The many and complex chemical reactions that are necessary to maintain life can take place only in a delicately balanced environment. 

Living cells, such as the human cheek cells shown on the top border of this page, are the life support chambers that maintain this special environment.  Cheek cells are frequently the cells that are collected for use in DNA testing for genealogy. At the right (or below if you are viewing on a cell phone screen), Mr. James Dorsey collects cheek cells as part of the launch of the Dorsey DNA Project, November 2001.

The human body contains approximately 37.2 trillion cells, most of which are less than a tenth of a millimeter across.

There are two types of cells in nature. Prokaryotic cells are the simplest cells. Each cell is an individual organism. A prokaryotic cell is basically a membrane sack filled with cytoplasm—a thick jelly like substance where all the chemical reactions required for the life of the organism take place.

Eukaryotic cells are more advanced on the evolutionary scale and are more complex in both structure and function. Eukaryotic cells can be either single cellular or multicellular.

Figure 1 Jim Dorsey collecting cheek cells for the launch of the Dorsey DNA Project, November 2001

Cell Theory

All living organisms are composed of one or more cells.

The cell is the basic unit of structure and organization in organisms.

All cells arise from pre-existing cells.