Nucleoid

One of the main features that distinguish a prokaryotic cell from a eukaryotic cell is the absence of a nuclear membrane. In eukaryotic cells, chromosomes are contained within a membrane-delimited organelle known as the nucleus. On the contrary, prokaryotes lack this membrane-delimited nucleus. In prokaryotes, chromosome is located in nucleoid which is an irregularly shaped region. Nucleoid is the central region in the prokaryotic cell that contains the genetic material i.e. DNA and is not surrounded by a membrane. Nucleoid is generally composed of 60% DNA, 30% RNA and 10% protein by weight. Transcription and replication of DNA takes place within nucleoid.

The nucleoid is visible under light microscope; after staining with DNA-specific stains such as the Feulgen or Giemsa stain. Both phase contrast microscopy and fluorescence microscopy have been used to visualize the nucleoid. Electron microscopic studies have revealed that nucleoid is associated with either plasma membrane or mesosome. Membranes are also found attached to the isolated nucleoid. This proves that bacterial DNA is attached to cell membrane and also cell membrane may play some role in the separation of DNA into daughter cells during division.