Bacterial genetic recombination


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Bacterial genetic recombination

  1. 1. Genetic RecombinationBacterial GenomeAsexual ReproductionConjugation & Transformation
  2. 2. Bacterial Genome Circular, double stranded Nucleoid region  where DNA is tightly packed  Similar to nucleus in eukaryote but not surrounded by a membrane Genome contains one chromosomal DNA and many plasmids
  3. 3. Plasmids Small, circular, self-replicating pieces of DNA (separate from the bacterial chromosome) Contain a small number of genes Can incorporate themselves into the bacterial chromosome Episome: genetic elements that can exist either as a plasmid or as part of the bacterial chromosome
  4. 4. Advantage of Plasmids Plasmids are not required for bacterial cells to survive under normal conditions Under stress, genes on plasmids can confer advantages (e.g. resistance) Plasmids increase genetic variation and thus the likelihood of survival in bacteria
  5. 5. Important Plasmids R (resistance) plasmid  genes that make bacteria resistant to antibiotics  Also carries genes encoding the sex pili F (fertility) plasmid facilitates genetic recombination  Genes for production of sex pili  Fig 18.15a (male = F+, female = F-)  female become male if receive F plasmid from male
  6. 6. Asexual Bacterial Reproduction Binary fission Asexual form of reproduction rapid Bacterial colony = identical clone
  7. 7. Origins of Mutation Although new mutations are individually rare (low probability of spontaneous mutation) bacterial proliferation (growth rate) is high Thus, new mutations can have a significant impact on genetic diversity when reproduction rate is high Contrast with slow reproductive organisms (e.g. humans) where heritable variation is not due to new mutations but due to sexual recombination of existing genetic information
  8. 8. Mathematical Mutation E.coli reproduction  ~ 2 x 1010 new cells per day Spontaneous mutation rate  ~ 10-7 mutations per division (1 in 10 million) E. coli mutation rate  2 x 1010 divisions per day x 10-7 mutations per division  2000 bacteria with mutations per day
  9. 9. Thought Question Can spontaneous mutations alone along with a high reproduction rate account for the diversity seen in bacteria? Experiments: Lederberg and Tatum
  10. 10. Lederberg & Tatum Experiment (1946)
  11. 11. Hypothesis If spontaneous mutations was the sole cause of diversity, then after many reproductive cycles:  bacterial mutants that normally can not survive on minimal media should be able to  should see living cells in minimal media
  12. 12. Lederberg & Tatum Experiment (1946)
  13. 13. Example in textbook: Fig 18.12 Mutant Mutant E. coli need both makes arg makes trp not trp not arg arg and trp to survive Mutants that can’t make arg or trp will die unless provided Grow on minimal medium in the medium (solution of glucose and salts without arg and trp) X X No colonies No colonies
  14. 14. Thought Question What evidence is there that something else helps increase the genetic diversity in bacteria? Experiments:  Lederberg and Tatum  Recall: Griffith
  15. 15. Lederberg & Tatum Experiment (1946)
  16. 16. Lederberg & Tatum Experiment (1946)
  17. 17. Lederberg & Tatum Experiment (1946)
  18. 18. Lederberg & Tatum Experiment (1946)
  19. 19. Example in textbook: Fig 18.12 Mutant Mutant E. coli need both arg makes arg Mixture of makes trp and trp to survive not trp mutants not arg Mutants that can’t make arg or trp will die unless provided in the medium Add both mutants together into same Grow on minimal medium (solution of glucose and salts without arg and trp) minimal medium  None should live… but some did!!! No colonies Colonies No colonies
  20. 20. Recall: Griffith’s Experiment
  21. 21. Recall: Griffith’s Experiment Recombinant
  22. 22. Evidence of GeneticRecombination in Bacteria Spontaneous mutation alone could not explain why the mixture of cells could survive in minimal medium Cells that survived must have acquired genes from the other mutant strain Even though new mutations are a major source of genetic variation in bacteria, genetic recombination adds more diversity
  23. 23. Mechanism of GeneticRecombination Eukaryote:  Sexual reproduction / fertilization  Crossing over / meiosis Prokaryotes:  Conjugation  Transformation  Transduction
  24. 24. Sexual Bacterial Reproduction Conjugation: direct transfer of genetic material between two joined bacterial cells One way DNA transfer:  “male” donor  “female” recipient Mechanism  Donor extends sex pili to recipient  Sex pilus retracts, pulling cells together  DNA transferred through cytoplasmic bridge
  25. 25. Conjugation
  26. 26. Transformation Alteration of a bacterial cell’s genotype by the uptake of naked foreign DNA When two different strains of bacteria are mixed together, eventually they will show characteristics of both strains
  27. 27. Transformation
  28. 28. Conditions for Transformation Some bacteria have surface proteins that recognize and transport DNA from closely related species Some bacteria only transform when placed in a specific type of environment  E.g. E.coli in high calcium concentration
  29. 29. HW Question Describe the major similarity between conjugation and transformation. Explain the major difference between what is required for conjugation to occur compared to transformation.
  30. 30. Comparing Mechanism ofBacterial Genetic Recombination State ofMechanism Requirement State of donor recipient Physical contactConjugation via sex pili; Living Living F plasmid does not Free DNA in theTransformation require a cell or Living environment cell is dead Killed byTransduction Bacteriophage Living bacteriophage