Chapter 23 : The Central Role of DNA: New Vistas in Microbial Biotechnology

Figure 36

Schematic representation of the DNA double helix. The helical strands are composed of alternating units of phosphate and the five-carbon sugar deoxyribose. The “rungs” represent chemical bonds between pairs of nitrogen-containing nucleic acid bases that extend toward the axis of the helix (which is indicated by the dashed line).

Figure 37

Diagram of a portion of a DNA molecule (uncoiled and flattened). The “uprights” of the molecule's ladder-like structure are strands composed of alternating deoxyribose units (pentagons) and phosphate groups (black dots). The “cross-rungs” of the ladder are composed of pairs of complementary nucleic acid bases, each pair linked by a chemical bond.

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Figure 38

Conjugation between F⁺ and F⁻ cells. In this diagram, only a few genes (A, B, C, D, and E) of the bacterial chromosome are indicated. The F⁺ cell contains normal DNA; the F⁻ cell contains two damaged genes, “b” and “d.” In the center section, the chromosome from the F⁺ cell is seen threading into the F⁻ cell through a conjugation bridge. After the plasmid carrying normal genes A, B, and C has entered the cell, it is incorporated into the F⁻ cell's chromosome and replaces the damaged genes with normal ones. The recipient cell is now an F⁺ genetic recombinant. If genes B and D happened to be genes controlling production of two particular vitamins, the recombinant cell, previously deficient, would have acquired the ability to make the vitamins.

Figure 39a

Figure 39b

Figure 40

DNA fingerprinting of strains of Campylobacter jejuni, a bacterium that occurs in poultry and other animals and can cause food poisoning. Lane 7 contains standardized markers for comparison.