Dictionary > Nucleoid



plural: nucleoids
nu·cle·oid, ˈnjuːklɪˌɔɪd
The portion within a prokaryotic cell where the genophore (genetic material) is to be found



A prokaryotic cell is characterized by the lack of a ‘’true’’ nucleus. Instead of a nucleus, the cell has a region where their genetic material is localized and this region is called nucleoid.


The nucleoid, which means nucleus-like, is an irregularly shaped area containing the genetic material of the prokaryotic cell. It is different from the nucleus of a eukaryotic cell in a way that the genetic material is not enclosed in a membrane to separate it from the cytoplasm. Most of the genetic materials found in the nucleoid are the DNAs. They comprise about 60%. The remaining percentages are from the amounts of RNAs (e.g. mRNAs) and proteins (e.g. transcription factor proteins and nucleoid-associated proteins).

Biological importance

The nucleoid contains the genophore, i.e. the genetic material of a prokaryotic cell. The DNA of the prokaryote is circular and double stranded. A single cell may contain several copies of DNA at a time. And just like any other cell, the genetic material must be contained inside the cell in a compacted form so as to fit everything inside.

Common biological reactions

Common biological reactions

NAPs or nucleoid-associated proteins (or simply nucleoid proteins) are proteins in the nucleoid such as HU, H-NS, and Fis. These proteins are involved in nucleoid condensation, a process whereby the genetic material is compacted into a nucleoid region. Unlike histones of eukaryotes that form nucleosomes as a mechanism for DNA compaction, NAPs promote compaction by DNA looping (i.e. DNA bending, bridging, and aggregation).1 HU is one of the most abundant types of NAP. It is typically 20kDa and a heterodimer (e.g. in Escherichia coli, comprised of HUα and HUβ).2 Hu compacts DNA by constraining the toroidal DNA supercoils while not impeding transcription as opposed to H-NS that does.3 H-NS (i.e. histone-like nucleoid structuring protein) is a type of NAP that promotes DNA compaction by forming complexes with one another and then binding to the different sections of DNAs to bring them together. While doing this, H-NS prevents gene expression (by binding to AT rich DNA). Fis is a NAP (as well as a global regulatory protein in E. coli). As a NAP, it is suggested to have a role in defining the nucleoid structure, in modulating bacterial chromatin structure, and in initiating DNA replication.


The nucleoid can be visualized using an electron micrograph. It can also be seen under a light microscope when the specimen is stained with the Fuelgen stain that makes the DNA visible. Another method is by fluorescence microscopy and staining techniques using DAPI and ethidium bromide.



  • nucleo- + -oid; thus “like a nucleus”

Further reading


See also

  • prokaryote
  • Reference

    1. Dame, R. T., Kalmykowa, O. J., & Grainger, D. C. (2011). “Chromosomal macrodomains and associated proteins: implications for DNA organization and replication in gram negative bacteria”. PLOS Genetics. 7 (6): e1002123.
    2. Lee, S. F., Thompson, M. A., Schwartz, M. A., Shapiro, L., & Moerner, W. E. (2011). Super-Resolution Imaging of the Nucleoid-Associated Protein HU in Caulobacter crescentus. Biophysical Journal, 100(7), L31–L33. https://doi.org/10.1016/j.bpj.2011.02.022
    3. Berger, M., Gerganova, V., Berger, P., Rapiteanu, R., Lisicovas, V., & Dobrindt, U. (2016). Genes on a Wire: The Nucleoid-Associated Protein HU Insulates Transcription Units in Escherichia coli. Scientific Reports, 6(1). https://doi.org/10.1038/srep31512

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