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 of prokaryotes where the genetic material is localized.[1] 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.
A genophore is the DNA of a prokaryote. This is commonly referred to as a prokaryotic chromosome. The term chromosome is misleading for a genophore because the genophore lacks chromatin[2]. The genophore is compacted through a mechanism known as supercoiling[3]. Where a chromosome is compacted via chromatin. The genophore is circular in most prokaryotes, and linear in very few. The circular nature of the genophore allows replication to occur without telomeres.[4] Genophores are generally of a much smaller size than Eukaryotic chromosomes. A genophore of a true organism can be as small as 580,073 base pairs (Mycoplasma genitalium). Many eukaryotes (such as plants and animals) carry genophores in organelles such as mitochondria and chloroplasts. These organelles are very similar to true prokaryotes.[5]
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.
^ Thanbichler M, Wang S, Shapiro L (2005). "The bacterial nucleoid: a highly organized and dynamic structure". J Cell Biochem96 (3): 506–21. doi:10.1002/jcb.20519. PMID 15988757.
^ Ris, H. (1961). Ultrastructure and molecular organization of genetic systems. Can. J. Genet. Cytol. 3, 95-120.
