The prokaryotic cell

The model Bacterium is Escherichia coli, normally found in the colon of humans and other mammals. It has a limited capacity for independent existence, is motile and rod- shaped, about 2 mm long and 1 mm wide.  Its cell wall is Gram-negative in structure.  Morphologically E. coli resembles many other Bacteria and some members of the Archaea, although the fine structure of the cell wall, biochemistry, and molecular biology differ considerably. The single circular chromosome is attached to the cytoplasmic membrane in Figure 1 and this DNA is free in the cytoplasm. The word prokaryote is derived from the Greek pro and karyote so this prenucleate state defines the prokaryotes. There are generally no discernible subcellular components in Bacterial cells the exception being the presence of layered membrane bodies of unknown function in the methanotrophic and nitrifying Bacteria. The main subcellular differences are that the following are absent from Archaeal and Bacterial cells:

? Nuclear membrane (except Planctomycetes)

? Chloroplasts and mitochondria energy generation takes place across the cell membrane rather than in some specialized bodies

? Endoplasmic reticulum or Golgi apparatus free ribosomes in the cytoplasm translate mRNA

Archaea and Bacteria come in a variety of shapes and sizes and are classified according to their biochemical rather than morphological differences. The differences among the Bacteria and Archaea are wide covering the machinery used for transcription, replication, translation, and central metabolism. Overall they are linked through a general cell structure and share many ecological niches, but in this respect are a product of convergent evolution.