Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 93
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M. Kurnit. 1988. Trisomy 21: Molecular and cytogenetic studies ofnondisjunction. Advances in Human Genetics 17: 99.Trichopoulos, D., F. P. Li, and D. J. Hunter. 1996. What causes cancer? Scientific American, September.Vogt, P. K., ed. 1997. Chromosomal Translocations and Oncogenic Transcription Factors. New York: SpringerVerlag.Wagner, R. P., M. P. Maguire, and R. L. Stallings. 1993. Chromosomes. New York: Wiley-Liss.Wang, J.
Y. J. 1997. Retinoblastoma protein in growth suppression and death protection. Current Opinion inGenetics θ Development 7: 39.Weinberg, R. A. 1996. How cancer arises. Scientific American, September.White, M. J. D. 1977. Animal Cytology and Evolution. London: Cambridge University Press.Page 306Two cells of Escherichia coli caught in the act of mating (conjugation). The male cell is at the lower left, and thefemale cell, at the upper right.
The connecting tube is an F pilus, encoded by genes in the Fplasmid, through which DNA from the male cell enters the female cell. (The pilus shortens somewhatprior to DNA transfer.) In this case, for ease of visualization, the F pilus has been coated with particles of amale-specific bacteriophage.
The male cell has numerous other appendages, not F pili, thatare used in colonizing the intestine.[Courtesy of C. C. Brinton, Jr., and J. Carnahan.]Page 307Chapter 8—The Genetics of Bacteria and VirusesCHAPTER OUTLINE8-1 The Genetic Organization of Bacteria and Viruses8-2 Bacterial Mutants8-3 Bacterial Transformation8-4 ConjugationPlasmidsHfr CellsTime-of-Entry MappingF' Plasmids8-5 Transduction8-6 Bacteriophage GeneticsPlaque Formation and Phage MutantsGenetic Recombination in Virulent BacteriophagesFine Structure of the rII Gene in Bacteriophage T48-7 Genetic Recombination in Temperate BacteriophagesLysogenySpecialized Transducing Phage8-8 Transposable ElementsTransposons in Genetic AnalysisChapter SummaryKey TermsReview the BasicsGuide to Problem SolvingAnalysis and ApplicationsChallenge ProblemsFurther ReadingGeNETics on the webPRINCIPLES• Some bacteria are capable of DNA transfer and genetic recombination.• In transformation, bacterial cells take up DNA from the surrounding medium and incorporate it into their genomeby homologous recombination, replacing some host genes in the process.• In E.
coli, the F (fertility) plasmid can mobilize the chromosome for transfer to another cell in the process ofconjugation.• Some types of bacteriophages can incorporate bacterial genes and transfer them into new host cells in the processof transduction.• DNA molecules from related bacteriophages that are present in the same host cell can undergo geneticrecombination.• Transposable elements and plasmids are widely used for genetic analysis and DNA manipulation in bacteria.CONNECTIONSCONNECTION: The Sex Life of BacteriaJoshua Lederberg And Edward L.
Tatum 1946Gene Recombination In Escherichia ColiCONNECTION: Is a Bacteriophage an "Organism"?Alfred D. Hershey And Raquel Rotman 1948Genetic Recombination Between Host-Range And Plaque-Type Mutants Of Bacteriophage In Single BacterialCellsCONNECTION: ArtooSeymour Benzer 1955Fine Structure Of A Genetic Region In BacteriophagePage 308In earlier chapters, we examined the genetic properties of representative eukaryotic organisms.
The genomes ofthese organisms consist of multiple chromosomes. In the course of sexual reproduction, genotypic variation amongthe progeny is achieved both by random assortment of the chromosomes and by crossing-over, processes that takeplace in meiosis. Two important features of crossing-over in eukaryotes are that (1) it results in a reciprocalexchange of material between two homologous chromosomes and (2) both products of a single exchange can oftenbe recovered in different progeny. The situation is quite different in prokaryotes, as we will see in this chapter.Bacteria and viruses bring to traditional types of genetic experiments four important advantages over multicellularplants and animals.
