Every living cell goes through a cycle. The cycle is made up of a time of growth and a time of dividing. Most of the cell's cycle is spent growing and developing. During this time the cell prepares to divide.

During cell division, the nucleus of a cell divides into two identical nuclei. This division of a nucleus into two during the process of cell division is called mitosis. Gradually the cytoplasm divides, and the cell division is complete.

During mitosis, several stages, or phases, may be observed. During interphase, the nucleus can be seen clearly. Chromosomes make copies of themselves. Chromosomes become visible during prophase. During metaphase, the chromosome pairs line up to split apart during anaphase. Finally, in telophase, a nuclear membrane forms around each set of chromosomes.

Reproduction is the process in which organisms produce more of their own kind. Reproduction usually takes place with one or two parents. The production of a new organism from one parent is called asexual reproduction. An amoeba reproduces from only one parent-the original cell.

Reproduction from two parents is called sexual reproduction. Humans, for example, reproduce from two parents. Each parent produces a sex cell. The male sex cell is a sperm. The female sex cell is an egg. During sexual reproduction these two cells join into one cell.

In a process called meiosis, the nucleus of a sex cell divides twice. As a result, the mature sex cells have only half as many chromosomes as the other cells of an organism. In the case of a human, the sex cells contain 23 chromosomes each.

Just before they join, a human sperm cell and an egg cell each have 23 chromosomes. When they join they become one cell with 46 chromosomes. This cell has received half the number of chromosomes from each parent cell. The joining of a sperm and an egg is called fertilization. The cell that is formed is called a zygote. The zygote begins to divide by mitosis. Each new human cell continues to have 46 chromosomes. The zygote develops and grows into a new organism.

Cancer is any of a group of diseases in which cells divide faster and more often than normal cells. When cells divide rapidly, they may form growths or tumors, and may spread through the body. Some tumors are benign and cause no harm to other cells. But a malignant tumor may spread through the body and damage healthy organs. Researchers identify certain chemicals as triggers of cancer.