The basic building or unit of life is known as a “cell”. Robert Hooke in 1665 discovered a cell from a cork cell. Lee Wenhoek in 1674 discovered cells from living cells in pond water, and in 1833 Robert Brown coined the term nucleus. The nucleoid is majorly composed of copies of DNA, with the addition of 40% of RNA and proteins. The DNA in prokaryotes is double-stranded. The DNA has a circular shape. 

Single-celled microorganisms are called “unicellular organisms”. Examples include amoeba, bacterium, paramecium, etc. Organisms in which many cells are grouped in a single body are called “multicellular organisms”. For example, animals, plants, fungi, etc. 

Difference between prokaryotic and eukaryotic cell

A cell is made up of a plasma membrane, nucleus, and cytoplasm. The nucleus of a cell helps in cell division, controls various cytoplasmic activities, carries inheritance of characters, etc. 

Each cell performs certain basic functions. According to the cellular structure, organisms are of 2 types:

• Eukaryotic Cells

Eukaryotic cells are those that have a ‘true’ nucleus or central compartment for DNA, and other organelles contain membranes. The nucleus is known as the ‘control centre’ of the eukaryotic cell. 

• Prokaryotic Cells

Prokaryotes are those that lack organelles and membrane-bound structures within the cell. In the prokaryotic cells, the nucleus and nuclear membrane are absent. It means in prokaryotic cells, the nuclear region is not well-defined. This is known as “nucleoid”. 

• Prokaryotic comes from the word pro+ karyon. Pro means primitive and karyon means nucleus. 
• These are smaller (1-10 micrometres). 
• Prokaryotes are divided into bacteria and archaea. These are unicellular organisms and do not contain membrane-bound organelles. 
• The nucleoid has no membrane around it. It is attached to the cell membrane. The nucleoid has not a uniform shape and specific size. We can identify nucleoids from the rest of the cell under a light microscope. 

Nucleoid

Genetic materials found in nucleoids are called DNAs. The prokaryotic nucleoid comprises about 60% of DNA, 40% of RNAs and proteins. This region in the prokaryotic cell contains the main DNA material. 

A nucleoid comprises most of the genetic material termed as “genophore”. In a prokaryotic cell, the primary material will be in the nucleoid. The nucleus of the eukaryotic cell is circular, but the nucleoid has an irregular shape. In prokaryotic cells, the nucleoid is not bounded by a nuclear membrane. It lies in direct contact with the cytoplasm. DNA in the nucleoid may be circular and supercoiled. As the cells grow, the DNA in the nucleoid (prokaryotic cell) may extend into the cytosol/cellular fluid.

A single cell constitutes multiple copies and various lengths of a genome. For example, Escherichia Coli (E. Coli) does not contain any structure of a bacterial nucleoid. It consists of features in it. The loops are found in the DNA sites.

How to view a nucleoid

• The nucleoid of the prokaryotic cell can be viewed using an “electron micrograph”. 
• When the specimen is stained with the Fuelgen stain, the nucleoid can be seen under a “light microscope”. The Fuelgen stain makes the DNA visible.
• When the specimen is stained with DAPI and ethidium bromide,  the nucleoid can be seen by fluorescence microscopy and staining techniques.

Functions of a nucleoid

• The nucleoid is essential for controlling the activity and reproduction of the prokaryotic cell.
• In this place transcription and replication of DNA take place. 
• Nucleoid binding proteins are abundant and have a lower molecular mass.
• Nucleoid binding proteins bind DNA and alter its shape. 
• Nucleoid helps in the arrangements and reproduction of DNA. 

Conclusion

Nucleoid is present in the region of transcription and the replication of the DNA in a cell. An electron micrograph at a high magnification helps view the nucleoid. Feulgen stain helps in specifically staining the DNA, and is often used to view DNA under a light microscope.