Basically, animals and plants alike multiply repeatedly from only one fertilized ovum to form a body comprising many cells. Cells do not proliferate by multiplying many-fold all at once, but by doubling. The mechanism supporting this proliferation by doubling is called cell division (Fig. 2-8).
In order for cells to divide, the DNA genetic information must double, the chromosomes containing the doubled DNA (see Chapter 3, Fig. 3-2) must separate into two equal sets, and each set of chromosomes must be conveyed into one of the two newly formed cells. The replication and equal distribution of this genetic information is strongly controlled; thus, the cells can multiply repeatedly. This mechanism can be found in all organisms, from unicellular yeasts to multicellular humans. Human cells are thought to take about one day per cell division. However, the cells of multicellular organisms such as humans cannot continue to multiply indefinitely and must stop dividing after a certain number of divisions. The cells divide or stop dividing depending on influences from the environment or surrounding cells. Abnormality in the control mechanism of cell division leads to diseases such as cancer (see Chapter 7).