《分子生物学》5cha.ppt
Chapter 10,Homologous recombination at the molecular level,All DNA is recombinant DNA.Genetic exchange works constantly to blend and rearrange chromosomes,most obviously during meiosis,when homologous chromosomes pair prior to the first nuclear division.During this pairing,genetic exchange between the chromosomes occurs.This exchange,classically termed crossing over,is one of the results of homologous recombination.,Models for Homologous Recombination Homologous Recombination Protein MachinesHomologous Recombination in Eukaryotes Mating-Type Switching,in prokaryotes,Alignment:Breaks:Strand invasionHolliday junctionBranch migration:Movement of the Holliday junctionResolution:Cleavage of the Holliday junction,Models for homologous recombination,1.The Holliday Model,2.The Double-Strand Break Repair Model,Double-straned breaks in DNA arise quite frequently In bacteria,the major biological role of homologous recombination is to repair DSBsIn addition to repairing DSBs in chromosomal DNA,homologous recombination promotes genetic exchange in bacteriaIn eukaryotic cells,homologous recombination is critical for repairing DNA,Double stranded DNA breaks arise by numerous means and initiate homologous recombination,Damage in the DNA template can lead To DSB formation during DNA replication,Homologous recombination protein machines,Homologous recombination in prokaryotes,The RecBCD helicase/nuclease processes broken DNA molecules for recombination RecA protein assembles on single-stranded DNA and promotes strand invasionNewly base-paired partners are established within the RecA filamentRuvAB complex specifically recognizes Holliday junctions and promotes branch migrationRuvC cleaves specific DNA strands at the Holliday junction to finish recombination,Steps of DNA processing by RecBCD,Polar action of chi,RecA Protein:strand-exchange proteinThe active form of recA is a protein-DNA filament many subunits+many strands Coated with RecA make DNA highly extendedPolarity of RecA assembly,RecA Protein Assembles on Single-stranded DNA and Promotes Strand Invasion,RecA Stabilises single stranded DNA,RecA prevents secondary structure within the single stranded DNA RecA forces the single stranded DNA to take on a helical shape,effectively lowering the energy barrier to recombination,RecA Protein Assembles on Single-stranded DNA and Promotes Strand Invasion,Polarity of RecA assembly,Newly base-paired partners are established within the RecA filament,cross-section of single DNA strand bound to RecA proteinHomology search is promoted by RecA.DNA in secondary site is testes for complementarityJoint molecular:Base-paring between strands is switchedRecA is prone to bind the DNA products after strand exchange.,RecA-catalyzed strand exchange can be divided into different stages,Model of two steps in search for homology and DNA strand exchange within the RecA filament,Homologous recombination in prokaryotes,The RecBCD helicase/nuclease processes broken DNA molecules for recombination RecA protein assembles on single-stranded DNA and promotes strand invasionNewly base-paired partners are established within the RecA filamentRuvAB complex specifically recognizes Holliday junctions and promotes branch migrationRuvC cleaves specific DNA strands at the Holliday junction to finish recombination,RuvAB complex specifically recognizes Holliday junctions and promotes branch migration,RuvA protein is a Holliday junction specific DNA-binging protein that recognizes the structure of the DNA junctionRuvB is a hexametric ATPase:drive the exchange of base pair that move the DNA branch,High resolution structure of the RuvA-DNA complex and schematic model of the RuvAB complex bound to Holliday junction DNA,RuvC cleaves specific DNA strands at the Holliday junction to finish recombination,Completion of recombination requires that the Holliday junction between the two recombining DNA molecules be resolved RuvC specifically nicks two of the homologous DNA strands that have the same polarity RuvC recognize the structure rather than a special sequence,but still with modest sequence specificity,High resolution structure of the RuvC resolves and schematic model of the RuvC dimer bound to Holliday junction DNA,Models for Homologous Recombination Homologous Recombination Protein MachinesHomologous Recombination in Eukaryotes Mating-Type Switching Genetic Consequences of the Mechanism of Homologous Recombination,in prokaryotes,Homologous recombination has additional functions in eukaryotes,Homologous recombination can repair double stranded breaks in DNA,to restart collapsed replication forks,and to allow a cells chromosomal DNA to recombine with DNA that enters via phage infection or junctionDuring meiosis,homologous recombination is required for proper chromosome pairing,Recombination must be complete before the first nuclear division to allow the homologs to properly align and then separate The improper segregation of chromosomes,called nondisjunction,leads to a large number of gametes without the correct chromosome complementThe homologous recombination events that occur during meiosis are called meiotic recombination,Homologous recombination is required for chromosome segregation during meiosis,DNA dynamics during meiosis,Homologous recombination in eukaryotes,Double stranded DNA breaks MRX protein:RecBCD-like protein Dmc1:RecA-like protein Many protein together:Rad51 Rad52,Mechanism of cleavage by spo11,Overview of meiotic recombination pathway,Dmc1-dependent recombination occurs preferentially between nonsister homologous chromatids,Many protein function together to promote meiotic recombination,Some large protein-DNA complexes,known as recombination factoriesRad51,Dmc1,Rad52,Models for Homologous Recombination Homologous Recombination Protein MachinesHomologous Recombination in Eukaryotes Mating-Type Switching Genetic Consequences of the Mechanism of Homologous Recombination,in prokaryotes,Genetic loci encoding mating-type information,Models for Homologous Recombination Homologous Recombination Protein MachinesHomologous Recombination in Eukaryotes Mating-Type Switching,in prokaryotes,
