DNA in prokaryotes

Many bacterial species have only one chromosome per cell and, in nearly all cases, each chromosome contains only one copy of each gene. A very few genes, such as those for rRNAs, are repeated several times. Genes and regulatory sequences account for almost all the DNA in prokaryotes. Moreover, almost every gene is precisely colinear with the amino acid sequence (or RNA sequence) for which it codes (Fig. 24-2). 

Repressors bind to specific sites on the DNA. In prokaryotic cells, such binding sites, called operators, are generally near a promoter. RNA polymerase binding,... 

The duplex is a right-handed double helix with 10 bases per turn. The diameter of the helix is 20 A (2 nm) and the pitch is 34 A (3.4 nm). The sugar-phosphate backbone is on the outside of the helix, and the two antiparallel chains are connected by the hydrogen-bonded bases. The DNA in prokaryotes and eukaryotes is generally found in the duplex form, although there are some single-stranded DNA viruses. DNA is a very robust molecule in comparison with many proteins. The simple double-helical secondary structure is readily reassembled after denaturation, unlike the complex tertiary protein structures that can denature... 

The DNA of prokaryotes is not complexed with proteins in extensive arrays with specified architecture, as is the DNA of eukaryotes. In general, there is only a single, closed, circular molecule of DNA in prokaryotes. This circle of DNA, which is the genome, is attached to the cell membrane. Before a prokaryotic cell divides, the DNA replicates itself, and both DNA circles are bound to the plasma membrane. The cell then divides, and each of the two daughter cells receives one copy of the DNA (Figure 1.10). 

An alternative possibility, which may have occurred in later stages of molecular evolution, is based on a mechanism similar to the rolling circle model devised to explain the replication of circular DNA in prokaryotic organisms. However, this slippage model would have required the cyclization of oligomers with about 20 units or more, which, so far, has only been accomplished with the help of enzymes. (See also references 29, 30 and 31.)... 

In animal, yeast, and fungal cells, DNA is present in two organelles, the nucleus and the mitochondria. In plant and algal cells, DNA is present in plastids (of which chloroplasts are one example) as well as in mitochondria and the nucleus. Unlike the DNA in the nucleus, which is packaged into chromosomes, plastid DNA and mitochondrial DNA are circular and thus resemble the DNA in prokaryotes (e.g., bacteria). 

Legros, M. Kepes, A. One-step fluorometric microassay of DNA in prokaryotes. Anal. Biochem. 1985,147,497-502. 

TLRs are the basis for the fact that plasmid DNA of prokaryotic origin, as is used in all DNA vaccination... 

Figure 36-16. The discontinuous poiymerization of deoxyribonucleotides on the lagging strand formation of Okazaki fragments during iagging strand DNA synthesis is illustrated. Okazaki fragments are 100-250 nt iong in eukaryotes, 1000-2000 bp in prokaryotes.

Eukaryotic DNA that is in an active region of chromatin can be transcribed. As in prokaryotic cells, a... 

Methods of DNA manipulation now make it possible to insert DNA into prokaryotic, eukaryotic, or viral hosts, creating versatile marker systems that allow as- sessment of the survival and spread of strains, studies on gene transfer, and determinations of cell activity. A potential marker gene must be absent from the strain used in the study, and either absent or in sufficiently low abundance in the microbial population under study to allow detection of marked cells at an appropriate level. 

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