6.2 接合.ppt

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1、Conjugation(接合接合):the second mechanism thatbacteria use to exchange genesE.Coli-the ultimate lab rat Plasmid(质粒质粒):Extrachromosomal genetic elements that are capable of autonomous replication(replicon)(1)Extrachromosomal DNA,usually circular(2)Usually encode ancillary functions for in vitro growth.(

2、3)Can be essential for specific environments:virulence,antibiotics resistance,use of unusual nutrients,production of bacteriocins(细菌素).(4)Must be a replicon-self-replicating genetic unit.Classification of Plasmids(1)Conjugative plasmid Carries genes for sex pili and transfer of the plasmid,F plasmid

3、(2)Dissimilation plasmids Encode enzymes for catabolism of unusual compounds(3)R factor Encode antibiotic resistanceFigure 8.29有性纤毛基因:PilusConjugative plasmidF plasmid(1)large(100 kb)(2)low copy(1-2 copies/cell)(3)self transmissible(传递)(4)requires protein synthesis RepE gene encodes RepE protein Rep

4、E protein binds to origin of replication and initiates DNA replication.RepE binds to the repE promoter and activates transcription.Bacterial PhenotypesBacterial Cells Can RecombineDiscovered by Lederberg and Tatum(40s)野生的野生的Bacterial Cells Can RecombineWhy did they use multiple different mutations?R

5、ecombination Requires Physical ContactThis shows that it is not the transfer of a molecule through the medium过滤器过滤器 Recombination Requires Physical ContactThe process is called congugation(接合接合),and it involves the transfer of a plasmid(F factor)from one cell to anotherF Factor Integration Yields Hf

6、r StrainsIf the F factor integrates at random points in the chromosome Hfr(high frequency of recombination)strains are generated.Episome(附加体附加体):a plasmid that can integrate into the chromosomeTransferred DNA May RecombineBut the transfer is unidirectional,unlike eukaryotic sex.Partial diploid(部分二倍部

7、分二倍体体)or meorzygote(部分部分合子合子)p 227接合后体接合后体外基因子外基因子 重组子重组子 内基因子内基因子 The Order of Transfer Reflects Gene OrderThis can be used to map genes in E.coli.接接合合后后体体阿奇霉素阿奇霉素 噬菌体噬菌体T1T1抗性抗性 乳糖乳糖 半乳糖半乳糖 The Order of Transfer Reflects Gene OrderThis can be used to map genes in E.coli.Note:F factor is the lastes

8、tMultiple Integration SitesThe F plasmid can integrate into the E.coli genome at a number of different places.Thus,one can map the whole E.coli genome by using different Hfr strains.Multiple Integration SitesOne can map the whole E.coli genome by usingdifferent Hfr strains.DonorRecipientConjugationD

9、efinition:Gene transfer from a donor to a recipient by direct physical contact between cellsMating types in bacteriaDonor (i.e.F+)F factor(Fertility factor)F(sex)pilusRecipient(i.e.F-)Lacks an F factorF+Physiological States of F FactorAutonomous(自主体)(F+)Characteristics of F+x F-crosses:(1)F-becomes

10、F+while F+remains F-(2)Low transfer of donor chromosomal genesF+F-F+F-F+F+F+F+Mechanism of F+x F-CrossesDNA transfer:(1)Origin of transfer(2)Rolling circle replicationPair formation:Conjugation bridgeF+HfrPhysiological States of F FactorIntegrated(Hfr)Characteristics of Hfr x F-crosses(1)F-rarely be

11、comes Hfr while Hfr remains Hfr(2)High transfer of certain donor chromosomal genesHfrF-HfrF-HfrF-HfrF-Which is a partial diploid in this figure?DNA transfer:(1)Origin of transfer(2)Rolling circle replicationHomologous recombination Pair formation:Conjugation bridgeMechanism of Hfr x F-CrossesHfrFAut

12、onomous with donor genes(F)Characteristics of F x F-crosses:(1)F-becomes F while F remains F(2)High transfer of donor genes on F and low transfer of other donor chromosomal genesPhysiological States of F FactorF 因子因子FFFFFF-FF-Called sexduction or F-duction（性导性导）Does the sexduction form a partial dip

13、loid？DNA transfer:(1)Origin of transfer(2)Rolling circle replication Pair formation:Conjugation bridgeMechanism of F x F-CrossesPutting it all togetherF+cells can transferthe F plasmid to F-cells.The F plasmid can integrate,generating Hfr strains.These strains can transfer parts of the bacterial genome.