Recombination at the Molecular Level

• three types– homologous recombination– site specific recombination– transposition

Reciprocal Homologous Recombination:

Double-Strand Break Model

Nonreciprocal Homologous Recombination: Fox Model

• incorporation of single strand of DNA into chromosome, forming heteroduplex DNA

• thought to occur during bacterial transformation

Site-specific recombination

• important in insertion of viral genomes into host chromosomes

• there is only a small region of homology between inserted genetic material & host chromosome

transposition & transposable elements

transposable elements = transposons = mobile genetic elements = “jumping genes”

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replicativetransposition

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R plasmid w/ integrated Tn3 transposon

What are potential effects of transposons?

Bacterial Conjugation

• Transfer of genes between bacteria that depends on– direct cell to cell

contact mediated by a sex pilus

– type IV secretion system

Evidence for Bacterial Conjugation (Lederberg & Tatum, 1946)

The U-Tube Experiment (Davis, 1950)

Types of Conjugation

1. F+ × F-

2. Hfr × F-

3. F’

F+ x F- Mating

– copy of F factor is transferred to recipient & does not integrate into the host chromosome

– donor genes usually not transferred

F+ x F- Mating

rolling-circle replication

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Hfr Conjugation

– donor has F factor integrated into its chromosome (Hfr cell)

– donor genes are transferred to recipient cell– complete copy of the F factor is usually not

transferred

Figure 14.24a

Figure 14.24b

F’ Conjugation

• F factor incorrectly leaves host chromosome

• some of the F factor is left behind in host chromosome

• some host genes have been removed along w/ some of the F factor– these genes can be transferred to a second host

cell by conjugation

Figure 14.25

Bacterial Transformation

• uptake of naked DNA by a competent cell followed by incorporation of the DNA into the recipient cell’s genome

Transduction

• transfer of bacterial genes by viruses• viruses (bacteriophages) can carry out the

lytic cycle in which the host cell is destroyed or the viral DNA can integrate into the host genome, becoming a latent prophage

Generalized Transduction

• any part of bacterial genome can be transferred

• occurs during lytic cycle• during assembly, fragments of host DNA

mistakenly packaged into phage head– generalized transducing particle

Generalized transduction

abortive transductants = bacteria w/ nonintegrated transduced DNA

Specialized Transduction

• a.k.a. restricted transduction• carried out only by lysogenic/temperate

phages• only specific portion of bacterial genome is

transferred• occurs when prophage incorrectly excises

Specialized/restricted transduction

Phage lambda

low-frequencytransducing lysate

insert Figure 13.21

high-frequencytransducing lysate

For Tuesday:Read Ch 17

For Lab Next Week:Identify unknowns to Group level

Collect materials needed for Team Projects