<P> The bacterial flagellum consists of three basic components: a whip - like filament, a motor complex, and a hook that connects them . The filament is approximately 20 nm in diameter and consists of several protofilaments, each made up of thousands of flagellin subunits . The bundle is held together by a cap and may or may not be encapsulated . The motor complex consists of a series of rings anchoring the flagellum in the inner and outer membranes, followed by a proton - driven motor that drives rotational movement in the filament . </P> <P> In comparison to eukaryotes, the intracellular features of the bacterial cell are extremely simple . Bacteria do not contain organelles in the same sense as eukaryotes . Instead, the chromosome and perhaps ribosomes are the only easily observable intracellular structures found in all bacteria . There do exist, however, specialized groups of bacteria that contain more complex intracellular structures, some of which are discussed below . </P> <P> Unlike eukaryotes, the bacterial DNA is not enclosed inside of a membrane - bound nucleus but instead resides inside the bacterial cytoplasm . This means that the transfer of cellular information through the processes of translation, transcription and DNA replication all occur within the same compartment and can interact with other cytoplasmic structures, most notably ribosomes . The bacterial DNA is not packaged using histones to form chromatin as in eukaryotes but instead exists as a highly compact supercoiled structure, the precise nature of which remains unclear . Most bacterial chromosomes are circular although some examples of linear DNA exist (e.g. Borrelia burgdorferi). Along with chromosomal DNA, most bacteria also contain small independent pieces of DNA called plasmids that often encode for traits that are advantageous but not essential to their bacterial host . Plasmids can be easily gained or lost by a bacterium and can be transferred between bacteria as a form of horizontal gene transfer . So plasmids can be described as an extra chromosomal DNA in a bacterial cell . </P> <P> In most bacteria the most numerous intracellular structure is the ribosome, the site of protein synthesis in all living organisms . All prokaryotes have 70S (where S = Svedberg units) ribosomes while eukaryotes contain larger 80S ribosomes in their cytosol . The 70S ribosome is made up of a 50S and 30S subunits . The 50S subunit contains the 23S and 5S rRNA while the 30S subunit contains the 16S rRNA . These rRNA molecules differ in size in eukaryotes and are complexed with a large number of ribosomal proteins, the number and type of which can vary slightly between organisms . While the ribosome is the most commonly observed intracellular multiprotein complex in bacteria other large complexes do occur and can sometimes be seen using microscopy . </P>

Small molecules of dna exist in bacteria as circular units known as
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