Replication of phage genetic material and breakdown of host's genetic material in Software

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Replication of phage genetic material and breakdown of host's genetic material
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Manufacture of phage proteins
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Assembly of new phage
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Lysis of cell Figure 7.21 Phage
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Figure 7.20 The viral life cycle, using T4 infection of E. coli as
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. Magni cation 167,300 . Note that phage lacks the tail bers and base plate of phage T2 (see g. 7.1). (Courtesy of Robley Williams.)
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Tamarin: Principles of Genetics, Seventh Edition
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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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The McGraw Hill Companies, 2001
Seven
Linkage and Mapping in Prokaryotes and Bacterial Viruses
h+r+
E. coli Tto s
h+r+
Figure 7.22 Four types of plaques produced by mixed phage
T2 on a mixed lawn of E. coli. (From Molecular Biology of Bacterial
Viruses by Gunther S. Stent, 1963, 1978 by W. H. Freeman & Company. Used with permission.)
E. coli mixed-indicator lawn (Tto r + Tto s)
From the wild stock of phages, we can isolate hostrange mutants by looking for plaques on a Ttor bacterial lawn. Only h mutants will grow. These phages can then be tested for the r phenotype and the double mutants isolated. Once the two strains (double mutant and wildtype) are available, they can be added in large numbers to sensitive bacteria ( g. 7.23). Large numbers of phages are used to ensure that each bacterium is infected by at least one of each phage type, creating the possibility of recombination within the host bacterium. After a round of phage multiplication, the phages are isolated and plated out on Delbr ck s mixed-indicator stock. From this growth, the phenotype (and, hence, genotype) of each phage can be recorded. The percentage of recombinants can be read directly from the plate. For example, on a given petri plate (e.g., g. 7.22) there might be hr h r 46 34 h r hr 52 26
Clear, large
Clear, small
Turbid, large
Turbid, small
Figure 7.23 Crossing hr and h r phage. Enough of both types are added to sensitive bacterial cells (Ttos) to ensure multiple infections. The lysate, consisting of four genotypes, is grown on a mixed-indicator bacterial lawn (Ttos and Ttor). Plaques of four types appear (see g. 7.22), indicating the genotypes of the parental and recombinant phages.
which recombination could have taken place. The proportion of recombinants is (34 26)/(46 52 34 26) 60/158 0.38 or 38% or 38 map units This percentage recombination is the map distance, which (as in eukaryotes) is a relative index of distance between loci: The greater the physical distance, the greater the amount of recombination, and thus the larger the map distance. One map unit (1 centimorgan) is equal to 1% recombinant offspring.
The rst two, hr and h r , are the original, or parental, phage genotypes. The second two categories result from recombination between the h and r loci on the phage chromosome. A single crossover in this region produces the recombinants. Note that with phage recombination, parental phages are counted, since every opportunity was provided for recombination within each bacterium. Thus, every progeny phage arises from a situation in
Tamarin: Principles of Genetics, Seventh Edition
II. Mendelism and the Chromosomal Theory
7. Linkage and Mapping in Prokaryotes and Bacterial Viruses
The McGraw Hill Companies, 2001
Transduction
Curing (loss of prophage)
Lytic response
Lysogenic response Lysogenic cell (prophage present)
Induction Vegetative growth Lysogenic growth
Figure 7.24 Alternative life-cycle stages of a temperate phage (lysogenic and vegetative growth).
Lysogeny
Certain phages are capable of two different life-cycle stages. Some of the time, they replicate in the host cytoplasm and destroy the host cell. At other times, these phages are capable of surviving in the host cell. The host is then referred to as lysogenic and the phage as temperate. (The term lysogeny means giving birth to lysis. A lysogenic bacterium can be induced to initiate the virulent phase of the phage life cycle.) The majority of research on lysogeny has been done on phage (see g. 7.21).The prophage integrates into the host chromosome; other prophages, like P1, exist as independent plasmids. Phage , unlike the F factor, attaches at a speci c point, termed att . This locus can be mapped on the E. coli chromosome; it lies between the galactose ( gal) and biotin (bio) loci. When the phage is integrated, it protects the host from further infection (superinfection) by other phages. The integrated phage is now termed a prophage. Presumably it becomes integrated by a single crossover between itself and the host after apposition at the att site. (This process resembles the F-factor integration shown in g. 7.18.) A prophage can enter the lytic cycle of growth by a process of induction, which involves the excision of the prophage followed by the virulent or lytic stage of the viral life cycle. We consider the interesting and complex control mechanisms of life cycle in detail in chapter 14. Induction can take place through a variety of mechanisms, including UV irradiation and passage of the integrated prophage during conjugation (zygotic induc-
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