Probably the most striking difference between procaryotes and eucaryotes is the way in which their genetic material is packaged. Eucaryotic cells have two or more chromosomes contained within a membrane-delimited organelle, the nucleus. In contrast, procaryotes lack a membrane-delimited nucleus. The procaryotic chromosome is located in an irregularly shaped region called the nucleoid (other names are also used: the nuclear body, chromatin body, nuclear region). Usually procaryotes contain a single circle of double-stranded deoxyribonucleic acid (DNA), but some have a linear DNA chromosome. Recently it has been discovered that some bacteria such as Vibrio cholerae have more than one chromosome. Although nucleoid appearance varies with the method of fixation and staining, fibers often are seen in electron micrographs (figure 3.11 and figure 3.14) and are probably DNA. The nucleoid also is visible in the light microscope after staining with the Feulgen stain, which specifically reacts with DNA. A cell can have more than one nucleoid when cell division occurs after the genetic material has been duplicated (figure 3.14a). In actively growing bacteria, the nucleoid has projections that extend into the cytoplasmic matrix (figure 3.14b,c). Presumably these projections contain DNA that is being actively transcribed to produce mRNA. Careful electron microscopic studies often have shown the nucleoid in contact with either the mesosome or the plasma membrane. Membranes also are found attached to isolated nucleoids. Thus there is evidence that bacterial DNA is attached to cell membranes, and membranes may be involved in the separation of DNA into daughter cells during division. Nucleoids have been isolated intact and free from membranes. Chemical analysis reveals that they are composed of about 60% DNA, 30% RNA, and 10% protein by weight. In Escherichia coli, a rod-shaped cell about 2 to 6 m long, the closed DNA circle measures approximately 1,400 m. Obviously it must be very efficiently packaged to fit within the nucleoid. The DNA is looped and coiled extensively (see figure 11.8), probably with the aid of RNA and nucleoid proteins (these proteins differ from the histone proteins present in eucaryotic nuclei). There are a few exceptions to the above picture. Membranebound DNA-containing regions are present in two genera of planctomycetes. Pirellula has a single membrane that surrounds a region, the pirellulosome, which contains a fibrillar nucleoid and ribosome-like particles. The nuclear body of Gemmata obscuriglobus is bounded by two membranes (see figure 21.12). More work will be required to determine the functions of these membranes and how widespread this phenomenon is. The cell cycle and cell division (pp. 285–86). Procaryotic DNA and its function (chapters 11 and 12) Many bacteria possess plasmids in addition to their chromosome. These are double stranded DNA molecules, usually circular, that can exist and replicate independently of the chromosome or may be integrated with it; in either case they normally are inherited or passed on to the progeny. However, plasmids are not usually attached to the plasma membrane and sometimes are lost to one of the progeny cells during division. Plasmids are not required for host growth and reproduction, although they may carry genes that give their bacterial host a selective advantage. Plasmid genes can render bacteria drug-resistant, give them new metabolic abilities, make them pathogenic, or endow them with a number of other properties. Because plasmids often move between bacteria, properties such as drug resistance can spread throughout a population.The Nucleoid
Probably the most striking difference between procaryotes and eucaryotes is the way in which their genetic material is packaged. Eucaryotic cells have two or more chromosomes contained within a membrane-delimited organelle, the nucleus. In contrast, procaryotes lack a membrane-delimited nucleus. The procaryotic chromosome is located in an irregularly shaped region called the nucleoid (other names are also used: the nuclear body, chromatin body, nuclear region). Usually procaryotes contain a single circle of double-stranded deoxyribonucleic acid (DNA), but some have a linear DNA chromosome. Recently it has been discovered that some bacteria such as Vibrio cholerae have more than one chromosome. Although nucleoid appearance varies with the method of fixation and staining, fibers often are seen in electron micrographs (figure 3.11 and figure 3.14) and are probably DNA. The nucleoid also is visible in the light microscope after staining with the Feulgen stain, which specifically reacts with DNA. A cell can have more than one nucleoid when cell division occurs after the genetic material has been duplicated (figure 3.14a). In actively growing bacteria, the nucleoid has projections that extend into the cytoplasmic matrix (figure 3.14b,c). Presumably these projections contain DNA that is being actively transcribed to produce mRNA. Careful electron microscopic studies often have shown the nucleoid in contact with either the mesosome or the plasma membrane. Membranes also are found attached to isolated nucleoids. Thus there is evidence that bacterial DNA is attached to cell membranes, and membranes may be involved in the separation of DNA into daughter cells during division. Nucleoids have been isolated intact and free from membranes. Chemical analysis reveals that they are composed of about 60% DNA, 30% RNA, and 10% protein by weight. In Escherichia coli, a rod-shaped cell about 2 to 6 m long, the closed DNA circle measures approximately 1,400 m. Obviously it must be very efficiently packaged to fit within the nucleoid. The DNA is looped and coiled extensively (see figure 11.8), probably with the aid of RNA and nucleoid proteins (these proteins differ from the histone proteins present in eucaryotic nuclei). There are a few exceptions to the above picture. Membranebound DNA-containing regions are present in two genera of planctomycetes. Pirellula has a single membrane that surrounds a region, the pirellulosome, which contains a fibrillar nucleoid and ribosome-like particles. The nuclear body of Gemmata obscuriglobus is bounded by two membranes (see figure 21.12). More work will be required to determine the functions of these membranes and how widespread this phenomenon is. The cell cycle and cell division (pp. 285–86). Procaryotic DNA and its function (chapters 11 and 12) Many bacteria possess plasmids in addition to their chromosome. These are double stranded DNA molecules, usually circular, that can exist and replicate independently of the chromosome or may be integrated with it; in either case they normally are inherited or passed on to the progeny. However, plasmids are not usually attached to the plasma membrane and sometimes are lost to one of the progeny cells during division. Plasmids are not required for host growth and reproduction, although they may carry genes that give their bacterial host a selective advantage. Plasmid genes can render bacteria drug-resistant, give them new metabolic abilities, make them pathogenic, or endow them with a number of other properties. Because plasmids often move between bacteria, properties such as drug resistance can spread throughout a population.