First, they are haploid, so dominance or recessiveness of alleles is not a complication inidentifying genotype. Second, a new generation is produced in minutes rather than weeks or months, which vastlyincreases the rate of accumulation of data. Third, they are easy to grow in enormous numbers under controlledlaboratory conditions, which facilitates molecular studies and the analysis of rare genetic events. Fourth, theindividual members of these large populations are genetically identical; that is, each laboratory population is aclone of genetically identical cells.8.1—The Genetic Organization of Bacteria and VirusesThe organization of a typical prokaryotic cell is exemplified by the bacterial cell in Figure 8.1A. The geneticmaterial is located in a region that lacks clear boundaries and is called the nucleoid.
Contrast this relativelyunstructured organization with the eukaryotic cell in Figure 8.1B. In eukaryotes, the genetic material is enclosed inthe nucleus by a membrane envelope connected in special ways to the cytoplasm; eukaryotic cells also have othermembrane systems that subdivide the cytoplasm into regions of specialized function.A bacterial cell contains a major DNA molecule that almost never encounters another complete DNA molecule.Instead, genetic exchange is usually between a chromosomal fragment from one cell and an intact chromosomefrom another cell.
Furthermore, a clear donor-recipient relationship exists: The donor cell is the source of a DNAfragment, which is transferred to the recipient cell by one of several mechanisms, and exchange of genetic materialtakes place in the recipient by means of reciprocal recombination between homologous DNA sequences.Incorporation of a part of the donor DNA into the chromosome requires at least two exchange events, one at eachend.
Because the recipient molecule is circular, only an even number of exchanges results in a viable product. Theusual outcome of these events is the recovery of only one of the crossover products. However, in some situations,the transferred DNA is also circular, and a single exchange results in total incorporation of the circular donor DNAinto the chromosome of the recipient.Three major types of genetic transfer are found in bacteria: transformation, in which a DNA molecule is taken upfrom the external environment and incorporated into the genome; transduction, in which DNA is transferred fromone bacterial cell to another by a bacterial virus; and conjugation, in which donor DNA is transferred from onebacterial cell to another by direct contact. Recombination frequencies that result from these processes are used toproduce genetic maps of bacteria.
Although the maps are exceedingly useful, they differ in major respects from thetypes of maps obtained from crosses in eukaryotes because genetic maps in eukaryotes are based on frequencies ofcrossing-over in meiosis.A virus is a small particle, considerably smaller than a cell, that is able to infect a susceptible cell and multiplywithin it to form a large number of progeny virus particles.
Few, if any, organisms are not subject to viral infection.Many human diseases are caused by viruses, including influenza, measles, AIDS, and the common cold. Virusesthat infect bacterial cells are called bacteriophages or simply phages. Most viruses consist of a single molecule ofPage 309genetic material enclosed in a protective coat composed of one or more kinds of protein molecules; however, theirsize, molecular constituents, and structural complexity vary greatly (Figure 8.2).Isolated viruses possess no metabolic systems, so a virus can multiply only within a cell. It is "living" only in thesense that its genetic material directs its own multiplication; outside its host cell, a virus is an inert particle.Nevertheless, within an infected cell, many viruses and bacteriophages possess mechanisms by which their DNAmolecules exchange genetic material resulting in genetic recombination.
In the life cycle of a bacteriophage, aphage particle attaches to a host bacterium and injects its nucleic acid into the cell; the nucleic acid replicates manytimes; finally, newly synthesized nucleic acid molecules are packaged into protein shells (forming progeny phage),and then the particles are released from the cell (Figure 8.3). Except for new mutations, phage progeny from abacterium infectedFigure 8.1The organization of a prokaryotic cell and a eukaryotic cell.(A) An electron micrograph of a dividing bacterial cell, showingthe dispersed genetic material (light areas). (B) An electronmicrograph of a section through a cell of the eukaryoticalga Tetraspora, showing the membrane-boundednucleus, the nucleolus (dark central body), and othermembrane systems (chloroplasts and mitochondria) thatsubdivide the cytoplasm into regions of specialized function.The dark sharply bounded regions in part B arestarch storage granules, which store carbohydrate.
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