Procaryotic Domains

Procaryotes are single cells or simple associations of similar cells (usually 0.2–10.0 µm in smallest dimension, although some are much larger) forming a group defined by cellular, not organismal, properties (i.e., by the structure and components of the cells of an organism rather than by the properties of the organism as a whole). Some differential characteristics of procaryotes and eucaryotes are described in detail. Recent phylogenetic analyses of the Bacteria , Archaea , and Eucarya using conserved protein sequences have shown that the majority of trees support a closer relationship between the Archaea and Eucarya than between Archaea and Bacteria . For practical purposes, the Bacteria may be divided into three phenotypic subgroups: (1) those that are Gram‐negative and have a cell wall, (2) those that are Gram‐positive and have a cell wall, and (3) those that lack any cell wall. The major groups of the Archaea include (a) the methanogenic Archaea , (b) the sulfate‐reducing Archaea , (c) the extremely halophilic Archaea , (d) the Archaea lacking cell walls, and (e) the extremely thermophilic S 0 ‐metabolizing Archaea . Some characteristics differentiating Bacteria from Archaea are explained in detail . The apparent Archaea–Eucarya sisterhood raises interesting questions about the phylogenetic relationships between procaryotes and eucaryotes.