Nucleoid Definition, Functions, Characteristics

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The Nucleoid (meaning it’s like a nucleus) is an irregularly-shaped part of the prokaryotic cell which is home to all or most part of the gene material. The chromosome of a protokaryote is circular and its length is big in comparison to cell dimensions, which is why it has to be compressed to be able to fit. Contrary to the nucleus in a prokaryotic cell, it’s not protected by the nuclear membrane.

In contrast, the nucleoid develops through condensing and functional arrangement by the use of chromosomal structural proteins and the RNA molecules and DNA supercoiling. Its length gene is variable (generally at most several thousand base pairs) and cells may have multiple copies.

There isn’t yet an extremely high-resolution structure of the nucleoid in a bacterial nucleoid. However certain aspects have been studied in Escherichia bacteria as an organism model. Within E. coli, the DNA chromosomal is negatively wrapped and folded into plectonemic loops. These are limited to specific physical regions, but seldom disperse into one another.


These loops spatially group into macrodomains of megabase size where DNA sites are often in contact however, interactions are very rare. The compacted as well as spatially organized DNA is shaped as a helical oval ellipsoid, which is tightly confined inside the cell. The 3D shape of DNA within the nuceoid is believed to alter based on the environment and is connected to the expression of genes, so that the nucleoid’s structure and transcription of genes are interdependent and affect one another reciprocally.

Definition of Nucleoid

The nucleoid space is inside a prokaryotic cell, in which the genetic information known as the genophore is stored. Prokaryotes can be classified into archaea and bacteria which are unicellular and contain no organelles that are membrane bound. The nucleoid is also devoid of a membrane surrounding it. It is attached directly to cell membrane and is in direct contact with the cells’ cytoplasm. The nucleoid isn’t the shape of a single piece and has no particular size. But we are able to identify it from other parts of the cell and see the nucleoid under a microscope.

The nucleoid is composed of multiple copies of DNA that form a continuous thread. It also has the addition of proteins and RNA. Prokaryotes’ DNA is double-stranded and typically has an oval shape. Be aware that DNA may be found in different regions beyond the nucleoid. To understand the situation one can consider the nucleoid’s eukaryotic cousin. 

Eukaryotes like animals and plants, have an nucleus that contains their genetic material. It is surrounded by the double membrane surrounding it, which we refer to as”the nuclear envelope. This membrane divides the nucleus’ contents and the cytoplasm. Similar to prokaryotes the DNA of Eukaryotes is double-stranded as well.

Characteristics of Nucleoid

  • The word “nucleoid,” which translates to nucleus-like, is a irregularly shaped space that contains DNA of prokaryotic cells.
  • It is distinct from the nucleus in a cells in the sense in that genetic information isn’t protected by a membrane to keep it separate from the cell’s cytoplasm.
  • Most of the genetic material that are found in the nucleoid is DNAs. They make up about 60%..
  • The remaining percentages are derived from the amount of proteins, RNAs (e.g. MRNAs) as well as proteins (e.g. transcription factor proteins and nucleoid associated proteins).

Function of Nucleoid

The nucleoid plays a crucial role in controlling the activities of cells as well as reproduction. It is the place where the transcription and replication of DNA happen. Within it, we could expect to see enzymes that function as biological catalysts and assist with replication along with others that play functions and structural roles for example, helping to form DNA, aiding in the growth of cells, and controlling the genetic material within the cell.

Common biological reactions

NAPs or nucleoid-associated protein (or the nucleoid-associated proteins) are proteins that reside in the nucleoid, such as H-NS, HU and Fis. These proteins play a role in nucleoid condensing, which is a process in which genetic material is compressed into a nucleoid area. Contrary to histones from eukaryotes which make nucleosomes, which are a mechanism for compaction of DNA NAPs facilitate compaction through looping of DNA (i.e. DNA bending, bridging and aggregation).1 Hu is one of the most prevalent varieties of NAP. 

It’s typically 20kDa and is a heterodimer (e.g. within Escherichia coli, composed of Hua and HUb).2 Hu compresses DNA by restricting the supercoils of toroidal DNA while not affecting transcription, as opposed to H-NS which does.3 It is also known as H-NS. (i.e. histone-like nucleoid structural protein) is a kind of NAP that helps in promoting DNA compaction through the formation of complexes that bind to one other and binding to different segments of DNA to bring them together. 

In doing so H-NS blocks gene expression (by the binding of AT abundant DNA). Fis is an NAP (as as a regulatory protein that is global within E. coli). As it is a NAP is believed that it plays a role in defining the structure of the nucleoid as well as in modulating the bacteria’s chromatin structure, as well as in triggering DNA replication.


  • Joyeux M. Preferential Localization of the Bacterial Nucleoid. Microorganisms. 2019; 7(7):204.
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