Cells come in a great variety of shapes and an amazing range of sizes. Despite their differences, all cells have two characteristics in common. They are surrounded by a barrier called a cell membrane; and, at some point in their lives, they contain DNA -- the molecule that carries biological information.
Cells fall into two broad categories, depending on whether or not they contain a nucleus. Prokaryotic cells have genetic material, or DNA, that is not contained in a nucleus. (Eukaryotes, on the other hand, do not have a nucleus and are covered in further detail in another section.) Prokaryotic cells are generally smaller and simpler than eukaryotic cells, although there are many exceptions to this rule. Despite their simplicity, prokaryotes carry out every activity associated with living things -- they grow, reproduce, respond to the environment, and some can even move by gliding along surfaces or swimming through liquids.
The unicellular organisms we call bacteria are prokaryotes.
Video Clip: Prokaryotic Cells
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Eukaryotic cells have a nucleus to contain their DNA, but prokaryotes, like the picture shown to the left, do not. The DNA in prokaryotic cells is concentrated in an area called the nucleoid.
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Escherichia coli (right), commonly known as E. coli, is a colony-forming bacterium that normally lives in the intestines of people and animals. Though there are thousands of different strains of E. coli, very few are pathogenic (disease-causing). These harmful strains cause food poisoning, hemorrhagic diarrhea and kidney failure. These strains originates from the intestines of some cattle and spreads to contaminate beef products and milk.
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This site was last updated on Saturday, November 22, 2008.
