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Chromosome mapping in bacteria

Chromosome mapping in bacteria. Bacterial colonies, each derived from a single cell. Mixing bacterial genotypes produces rare recombinants. Mixing bacterial genotypes produces rare recombinants. Conjugation between two auxotrophic strains A y B

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Chromosome mapping in bacteria

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  1. Chromosome mapping in bacteria

  2. Bacterial colonies, each derived from a single cell

  3. Mixing bacterial genotypes produces rare recombinants

  4. Mixing bacterial genotypes produces rare recombinants

  5. Conjugation between two auxotrophic strains A y B • Streptomycin: antibiotic “kill” bacteria, but it does not destroy them • Conclusion: both strains do not have the same role • Strain A: donor, with fertility factor F • Strain B: receptor, it must survive Hayes Experiment

  6. F plasmids transfer during conjugation • Integration of the F plasmid creates an Hfr strain • Isolated by Hayes y Cavalli-Sforza from strains F+ • Transfer gene frequency 1000 times higher than F+ • Do not transfer plasmid/fertility • Hfr (High Frequency of Recombinants) • Integration of the F plasmid F in the chromosome

  7. Mating • Hfr aziR tonR lac+ gal+ StrS • F- aziS tonS lac- gal- StrR • Each phenotype number is counted in every sample (exconjugants) • Colony number goes up as the sample extraction time The “blender” experiment”Wollman y Jacob, 1957

  8. The F integration site determines the order of gene • transfer in HFRs

  9. Two types of DNA transfer can take place during conjugation

  10. A single crossover cannot produce a viable recombinant Figure 5-16

  11. The generation of various recombinants by crossing over in • different regions

  12. Faulty outlooping produces F´, an F plasmid that contains • chromosomal DNA

  13. Bacteria exchange DNA by several processes

  14. Cycle of a phage that lyses the host cells

  15. Lederberg y Zinder (1951) • Experiments on Salmonella • Transfer of genes between two strains without contact • If filter pore size was smaller than a virus, transfer did not take place • Transduction

  16. Generalised transduction by random incorporation of bacterial • DNA into phage heads

  17. Selected marker Unselected markers 1 leu+ 50% azir, 2% thr+ Frequency of cotransduction • Donor: leu+ thr+ azir • It is infected by the phage. Phage lysate collected removing surviving donor bacteria • Used to infect receptor bacteria • Receptor: leu– thr– azis • One of the markers (phenotypes) is selected • Then, checked colony phenotypes for the other genes Fcotransductionleu-azi= nº colonies leu+ azi+ nº colonies leu+ 2 thr+ 3% leu+, 0% azir 3 leu+ y thr+ 0% azir

  18. leu azi La FC es mayor entre thr-leu que entre thr-azi 2/3 50 Cotransductionmap • Frequency of cotransduction: frequency of colonies showing the selected marker and one of the not selected markers • The closer two genes are to each other, the more likely they are to be transducted by the same transductant particle • Cotransduction frequency is an inverse measure of distance thr thr

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