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Chapter 10

Chapter 10. Homologous Recombination at the Molecular Level ----- 生科类 彭灿 200431060171. OUTLINE. Models for Homologous Recombination( two models) ★ Homologous Recombination Protein Machines ★ Homologous Recombination in Eukaryotes Mating-Type Switching

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Chapter 10

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  1. Chapter 10 Homologous Recombination at the Molecular Level -----生科类 彭灿 200431060171

  2. OUTLINE • Models for Homologous Recombination( two models) ★ • Homologous Recombination Protein Machines★ • Homologous Recombination in Eukaryotes • Mating-Type Switching • Genetic Consequences of the Mechanism of Homologous Recombination

  3. Models for Homologous Recombination • Many models have been proposed to explain the molecular mechanism of genetic exchange. Key steps shared by these models: • 1 Alignment of two homologous DNA molecules. • 2 Introduction of breaks in the DNA. • 3 Formation of initial short regions of base pairing between the two recombination DNA molecules. • 4 Movement of the Holliday junction. • 5 Cleavage of the Holliday junction.

  4. The holliday model Figure 1

  5. (a) Two homologous DNA molecules line up • (b) Cuts in one strand of both DNAs • (c) The cut strands cross and join homologous strands, forming the Holliday structure (or Holliday junction). • (d) Heteroduplex region is formed by branch migration • (e) Resolution of the Holliday structure. • (f) The vertical cut will result in crossover between f-f' and F-F' regions.  The heteroduplex region will eventually be corrected by mismatch pair(g) The horizontal cut does not lead to crossover after mismatch repair.  However, it could cause gene conversion.

  6. Two ways of resolution of the Holliday model figure2

  7. The Double-Strand Break Repair Model Accurately Describes Many Recombination Events • As with the Holliday model, this pathway starts with aligned homologous chromosomes. But in this case, the initiating event is the introduction of a double-stranded break(DSB) in one of the two DNA molecules. The other DNA duplex remains intact. • Because double-stranded DNA breaks occur relatively frequently, this type of initiating event is attractive compared to the pair of aligned nickes that are proposed to initiate recombination by the Holliday model.

  8. DSB repair model for Homologous recombination Figure 3

  9. Double-stranded DNA Break Arise by Numerous Means and Initiante Homologous Recombination • Double-stranded breaks in DNA arise quite frequently. • The major mechanism used to repair DSBs in most cells is homologous recombination via DBS-repair pathway . • Homologous recombination repair DSBs in chromosomal DNA , and promote genetic exchange in bacteria.

  10. Homologous Recombination Protein Machines

  11. The RecBCD Helicase /Nuclease Processes Broken DNA Molecule for Recombination • RecBCD is composed of three subunits and has nuclease activities .It binds to DNA molecules at the site of a double-strand break and tract along DNA using the energy of ATP-hydrolysis. • The activities of RecBCD are controlled by specifi DNA sequence elements known as chi sites.

  12. Figure 4 Steps of DNA processing By RecBCD

  13. RecA Protein Assembles on Single-stranded DNA and Promotes Strand Invasion

  14. substrates for RecA strand exchange

  15. Newly Base-Paired Partners Are Established within the RecA Filament

  16. RuvAB Complex Specifically Recognizes Holliday Junctions and Promotes Branch Migration • Rave recognizes and binds to Holliday junctions and recruits the RuvB protein to this site. • The RuvB ATPase provides the energy to drive the exchange of base pairs that move the DNA branch.

  17. RuvC Cleaves Specific DNA Strands at the Holliday Junction to Finish Recombination

  18. Homologous Recombination in Eukaryotes

  19. Homologous Recombination Is Required for Chromosome Segregation during Meiosis

  20. Programmed Generation of Double-Strand DNA Breaks Occur during Meiosis SPO11 encodes a protein that introduces Double-strand breaks in chromosomal DNA to initiate meiotic recombination

  21. Overview of meiotic recombination pathway

  22. Mating-Type Switching

  23. The mating-type genes encode transcriptional regulators. • Mating-type switching is initiuated by the introduction of a DSB at the MAT locus, the action is completed by HO endonuclease. • Mating-type switching is unidirectional . • Mating-type switching is a gene conversion event.

  24. Genetic Consequence of the Mechanism of Homologous Recombination (exchange or conversion)

  25. THE END Thank you

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