Introduction

The basic building block of the human body as well as of all animals and plants is the cell. It is the smallest independent living entity and can live independently as a single-celled (unicellular) organism (e. g., flagellates, amebas). In multicellular organisms (metazoa) the cells organize in large units and become functional entities within an overarching framework. In unicellular organisms, such as bacteria and fungi, all the cells exhibit an identical basic structure. Multicellular organisms, such as plants, animals, and humans, also exhibit a fundamentally uniform organization. Here, however, there are great differences in the variety of tasks, and each type of cell specializes in the execution of a specific task within the organism. For instance, red blood cells (erythrocytes) transport oxygen, while other cells serve as conduits for stimuli (nerve cells) or serve reproduction (germ cells).
The actions of each individual cell in an organism depend on specific genetic information. In the cell this information is stored in certain sections of the substance termed deoxyribonucleic acid (DNA) in the genes. It consists of programs to direct cell reproduction as well as the synthesis of proteins. Both functions are essential to ensure that a fertilized ovum can develop into a multicellular organism and that cells differentiated in various ways, such as brain, lung, muscle, or liver cells, can develop from common precursor cells.