8 The Genetics of Bacteria and Their Viruses. Plasmids Plasmids are circular DNA molecules which replicate independently of the bacterial chromosome Plasmids.

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The Genetics of Bacteria and Their Viruses

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 = non-productive 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 site-specific 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