm cells, all basically the same size. In contrast, the female process of producing eggs, called oogenesis, makes four eggs, only one of which survives. This is because nature gives all the necessary cytoplasm (living material) and organelles (structures with particular functions) to only one egg, thereby increasing its chances of survival should it become fertilized.

[See alsoCell Division; Fertilization; Reproduction, Sexual ]