Prokaryotes are the group of organisms including bacteria and archaea that do not have a membrane-bound nucleus and instead have a circular, double-stranded molecule of DNA called a nucleoid, that is not contained in a nuclear envelope. This prokaryotic chromosome may vary in size from 160,000 base pairs in the bacterium Candidatus Carsonella ruddii, for example, to up to 12,200,000 base pairs in the soil-dwelling bacterium Sorangium Cellulosum. Some bacteria such as Borrellia Spirochetes (which causes Lyme disease) contain a linear chromosome rather than a circular one.
Within the well defined nucleus of the eukaryote, chromosomes have a sequence-based and well defined structure, while in prokaryotes there is a single point from where replication starts. In archaea, however, replication may originate at multiple points. The non-bound nuclei found in prokaryotes do not have a well defined DNA structure organized around proteins, with the exception of the archaea where DNA is packaged to form nucleosomes. The nucleoid occupies a central position in the bacterial cell as the nucleus does in eukaryotic cells. However, unlike the nucleus, the nucleoid constantly undergoes structural changes.
The chromosomes within the prokaryotes are also found in the form of small molecules of DNA called plasmids. Since these plasmids can be found attached to the plasma membrane, they can be easily isolated in the lab by using centrifugation to create small pellets of the plasma membrane along with the DNA. Plasmids are generally super coiled like eukaryotic DNA and this needs to be uncoiled to allow for transcription and translation of proteins.
Earlier it was believed that prokaryotes only ever contained a single chromosome. Recently, however, the number of chromosomes in prokaryotes has been determined using DNA fragment mapping and pulsed-field gel electrophoresis techniques. Results show that the Vibrio species of bacteria that causes diseases such as cholera in fact contains two large, circular-mapping chromosomes.
Reviewed by Sally Robertson, BSc