1. Microbial Genetics (Micr340)
Lecture 13
Molecular Basis of Recombination

2. Two Types of Recombination
Nonhomologous recombination
Occurs very rarely
Requires special proteins
Homologous recombination
Occurs much more often
Occurs between same or very similar sequences

3. Recombination requirements
Identical or very similar sequences in the cross-over region
Complementary base pairing between double-stranded DNA molecules
Recombination enzymes
Heteroduplex formation

4. Models of recombination
Holliday double-strand invasion model

5. Migration of Holliday junctions

6. Single-strand
Invasion
Model

7. Double-strand Break Repair Model

8. Molecular Basis for Recombination in E. coli
chi () Sites and the RecBCD nuclease
Synapse Formation and the RecA Protein
The Ruv and RecG proteins and the migration and cutting of Holliday Junctions

9. chi site and RecBCD enzyme

10. Synapse formation

11. Model for action of Ruv proteins

12. Model for action of Ruv proteins (continued)

13. Model for action of Ruv proteins (continued)

14. Phage Recombination Pathways
Rec Proteins and Phages T4 and T7
Gene 49 protein of T4 and gene 3 protein of T7 are similar to RuvC
Gene 46 and 47 products are similar to RecBCD
The RecE pathway of the rac prophage
RecE protein can substitute for RecBCD nuclease
The phage l red system
Red system requires l genes exo and bet.
Does not require RecA protein

15. Genetic Analysis of Recombination in Bacteria
Isolating Rec- mutants in E. coli
Other recombination genes
Gene conversion and other manifestations of heteroduplex formation during recombination

16. Repair of a mismatch

17. Repair of a mismatch (continued)

18. Outcome of a mismatch repair