Ch. 8: Presentation Slides - Indiana State University

Ch. 8: Presentation Slides - Indiana State University

8 The Genetics of Bacteria and Their Viruses 1 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

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 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

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 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

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 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 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 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 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 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 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 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 Temperate Bacteriophages Temperate bacteriophages have two life

cycles: lytic cycle=infection which results in production of progeny phage and bacterial cell lysis and lysogeny = nonproductive viral infection results in insertion of viral DNA into bacterial chromosome Viral DNA integration= site-specific insertion into bacterial chromosome

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 sitespecific recombination between the virus and bacterial cell 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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