Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 98
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(B) The transductants thatcould be formed by three possible types of transducing particles—one carrying gal+, one carrying bio+, and one carryingthe linked alleles gal+ bio+. The third type results in cotransduction. The distance between gal and bio, relative to thatbetween leu and gal, is greatly exaggerated, and the size of the DNA fragment in a transducing particle, relative tothe size of the bacterial chromosome, is not drawn to scale.markers in minutes on a conjugational map is related to the frequency of cotransduction approximately as follows:The implication of this equation is that the frequency of cotransduction falls off very rapidly as the distancebetween the markers increases.
In the formula, the shortest distance for which the cotransduction frequency equals0 is 2 minutes; hence any markers separated by more than 2 minutes will not be cotransduced by phage P1. For galand bio, which have a cotransduction frequency of about 12 percent, the distance equals 1 minute (see Figure 8.11).Page 329Connection Is a Bacteriophage an ''Organism"?Alfred D. Hershey and Raquel Rotman 1948Washington University,St. Louis, MissouriGenetic Recombination Between Host-range and Plaque-type Mutants of Bacteriophage in SingleBacterial CellsAre bacteriophages "organisms"? Well .
. . yes and no. If by the term organism you mean a cell orgroup of cells that function together as a whole to maintain life and its activities, then bacteriophagesare not organisms. By organism you might also mean "anything resembling a living thing in itscomplexity of structure and function." In this sense bacteriophages are organisms.
Even though theyrequire a bacterial host to survive and reproduce, their reproductive cycle conforms in all respects tothat of cellular organisms, including DNA as their genetic material, semiconservative replication ofDNA, heritable mutations, and, as shown first in this report, a process of recombination. Hershey andRotman were quite unsure whether phage recombination resulted from physical exchange of DNAmolecules or whether, when two molecules recombined, both reciprocal products of the exchange wereproduced. Later research showed that the exchanges are reciprocal and that they are physicalexchanges.We have shown that any two of several mutants of the bacterial virus T2 interact with each other, inbacterial cells infected with both, to give rise to wildtype and double mutant genetic recombinants .
. ..In principle, the experimental technique we have to describe is very similar to genetic crossing. Onestarts with a pair of mutants, each corresponding to a mutant haploid germ cell differing from wildtypeby a different unit change. Bacterial cells are infected with both members of the pair, and during viralgrowth the pair interact to produce viral progeny corresponding to germ cells of a new generation, butnow including some individuals differing from wildtypeThe analogy to other genetic recombination is obvious, and it is natural to lookfor a common mechanism.by both unit changes, and other individuals differing from wildtype not at all.
The analogy to othergenetic recombination is obvious, and it is natural to look for a common mechanism . . .. The crossesbetween h and r mutants yield the linkage system shown below.In the diagram, h refers to bacterial host range and r13 and r7 to two different, rapidly lysing mutants.The data show that the h locus is very closely linked to r13 (less than 1 percent of wildtype), and thatthe linkage relation between r13 and r7 is 6 percent . . .. The results further show that the tworecombinants [wildtype and double mutant] appear in equal numbers in any one cross, and that pairs ofreverse crosses yield equal numbers of recombinants. It is these relations that increase the resemblanceto simple types of Mendelian segregation .
. .. A hypothesis is proposed according to which onevisualizes genetic interaction not between two viral particles, but between two sets of independentlymultiplying chromosome-like structures. Genetic exchange occurs either by reassortment of thesestructures, or by something like crossing-over between homologous pairs, depending on the structuralrelation between the genetic factors concerned. The interpretation made brings the linkage relationsinto superficial agreement with the requirements of linear structure, but there is little evidence that thegenetic exchanges are reciprocal, and accordingly little evidence that they are material exchanges.Source: Genetics 34: 44–718.6—Bacteriophage GeneticsThe life cycles of phages fit into two distinct categories—the lytic and the lysogenic cycles. In the lytic cycle,phage nucleic acid enters a cell and replicates repeatedly, the bacterium is killed, and hundreds of phage progenyresult (see Figure 8.3).
All phage species can undergo a lytic cycle; a phage capable only of lytic growth is calledvirulent. In the alternative, lysogenic cycle, no progeny particles are produced, the bacterium survives, and aphage DNA molecule is transmitted to each bacterial daughter cell. In most cases, transmission of this kind isaccomplished by integration of the phage chromosome into the bacterial chromosome. A phage capable of such alife cycle is called temperate.Several phage particles can infect a single bacterium, and the DNA of eachPage 330particle can replicate. In a multiply infected cell in which a lytic cycle is underway, there are genetic exchangesbetween phage DNA molecules. If the infecting particles carry different mutations as genetic markers, thenrecombinant phages result.
Recombination in phage may be general recombination, which means that it can takeplace anywhere along two homologous DNA molecules. General recombination will be considered in this section.In temperate phages there is a second type of recombination called site-specific recombination because it takesplace only between a particular site in the phage and a particular site in the bacterial chromosome. The end result ofsite-specific recombination is not the production of recombinant phage chromosomes but the joining of phage andbacterial DNA molecules. This process is described in Section 8.7.Plaque Formation and Phage MutantsPhages are easily detected because in a lytic cycle, an infected cell breaks open (lysis) and releases phage particlesto the growth medium. The formation of plaques can be observed as outlined in Figure 8.16A.
A large number ofbacteria (about 108) areFigure 8.16Plaque formation. (A) In the absence of a phage, bacterial cells grow and form a translucent lawn. Bacterial cells depositedin the vicinity of a phage are infected and lyse. Progeny phages diffusing outward from the original site infect other cells andcause their lysis. Because of phage infection and lysis, no bacteria can grow in a small region around the site of each phageparticle originally present in the medium.
The area devoid of bacteria remains transparent and is called a plaque. (B)Large plaques in a lawn of E. coli formed by infection with a mutant of bacteriophage λ. Each plaque results from an initialinfection by a single bacteriophage.Page 331placed on a solid medium. After a period of growth, a continuous turbid layer of bacteria results. If a phage ispresent at the time the bacteria are placed on the medium, it adsorbs to a cell, and shortly afterward, the infectedcell lyses and releases many phages.
Each of these progeny adsorbs to a nearby bacterium, and after another lyticcycle, these bacteria in turn release phages that can infect still other bacteria in the vicinity. (Progeny phage remainclose to their site of origin because their size prevents diffusion in agar medium.) These cycles of infectioncontinue, and after several hours, the phages destroy all of the bacteria in a localized area, giving rise to a clear,transparent region—a plaque—in the otherwise turbid layer of confluent bacterial growth.
Phages can multiplyonly in growing bacterial cells, so exhaustion of nutrients in the growth medium limits phage multiplication and thesize of the plaque. Because a plaque is a result of an initial infection by one phage particle, the number ofindividual phages originally present on the medium can be counted.The genotypes of phage mutants can be determined by studying the plaques. In some cases, the appearance of theplaque is sufficient.
For example, phage mutations that decrease the number of phage progeny from infected cellsoften yield smaller plaques. Large plaques can be produced by mutants that cause premature lysis of infected cells,so that each round of infection proceeds more quickly (Figure 8.16B). Another type of phage mutation can beidentified by the ability or inability of the phage to form plaques on a particular bacterial strain.Genetic Recombination in Virulent BacteriophagesIf two phage particles with different genotypes infect a single bacterium, then some phage progeny are geneticallyrecombinant.
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