Meiosis and Mitosis

As described in the section on What are genes?, the human body is made up of billions of tiny building blocks called cells. Inside each cell is a complete copy of all of a person's genetic information. This page briefly describes sexual reproduction and human development from a genetic point of view.

Meiosis

The genetic information each person carries in his or her cells is a combination of some genetic information from the mother and some from the father. Human cells contain 46 chromosomes divided into 23 pairs. In each pair, one chromosome came from the mother and the other from the father. The genes from the mother are passed to the child in a special cell called an egg. The genes from the father are passed to the child in a special cell called a sperm. Meiosis is the name of the process in which a woman produces eggs or a man produces sperm.

The biological process of creating a child begins in the ovaries of women and the testicles of men, where meiosis takes place. In meiosis, a normal diploid cell (having 2 copies of each chromosome) undergoes a special form of cell division to create haploid gametes (having one copy of each chromosome). The female gamete is called an egg, and the male gamete is called a sperm. When an egg from the mother and a sperm from the father join together, they fuse to form a new diploid cell called a zygote, a process called fertilization. Based on this new arrangement of genetic information, combined from the mother and the father, the zygote develops into a unique individual. It is necessary to produce haploid cells through meiosis, because if a normal diploid male cell were combined with a normal diploid female cell, the resulting cell would have four copies of each chromosome! Because female cells have two X chromosomes, all eggs have one X chromosome. Male cells, on the other hand, have one X and one Y chromomsome, so each sperm can have either an X or a Y. The gender of the zygote formed depends on the sex chromosome of the sperm. For a more detailed description and illustrations of meiosis, link to Access Excellence at the National Health Museum.

Mitosis

As descibed above, when a sperm fertilizes an egg, the two cells join together to make the first cell of a developing baby. Mitosis is the name of the process by cells divide to make more cells.

In the first step of mitosis, all the chromosomes are copied, so that instead of two copies, the cell briefly has four copies of each chromosome. Shortly afterwards, the cell divides in half, resulting in two cells (sometimes called daughter cells) which each have a complete copy of all the genetic information. These cells grow larger and eventually also undergo mitosis, making two cells each for a total of four. This process continues many many times to make a human. For a more detailed description and illustrations of mitosis, link to Access Excellence at the National Health Museum. As there are more and more cells, the cells begin to differentiate, meaning they take on specific characteristics. For example, some cells will differentiate into blood cells. Others become the skin or the bones. This is a very complex process which is controlled mostly by genes. You will recall that there are about 40 000 different genes present in each cell of the body. However, in each part of the body, only some of those genes are expressed (turned on), and the others are silent. For example, in the eye, only genes important for the health and functioning of the eye are expressed. The area of study which addresses the development of a mature human (or other animal) from a single cell is called embryology or developmental biology.

Note: This website is meant only as an introduction and overview. If you have a medical question about genetics, please consult a health care professional.