This document discusses several types of molecular recombination: homologous recombination, site-specific recombination, and transposition. It describes reciprocal homologous recombination through the double-strand break model and nonreciprocal homologous recombination through the Fox model. Site-specific recombination is important for viral genome insertion and has small regions of homology. Transposition involves transposable elements like transposons that are mobile genetic elements.

1. Recombination at the Molecular Level
• three types
– homologous recombination
– site specific recombination
– transposition

2. Reciprocal Homologous
Recombination:
Double-Strand Break Model

3. Nonreciprocal Homologous Recombination: Fox Model
• incorporation of single
strand of DNA into
chromosome, forming
heteroduplex DNA
• thought to occur during
bacterial transformation

4. 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

5. transposition & transposable elements
transposable elements = transposons =
mobile genetic elements = “jumping genes”

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11. replicative
transposition

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13. R plasmid w/
integrated
Tn3 transposon

14. What are potential effects of
transposons?

16. Bacterial Conjugation
• Transfer of genes
between bacteria
that depends on
– direct cell to cell
contact mediated by
a sex pilus
– type IV secretion
system

17. Evidence for Bacterial Conjugation (Lederberg & Tatum, 1946)

18. The U-Tube Experiment (Davis, 1950)

19. Types of Conjugation
1. F+ × F2. Hfr × F3. F’

20. +
-
F x F Mating
– copy of F factor is transferred to recipient &
does not integrate into the host chromosome
– donor genes usually not transferred

21. +
-
F xF
Mating

22. rolling-circle
replication

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24. 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

26. Figure 14.24a

27. Figure 14.24b

28. 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

29. Figure 14.25

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

34. 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

36. 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

37. Generalized transduction
abortive transductants = bacteria w/
nonintegrated transduced DNA

38. 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

39. Specialized/restricted
transduction

41. Phage lambda
low-frequency
transducing lysate
insert Figure 13.21
high-frequency
transducing lysate

42. For Tuesday:
Read Ch 17
For Lab Next Week:
Identify unknowns to Group level
Collect materials needed for Team Projects