The classic genome organization of the bacterial chromosome is normally envisaged with all its genetic markers linked, thus forming a closed genetic circle of duplex stranded DNA (dsDNA) and several proteins in what it is called as "the bacterial nucleoid." This structure may be more or less corrugated depending on the physiological state of the bacterium (i.e., resting state or active growth) and is not surrounded by a double membrane as in eukayotic cells. The universality of the closed circle model in bacteria is however slowly changing, as new data emerge in different bacterial groups such as in Planctomycetes and related microorganisms, species of Borrelia, Streptomyces, Agrobacterium, or Phytoplasma. In these and possibly other microorganisms, the existence of complex formations of intracellular membranes or linear chromosomes is typical; all of these situations contributing to weakening the current cellular organization paradigm, i.e., prokaryotic vs eukaryotic cells.
Keywords: Borrelia; Linear bacterial chromosomes; Nucleoid; Nucleoid-associated proteins; Planctomycetes; Streptomyces.