Bacterial genetic recombination
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Bacterial genetic recombination






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Bacterial genetic recombination Bacterial genetic recombination Presentation Transcript

  • Genetic RecombinationBacterial GenomeAsexual ReproductionConjugation & Transformation
  • 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
  • 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
  • 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
  • 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
  • Asexual Bacterial Reproduction Binary fission Asexual form of reproduction rapid Bacterial colony = identical clone
  • 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
  • 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
  • Thought Question Can spontaneous mutations alone along with a high reproduction rate account for the diversity seen in bacteria? Experiments: Lederberg and Tatum
  • Lederberg & Tatum Experiment (1946)
  • 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
  • Lederberg & Tatum Experiment (1946)
  • 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
  • Thought Question What evidence is there that something else helps increase the genetic diversity in bacteria? Experiments:  Lederberg and Tatum  Recall: Griffith
  • Lederberg & Tatum Experiment (1946)
  • Lederberg & Tatum Experiment (1946)
  • Lederberg & Tatum Experiment (1946)
  • Lederberg & Tatum Experiment (1946)
  • 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
  • Recall: Griffith’s Experiment
  • Recall: Griffith’s Experiment Recombinant
  • 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
  • Mechanism of GeneticRecombination Eukaryote:  Sexual reproduction / fertilization  Crossing over / meiosis Prokaryotes:  Conjugation  Transformation  Transduction
  • 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
  • Conjugation
  • 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
  • Transformation
  • 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
  • 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.
  • 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