Nucleoid, bacteria

Prior, S. and B. Riemann. 1998. Effects of tributyltin, linear alkylbenzenesulfonates, and nutrients (nitrogen and phosphorus) on nucleoid-containing bacteria. Environ. Toxicol. Chem. 17 1473-1480. 

The cytoplasm of E. coli contains about 15,000 ribosomes, thousands of copies each of about 1,000 different enzymes, numerous metabolites and cofactors, and a variety of inorganic ions. The nucleoid contains a single, circular molecule of DNA, and the cytoplasm (like that of most bacteria) contains one or more smaller, circular segments of DNA called plasmids. In nature, some plasmids confer resistance to toxins and antibiotics in the environment. In the laboratory, these DNA segments are especially amenable to experimental manipulation and are extremely useful to molecular geneticists. 

In bacteria transcription and translation are closely linked. Polyribosomes may assemble on single DNA strands as shown in Fig. 28-5. It has often been assumed that RNA synthesis occurs on loops of DNA that extend out into the cytosol. However, recent studies indicate that most transcription occurs in the dense nucleoid and that assembly of ribosomes takes place in the cytosol.2683 In a similar way eukaryotic transcription occurs in the nucleus and protein synthesis in the cytosol. Nevertheless, some active ribosomes are present in the nucleus.26813... 

In addition to the nucleoid DNA, small circular molecules of double-stranded DNA occur in bacteria and replicate autonomously as the host cell proliferates. These are plasmids, and they carry the genes required for replication of themselves as well as for other functions, such as resistance to antibiotics. They can be readily separated from the nucleoid DNA, modified by splicing a piece of "foreign" DNA into it, and used as a vehicle for the cloning of the "foreign" DNA (Chapter 15). 

In eukaryotes, DNA does not exist free. It is complexed with an approximately equal mass of basic proteins called histones. For a long time it was thought that bacterial DNA did not form such complexes. While histones are absent from bacteria, there is increasing evidence for the presence of histonelike proteins in them that enable condensation of their DNA into its compact nucleoid form. 

Cytoplasm - The cytoplasm, or protoplasm, of bacterial cells is where the functions for cell growth, metabolism, and replication are carried out. It is a gellike matrix composed of water, enzymes, nutrients, wastes, and gases and contains cell structures such as ribosomes, a chromosome, and plasmids. The cell envelope encases the cytoplasm and all its components. Unlike the eukaryotic (true) cells, bacteria do not have a membrane enclosed nucleus. The chromosome, a single, continuous strand of DNA, is localized, but not contained, in a region of the cell called the nucleoid. All the other cellular components are scattered throughout the cytoplasm. 

Nucleoid - The nucleoid is a region of cytoplasm where the chromosomal DNA is located. It is not a membrane bound nucleus, but simply an area of the cytoplasm where the strands of DNA are found. Most bacteria have a single, circular chromosome that is responsible for replication, although a few species do have two or more. Smaller circular auxiliary DNA strands, called plasmids, are also found in the cytoplasm. 

A FIGURE 1-2 Prokaryotic cells have a simpler internal organization than eukaryotic cells, (a) Electron micrograph of a thin section of Escherichia coii, a common intestinal bacterium. The nucleoid, consisting of the bacterial DNA, is not enclosed within a membrane. E. coii and some other bacteria are surrounded by two membranes separated by the periplasmic space. The thin cell wall is adjacent to the inner membrane. 

Differences between bacteria and human cells. Different species of bacteria have some common structural features that distinguish them from animal cells. They are single-cell organisms that are prokaryotes ( before nucleus ). Their genetic material (DNA) is concentrated in the central region of the cell called a nucleoid, rather than a nucleus, because it is not separated from the rest of the cellular contents by a membrane. Likewise, bacteria contain no cytoplasmic organelles defined by membranes. They do have a plasma membrane that encloses the cytoplasm. External to this membrane is a peptidoglycan cell wall composed of extensively cross-linked polysaccharides that form a protective shield on the surface of the cell. 

Nuclear Region Central location of DNA, RNA, and some proteins in bacteria not a true nucleus. Known also as Nucleoid. 

---