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Genetic Analysis and Mapping in Bacteria and Bacteriophages

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1 Genetic Analysis and Mapping in Bacteria and Bacteriophages
PowerPoint® Lecture Presentation for Concepts of Genetics Ninth Edition Klug, Cummings, Spencer, Palladino Chapter 6 Genetic Analysis and Mapping in Bacteria and Bacteriophages Lectures by David Kass with contributions from John C. Osterman. Copyright © 2009 Pearson Education, Inc.

2 Section 6.1 6.1 Bacteria Mutate Spontaneously and Grow at an Exponential Rate Spontaneous mutation that occurs in the presence or absence of phage is considered the primary source of genetic variation in bacteria.

3 Section 6.1 Selection is the growth of the organism under conditions in which only the mutant of interest grows well, whereas the wild type does not.

4 Section 6.1 Prototroph Auxotroph
can synthesize all essential organic compounds, and therefore can be grown on minimal medium. Auxotroph through mutation, has lost the ability to synthesize one or more essential compounds, and must be provided with them in the medium if it is to grow.

5 Section 6.1 Bacteria have 4 phases when grown in culture: lag phase
log phase (exponential growth) stationary phase death phase


7 Section 6.2 6.2 Conjugation Is One Means of Genetic Recombination in Bacteria Bacteria undergo conjugation, in which genetic information from one bacterium is transferred to another it recombines with the second bacterium’s DNA

8 Figure 6-5 An electron micrograph of conjugation between an F+ and an F- E. coli cell and an cell. The sex pilus linking them is clearly visible. Figure 6.5

9 Section 6.2 In bacterial conjugation in E. coli, F+ cells serve as DNA donors and F– cells are the recipients (Figure 6.6). F+ cells contain a fertility factor (F factor) that confers the ability to donate DNA during conjugation. Recipient cells are converted to F+.

10 Figure 6-6 An mating, demonstrating how the recipient cell is converted to During conjugation, the DNA of the F factor is replicated with one new copy entering the recipient cell, converting it to To indicate the clockwise rotation during replication, a bar is shown on the F factor. Figure 6.6

11 Section 6.2 An Hfr (high-frequency recombination) strain has the F factor integrated. An Hfr strain can donate genetic information to an F– cell, but the recipient does not become F+.

12 Section 6.2 Interrupted matings demonstrated that specific genes in an Hfr strain are transferred and recombined sooner than others (Figure 6.7).

13 Figure 6-7 The progressive transfer during conjugation of various genes from a specific Hfr strain of E. coli to an strain. In this strain certain genes (azi and ton) are transferred sooner than others and recombine more frequently. Others (lac and gal) take longer to transfer and recombine with a lower frequency. Still others (thr and leu) are always transferred and are used in the initial screen for recombinants, but not shown in the histograms above. Figure 6.7

14 Figure 6-8 A time map of the genes studied in the experiment depicted in Figure 6–7.

15 Figure 6-9 (a) The order of gene transfer in four Hfr strains, suggesting that the E. coli chromosome is circular. (b) The point where transfer originates (O) is identified in each strain. Note that transfer can proceed in either direction, depending on the strain. The origin is determined by the point of integration of the F factor into the chromosome, and the direction of transfer is determined by the orientation of the F factor as it integrates. Figure 6.9

16 Figure 6-10 Conversion of to an Hfr state occurs by integration of the F factor into the bacterial chromosome (Step 1). The point of integration determines the origin (O) of transfer. During a subsequent conjugation (Steps 2–4), an enzyme nicks the F factor, now integrated into the host chromosome, initiating the transfer of the chromosome at that point. Conjugation is usually interrupted prior to complete transfer. Only the A and B genes are transferred to the cell (Steps 3–5), which may recombine with the host chromosome. Figure 6.10

17 Section 6.2 In some cases, an F factor is excised from the chromosome of an Hfr strain. In the process, the F factor (referred to as F’) often brings several adjoining genes with it (Figure 6.11). Transfer of an F’ to an F– cell results in a partially diploid cell called a merozygote.

