BACTERIA AND BACTERIAL VIRUSES IN GENETIC RESEARCH in Software

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BACTERIA AND BACTERIAL VIRUSES IN GENETIC RESEARCH
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Several properties of bacteria and viruses make them especially suitable for genetic research. First, bacteria and their viruses generally have a short generation time. Some viruses increase three-hundredfold in about a half hour; an Escherichia coli cell divides every twenty minutes. In contrast, generation time is fourteen days in fruit ies, a year in corn, and twenty years or so in human beings. (E. coli, the common intestinal bacterium, was discovered by Theodor Escherich in 1885.) Another reason bacteria and bacterial viruses are so well-suited for genetic research is because they have much less genetic material than eukaryotes do, and the organization of this material is much simpler. The term prokaryote arises from the lack of a true nucleus ( pro means before and karyon means kernel or nucleus); they have no nuclear membranes (see g. 3.2) and usually have only a single, relatively naked chromosome, so they are haploid. Bacteria may, however, contain small, auxiliary circles of DNA, called plasmids. Bacterial viruses are even simpler. Although animal and plant viruses, discussed in more detail later in the book (chapters 13 and 16), can be more complicated, the viruses we are inter-
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Collar Tail core Tail sheath Base plate Tail fiber (b) Figure 7.1
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Phage T2 and its chromosome. (a) The chromosome, which is about 50 m long, has burst from the head. (b) The intact phage. The phage attaches to a bacterium using its tail bers and base plate and then injects its genetic material into the host cell. ([a] A. K. Kleinschmidt, et al., Darstellung
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und Langen messungen des gesamten Deoxyribose-nucleinsaure Inhaltes von T2-Bacteriophagen Biochemica et Biophysica Acta, 61:857 64, 1962. Reproduced by permission of Elsevier Science Publishers.)
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II. Mendelism and the Chromosomal Theory
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7. Linkage and Mapping in Prokaryotes and Bacterial Viruses
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Seven
Linkage and Mapping in Prokaryotes and Bacterial Viruses
pathways later in the chapter. The smallest bacteriophages (e.g., R17) have RNA as their genetic material and contain just three genes, one each for a coat protein, an attachment protein, and an enzyme to replicate their RNA. The larger bacteriophages (T2, T4) have DNA as their genetic material and contain up to 130 genes. A third reason for the use of bacteria and viruses in genetic study is their ease of handling. A researcher can handle millions of bacteria in a single culture with a minimal amount of work compared with the effort required to grow the same number of eukaryotic organisms such as fruit ies or corn. (Some eukaryotes, such as yeast or Neurospora, can, of course, be handled using prokaryotic techniques, as we saw in chapter 6.) Let us look at an expansion of the techniques, introduced in chapter 6, that geneticists use in bacterial and viral studies.
TECHNIQUES O F C U L T I VA T I O N
All organisms need an energy source, a carbon source, nitrogen, sulfur, phosphorus, several metallic ions, and water. Those that require an organic form of carbon are termed heterotrophs. Those that can utilize carbon as carbon dioxide are termed autotrophs. All bacteria obtain their energy either by photosynthesis or chemical oxidation. Bacteria are usually grown in or on a chemically de ned synthetic medium, either in liquid in asks or test tubes, or on petri plates using an agar base to supply rigidity.When one cell is placed on the medium in the plate, it will begin to divide. After incubation, often overnight, a colony, or clone, will exist where previously was only one cell. Overlapping colonies form a con uent growth ( g. 7.2). A culture medium that has only the minimal necessities required by the bacterial species is referred to as minimal medium (table 7.1). Alternatively, bacteria can grow on a medium that supplies, in addition to their minimal requirements, the more
complex substances that the bacteria normally synthesize, including amino acids, vitamins, and so on. A medium of this kind allows the growth of strains of bacteria, called auxotrophs, that have particular nutritional requirements. (The parent, or wild-type, strain is referred to as a prototroph.) For example, a strain that has an enzyme defect in the pathway that produces the amino acid histidine will not grow on a minimal medium because it has no way of obtaining histidine; it is a histidine-requiring auxotroph. If, however, histidine were provided in the medium, the organisms could grow. This type of mutant is called a conditional-lethal mutant. The organism would normally die, but under appropriate conditions, such as the addition of histidine, the organism can survive. This histidine-requiring auxotrophic mutant can grow only on an enriched or complete medium, whereas the parent prototroph could grow on a minimal medium. Media are often enriched by adding complex mixtures of organic substances such as blood, beef extract, yeast extract, or peptone, a digestion product. Many media, however, are made up of a minimal medium with the addition of only one other substance, such as an amino acid or a vitamin. These are called selective media; we will discuss their uses later in the chapter. In addition to minimal, complete, and selective media, other media exist for speci c purposes such as aiding in counting colonies, helping maintain cells in a nongrowth phase, and so on.
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