8 The Genetics of Bacteria and Their Viruses. Plasmids Plasmids are circular DNA molecules which...
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Transcript of 8 The Genetics of Bacteria and Their Viruses. Plasmids Plasmids are circular DNA molecules which...
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8
The Genetics of Bacteria and Their Viruses
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Plasmids
• Plasmids are circular DNA molecules which replicate independently of the bacterial chromosome
• Plasmids often carry antibiotic resistance genes transferred to recipient cells by transformation
• Plasmids are used in genetic engineering as gene transfer vectors
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Conjugation
• Conjugation = process in which DNA is transferred from bacterial donor cell (F+) to a recipient cell (F-) by cell-to-cell contact
• F (fertility) factor = plasmid transferred by conjugation
• F factor = episome = genetic element that can insert into chromosome or replicate as circular plasmid
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Transposable Elements
• Transposable elements = DNA sequences present in multiple copies which are capable of movement within the genome
• Insertion (IS) elements = mobile elements contain transposase = enzyme catalyzes IS element transfer
• Transposons = IS elements which contain bacterial genes
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Transposable Elements
• Transposons can insert into plasmids which can be transferred to recipient cells by conjugation
Transposable elements are flanked by inverted repeats and often contain multiple antibiotic resistance genes= R plasmids
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Bacterial Genetics
Types of bacterial mutants:• Antibiotic-resistant mutants• Nutritional mutants: wildtype=prototroph
mutant=auxotroph which cannot grow in minimal media providing basic nutrients only
• carbon-source mutants=cannot use some carbon sources
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Bacterial Transformation
• Recipient cells acquire genes from DNA outside the cell
• DNA is taken up by cell and often recombines with genes on bacterial chromosome
• Bacterial transformation showed that DNA is the genetic material
• Transformation may alter phenotype of recipient cells
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Cotransformation of Linked Genes
• Donor DNA which contains genes located close together are often transferred as a unit to recipient cell = cotransformation
• The greater the distance between genes the less likely they will be transferred as a unit to recipient cell
• Cotransformation is used to map gene order
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Hfr
• Hfr (high frequency recombination) exchange
between donor cells F+ and
few cells where F factor
integrated into bacterial
chromosomes
• Recombination inserts Hfr genes into chromosome
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Chromosome Mapping
• Time-of-Entry mapping = method of mapping genes by Hfr X F- matings using interrupted mating technique
• A plot of time (minutes) versus # of recombinants is used to map genes as transfer order map
• Circular genetic map of E. coli shows map distances of genes in minutes
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Transduction
• Transduction = bacterial DNA fragment is transferred from one bacterial cell to another by a virus (phage) containing bacterial DNA = transducing phage
• Generalized transducing phage = transfers DNA derived from any part of the bacterial chromosome
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Transduction
• Phage P1 cuts bacterial chromosome into pieces and can package bacterial DNA into phage particles
• Transducing particle will insert ‘transduced” bacterial genes into recipient cell by infection
• Transduced genes may be inserted into recipient chromosome by homologous recombination
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Transduction
• Specialized transducing phage = particles contain phage and bacterial genes from a specific point of bacterial chromosome
• Cotransduction can be used to demonstrate linkage between bacterial genes
• Frequency of cotransduction is a measurement of linkage
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Transduction
• Specialized transducing phages transduce bacterial genes at the site of prophage insertion into the bacterial chromosome
• Transduction of bacterial genes occurs by aberrant excision of viral DNA which results in the incorporation of bacterial genes into phage chromosome
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Temperate Bacteriophages
• Temperate bacteriophages have two life cycles: lytic cycle=infection which results in production of progeny phage and bacterial cell lysis and lysogeny = non-productive viral infection results in insertion of viral DNA into bacterial chromosome
• Viral DNA integration= site-specific insertion into bacterial chromosome
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Lysogenic Bacteriohages
• In the lysogenic cycle, the viral DNA integrated into the bacterial chromosome is called a prophage
• Lysogen=bacterial cell containing integrated prophage
• Integration is catalyzed by a viral enzyme=integrase which carries out site-specific recombination between the virus and bacterial cell
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Lysogenic Bacteria
• Prophage induction=excision of prophage from bacterial chromosome and entry to lytic cycle
• Prophage induction results from damage to the bacterial chromosome by chemicals or radiation
• Excisionase=viral enzyme which removes prophage by site-specific recombination
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