Nucleoid

The nucleoid is the irregularly shaped region of a prokaryotic cell where the chromosome is located. Unlike the nucleus of eukaryotes it has no surrounding membrane, so the DNA lies in direct contact with the cytoplasm.

The chromosome of Escherichia coli is a circular DNA molecule of about 4.6 million base pairs, roughly 1.5 millimeters long when stretched out, yet the cell is only a couple of micrometers long. The DNA is compacted by supercoiling, introduced by enzymes such as topoisomerase (gyrase), and by small nucleoid-associated proteins like HU and H-NS, which play a role similar to that of histones in eukaryotes.

Because there is no nuclear envelope, transcription and translation can occur together, with ribosomes attaching to an mRNA as it is still being made. Replication begins at one origin and proceeds around the circle, and the two copies are then pulled to opposite ends of the cell before binary fission. Many bacteria also carry extra DNA in plasmids that are separate from the nucleoid.

Mitochondria and chloroplasts have nucleoids, which contain their own genomes, as expected from endosymbiotic theory; in mitochondria these hold the mitochondrial DNA. The nucleoid is a defining feature of prokaryotes, and its organization is of interest in studies of the bacteria and archaea.

• Prokaryotes — the cells that have one
• Nuclear Envelope — the membrane that a nucleoid lacks
• Topoisomerase — the enzyme that supercoils it