Lecture 21
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Transcript of Lecture 21
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