Nucleoid

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Prokaryote cell showing the nucleoid.

In prokaryotes, the nucleoid (meaning nucleus-like and also known as the nuclear region, nuclear body or chromatin body) is an irregularly-shaped region within the cell where the genetic material is localized. The nucleic acid is a circular, double-stranded piece of DNA, and multiple copies may exist. This method of genetic storage can be contrasted against that of the eukaryotes, where DNA is packed into chromatins and sequestered within a membrane-enclosed organelle called the nucleus.

Visualisation

The nucleoid can be clearly visualised on an electron micrograph at high magnification, where, although its appearance may differ, it is clearly visible against the cytosol. Sometimes even strands of what is thought to be DNA are visible. By staining with the Feulgen stain, which specifically stains DNA, the nucleoid can also be seen under a light microscope. The DNA-intercalating stains DAPI and ethidium bromide are widely used for fluorescence microscopy of nucleoids.

Composition

Experimental evidence suggests that the nucleoid is largely composed of DNA, about 60%, with a small amount of RNA and protein. The latter two constituents are likely to be mainly messenger RNA and the transcription factor proteins found regulating the bacterial genome. Proteins helping to maintain the supercoiled structure of the nucleic acid are known as nucleoid proteins or nucleoid associated proteins and are distinct from histones of eukaryotic nuclei. In contrast to histones, the DNA-binding proteins of the nucleoid do not form nucleosomes.

See also

• Cell nucleus
• Plasmid

References

• Prescott, L. (1993). Microbiology, Wm. C. Brown Publishers, ISBN 0-697-01372-3