Reviewed by: BD Editors

Nucleoid Definition

The nucleoid is the space within a prokaryotic cell where the genetic information, called the genophore, is found. Prokaryotes are divided into bacteria and archaea, which are both unicellular organisms that contain no membrane-bound organelles. The nucleoid, then, also has no membrane around it. It attached to the cell membrane and in immediate contact with the cytoplasm. The nucleoid also doesn’t take a uniform shape and has no specific size. However, we can still distinguish it from the rest of the cell and identify it under a light microscope.

The nucleoid is mostly composed of multiple compacted copies of DNA in a continuous thread, with the addition of some RNA and proteins. The DNA in prokaryotes is double-stranded and generally takes a circular shape. Keep in mind that DNA can sometimes also be found in other regions outside the nucleoid. To put things into perspective, we can look at the nucleoid’s eukaryotic counterpart. Eukaryotes, such as plants and animals, have a nucleus that houses their genetic material, with a surrounding double membrane, or what we call the nuclear envelope. This membrane separates the contents of the nucleus from the cytoplasm. As in prokaryotes, the DNA of eukaryotes is also double-stranded.

Function of Nucleoid

The nucleoid is essential for controlling the activity of the cell and reproduction. It is where transcription and replication of DNA take place. Within it, we can expect to find enzymes that serve as biological catalysts and help with replication, as well as other proteins that have other functional and structural roles, including assisting the formation of DNA, facilitating cell growth, and regulating the genetic material of the cell.

  • Chromatin – The substance, composed of DNA, RNA, and protein, that makes up the chromosome of eukaryote.
  • Mitochondrion – An organelle that produces energy for the cell in eukaryotic organisms.
  • Nucleolus – A small structure where ribosome assembly occurs within a nucleus.
  • Plasmid – A structure found in bacteria that contains DNA but is not associated with the chromosome.


1. Which of the following is true of a nucleoid?
A. It is surrounded by a double membrane.
B. It contains single-stranded DNA.
C. It is in contact with the cytoplasm.
D. It is found in bacteria and some plants.

Answer to Question #1
C is correct. The nucleoid is not enclosed by a membrane and therefore has immediate contact with the surrounding cytoplasm.

2. The genetic material in prokaryotes is:
A. Double-stranded and either linear or circular depending on the type of cell
B. Single-stranded and linear
C. Single-stranded and circular
D. Double-stranded and circular

Answer to Question #2
D is correct. The DNA found in a prokaryotic cell is double-stranded and circular.

3. Which of the following describes the contents of a nucleoid?
A. DNA with associated proteins
B. DNA, RNA, and proteins
C. DNA and histones
D. DNA and RNA

Answer to Question #3
B is correct. The nucleoid contains DNA, RNA, and proteins that have multiple functions, including regulating the replication of DNA.

Cite This Article

MLAAPAChicago Editors. "Nucleoid." Biology Dictionary,, 23 Dec. 2016, Editors. (2016, December 23). Nucleoid. Retrieved from Editors. "Nucleoid." Biology Dictionary., December 23, 2016.

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