18 Figure 6-11 Conversion of an Hfr bacterium to and its subsequent mating with an cell. The conversion occurs when the F factor loses its integrated status. During excision from the chromosome, the F factor may carry with it one or more chromosomal genes (A and E). Following conjugation with an cell, the recipient cell becomes partially diploid and is called a merozygote. It also behaves as an donor cell. Figure 6.11

19 Section 6.4 6.4 The F Factor Is an Example of a Plasmid
Plasmids contain one or more genes and replicate independently of the bacterial chromosome (Figure 6.12).

20 Figure 6-12 (a) Electron micrograph of a plasmid isolated from E. coli
Figure 6-12 (a) Electron micrograph of a plasmid isolated from E. coli. (b) Diagrammatic representation of an R plasmid containing resistance transfer factors (RTFs) and multiple r-determinants (Tc, tetracycline; Kan, kanamycin; Sm, streptomycin; Su, sulfonamide; Amp, ampicillin; and Hg, mercury). Figure 6.12

21 Section 6.4 F factors confer fertility.
R plasmids confer antibiotic resistance. Col plasmids encode colicins that can kill neighboring bacteria.

22 Section 6.5 6.5 Transformation Is Another Process Leading to Genetic Recombination in Bacteria In transformation, small pieces of extracellular DNA are taken up by a living bacterial cell and integrated stably into the chromosome (Figure 6.13).

23 Figure 6-13 Proposed steps for transformation of a bacterial cell by exogenous DNA. Only one of the two strands of the entering DNA is involved in the transformation event, which is completed following cell division. Figure 6.13

24 Section 6.6 6.6 Bacteriophages Are Bacterial Viruses
Bacteriophages can infect a host bacterium by injecting their DNA. Transduction Type of bacterial genetic recombination caused by the infection of a bacteriophage

25 Figure 6-14 The structure of bacteriophage T4, including an icosahedral head filled with DNA, a tail consisting of a collar, tube, sheath, base plate, and tail fibers. During assembly, the tail components are added to the head and then tail fibers are added. Figure 6.14

26 Figure 6-15 Life cycle of bacteriophage T4.

27 Section 6.6 Lysogeny occurs when:
the phage DNA integrates into the bacterial chromosome it is replicated along with the chromosome it is passed to daughter cells Bacteria containing a prophage are lysogenic and can grow and divide stably until viral reproduction is induced.

28 Section 6.7 6.7 Transduction Is Virus-Mediated Bacterial DNA Transfer
Bacteriophages, which can themselves undergo genetic recombination, can be involved in a mode of bacterial genetic recombination called transduction.

29 Section 6.7 The Lederberg-Zinder experiment led to the discovery of phage transduction in bacteria (Figure 6.17).

30 Figure 6-17 The Lederberg–Zinder experiment using Salmonella
Figure 6-17 The Lederberg–Zinder experiment using Salmonella. After placing two auxotrophic strains on opposite sides of a Davis U-tube, Lederberg and Zinder recovered prototrophs from the side containing the LA-22 strain, but not from the side containing the LA-2 strain. These initial observations led to the discovery of the phenomenon called transduction. Figure 6.17

31 Figure 6-18 Generalized transduction.

32 Section 6.7 In generalized transduction, bacterial DNA instead of phage DNA is packaged in a phage particle and transferred to a recipient host (Figure 6.18). In specialized transduction, a small piece of bacterial DNA is packaged along with the phage DNA.

33 Section 6.8 6.8 Bacteriophages Undergo Intergenic Recombination
Phage mutations often affect plaque morphology (Figure 6.19 and Table 6.1). Such mutations have been important in understanding genetic phenomena in phages.

34 Figure 6-19 Plaque morphology phenotypes observed following simultaneous infection of E. coli by two strains of phage T2, and In addition to the parental genotypes, recombinant plaques hr and were also recovered. Figure 6.19

35 Section 6.8 Mapping in Bacteriophages
Mixed infection experiments demonstrated that intergenic recombination occurs in bacteriophages.

36 Section 6.9 6.9 Intragenic Recombination Occurs in Phage T4
Seymour Benzer’s (1950s) detailed examination of the rII locus of phage T4 allowed him to produce a genetic map of this locus.

37 The End

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