Prokaryotes include cells of two domains, the Archaea and the Bacteria, which diverged early in evolution. The Archaea include cells that live in extreme environments that are unusual today but may have been preva lent in primitive Earth. For example, thermoacidophiles live in hot sulfur springs with temperatures as high as 80°C and pH values as low as 2. The Bacteria include the common forms of present-day prokaryotes a large group of organisms that live in a wide range of environments, including soil, water, and other organisms (e.g., human pathogens).
Prokaryotic cells are smaller and simpler than most eukaryotic cells, their genomes are less complex, and they do not contain nuclei or cytoplasmic organelles (Table 1.1). Most prokaryotic cells are spherical, rod-shaped, or spiral, with diameters of 1 to 10 μm. Their DNA contents range from about 0.6 million to 5 million base pairs, an amount sufficient to encode about 5000 different proteins. The largest and most complex prokaryotes are the cyanobacteria bacteria in which photosynthesis evolved.
The structure of a typical bacterial cell is illustrated by Escherichia coli (E. coli), a common inhabitant of the human intestinal tract (Figure 1.5). The cell is rod-shaped, about 1 μm in diameter and about 2 μm long. Like most other prokaryotes, E. coli is surrounded by a rigid cell wall composed of polysaccharides and peptides. Beneath the cell wall is the plasma membrane, which is a bilayer of phospholipids and associated proteins. Whereas the cell wall is porous and readily penetrated by a variety of molecules, the plasma membrane provides the functional separation between the inside of the cell and its external environment. The DNA of E. coli is a single circular molecule in the nucleoid, which, in contrast to the nucleus of eukaryotes, is not surrounded by a membrane separating it from the cytoplasm. The cytoplasm contains approximately 30,000 ribosomes (the sites of protein synthesis), which account for its granular appearance.