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Chapter 11 : On the Definition of Prokaryotes
As will be argued below, the present definition of a prokaryote is highly unsatisfactory. To give an example: a prokaryote is “a cell or organism lacking a nucleus and other membrane-enclosed organelles, usually having its DNA in a single circular molecule” (Brock, Biology of Microorganisms, 10th ed.). This seems a summary of the original definition of Stanier & van Niel (1962), which I quote for the sake of completeness: “The principle distinguishing features of the procaryotic cell are: 1. absence of internal membranes which separate the resting nucleus from the cytoplasm, and isolate the enzymatic machinery of photosynthesis and of respiration in specific organelles; 2. nuclear division by fission, not by mitosis, a character possibly related to the presence of a single structure which carries all the genetic information of the cell; and 3. the presence of a cell wall which contains a specific mucopeptide as its strengthening element.” Today’s perception of these points amounts largely, as indicated above, to the absence of a nuclear envelope in prokaryotes. It should be mentioned that Stanier & van Niel in the above paper also wished to differentiate a bacterium from a virus and to incorporate blue-green algae within the prokaryotic domain.